Principles of

U,M

/

ANIMAL ECOLOGY

By
W. C. ALLEE ORLANDO PARK

Late Professor of Zoology, The University of Chicago Professor of Zoology, Northwestern University

ALFRED E. EMERSON THOMAS PARK

Professor of Zoology, The University of Chicago Professor of Zoology, The University of Chicago

KARL P. SCHMIDT

Late Chief Curator of Zoology, Chicago Natural History Miisciiin

W. B. SAUNDERS COMPANY

PHILADELPHIA AND LONDON

Reprinted January, 1951, August, 1955, November, 1959, June, 1961,
September, 1963, February, 1965, May, 1967 and April, 1969

COPYMGHT, 1949, BY W. B. SAUNDERS COMPANY

■A-

COPYRIGHT UNDER THE INTERNATIONAL COPYRIGHT UNION

All rights reserved. This book is protected by copyright. No part of
it may be duplicated or reproduced in any manner without written
permission from the publisher. Made in the United States of
America at the Press of W. B. Saunders Company, Philadelphia.

TO

Marjorie Hill AUee
Winifred JelliflFe Emerson

Alberta Fritsche Park

Martha Whitehead Park

Margaret Wightman Schmidt

"As concerns 'Relations Physiology', i.e., the study of
the relations of the animal organism to the external
world, this in turn falls into two segments, ecology and
chorology. By ecology we mean the body of knowledge
concerning the economy of nature— the investigation of
the total relations of the animal both to its inorganic and
to its organic environment; including, above all, its
friendly and inimical relations with those animals and
plants with which it comes directly or indirectly into
contact— in a word, ecology is the study of all those com-
plex interrelations referred to by Darwin as the condi-
tions of the struggle for existence. This science of ecology,
often inaccurately referred to as 'biology' in a narrow
sense, has thus far formed the principal component of
what is commonly referred to as ''Natural History. As is
well shown by the numerous popular natural histories of
both early and modern times, this subject has developed
in the most close relations with systematic zoology. The
ecology of animals has been dealt with quite uncritically
in natural history; but natural history has in any case had
the merit of keeping alive a widespread interest in
zoology."

Ernst Haeckel, 1870

PREFACE

In writing this book we hope we have a
start at supplying the orientation of which
ecology, a subscience of biology, is in
need. The time seemed ripe for a group of
ecologists, approaching the science from
various points of view and with various
techniques, to attempt to gather together
fundamental concepts, supported in so far
as possible by well-verified evidence. Others
have accumulated many facts that we have
drawn upon freely, from both published
compilations and original research reports,
but our effort has been directed primarily
towards the presentation and documentation
of general ecological principles. We have
not been wholly successful. Many concepts
and principles of a future science of ecology
are only beginning to be recognized, and
many important ideas that will be taught
to future classes in biology have not yet
been conceived by the present generation
of ecologists.

We hope that, as a result of our efiForts,
the general biologist may more easily grasp
the scope and implications of ecology and
that profitable lines of investigation will be
more readily apparent to interested stu-
dents. We are encouraged by remembering
the stimulus gained some years ago from
Elton's small books, in which he emphasized
ecological principles.

From our point of view there is an ur-
gent demand for three different types of
books about ecology. On the one hand we
could well use an encyclopedic treatise of
present-day knowledge of the subject. In
distinct contrast, a brief statement of the
underlying principles would also be useful.
We felt that there was also a need for a
study of the underlving principles together
with a sampling of the evidence on which
they are based. This is the task we have
undertaken. So far as possible, no fact is
admitted to these pages for its own sake,
and although no general concept is stated

without the presentation of evidence sup-
porting it, an attempt has been made to
give no more than the necessary minimum
of factual support.

At one point we are immediately on the
defensive. In Umiting our discussion, at
least in certain chapters of the book, pri-
marily to the principles of animal ecology,
we appear to be recognizing a logical
dichotomy between ecological relations of
plants and of animals where none exists.
The decision not to extend our work to in-
clude the whole scope of ecology, the so-
called bio-ecology of some writers, was
based primarily on convenience and work-
ability. Yet, although this book stresses ani-
mal ecology, we have felt free, in fact we
have been compelled, to draw on ideas from
plant ecology and to make continued use of
the concepts in which plants and animals
are necessarily considered together. The dis-
tinction between our "animal ecology" and
ecology in the most comprehensive sense
lies in our emphasis on the animal factors.

We stress ecological generalizations from
two vantage points. First, there are those
principles concerned with the functions or
physiology of contemporary individuals and
ecological assemblages of whatever rank.
Second, there are those ecological principles
concerned with organic evolution. We are
not interested in helping to continue the
separation between these two aspects of
ecology. Rather, our aim is to point out
their essential interrelation, and we hope we
may have depicted ecology in better per-
spective in this connection.

In addition to attempting the correlation
of the shorter-term contemporary phenom-
ena with a longer-term evolutionary per-
spective, we have also been impressed by
the need for an historical approach to many
aspects of the subject. Besides the fairly full
section on ecological history, the historical
approach is frequently made elsewhere in

Vlll

PREFACE

the book. This emphasis has not necessarily
aflFected the selection of supporting ex-
amples, since neither the older, more widely
known illustrations nor the most recently
discovered ones have been regularly used.

We discuss ecological principles dealing
with the nonliving physical environment
more or less as a unit, whether they are
concerned primarily with the individual
(autecology) or with the population or the
community (synecology). The consideration
of the biotic environment of the individual
organism is less unified and perhaps less
comprehensive. It is hard to avoid some
duplication in dealing with the environmen-
tal relations of these different biological
units, and the inherent difficulties have not
been resolved formally and logically. In dis-
cussing principles dealing with the organism
in its nonliving physical environment, we
have anticipated many somewhat similar in-
terrelations with the higher ecological cate-
gories. In contrast, much of the discussion
of the biotic environment is given in direct
connection with populations, communities,
and evolution, rather than in a single part
of the book.

In our treatment of the ecological prin-
ciples that emerge with the population as
the unit of study, our attention centers first
on the population in both laboratorv and
field and, later, on aggregations and on
certain aspects of societies. The analysis of
functional contemporary principles leads
naturally to the examination of interspecies
groups. Here our primary concern is with
the underlying structure, organization, suc-
cessional development, and distribution of
the ecological community. In this section
our emphasis is on terminology only in
those instances in which the term itself is
a well-authenticated index of the principle.
The multiplication of terms represents a
juvem'le stage of the science as a whole,
and it is hoped that a critical definition and
sifting of the concepts that support the
terminology may lead to a reduction of
their complexity and to an advance toward
maturitv.

Finally, in examining the problems of
evolution we attempt to bring out those
ecological aspects that are particularly sig-
nificant, such as isolation, selection, adapta-
tion, distribution, regressive evolution, and
others insofar as they contribute to ecologi-
cal principles or as the ecological approach
aids in their solution.

The book was planned jointly. Each
author undertook primary responsibility for
preparing the first draft of sections or chap-
ters for the handling of which he showed
particular competence so far as our group
membership was concerned. Early working
outlines and successive copies of each chap-
ter or section were distributed to the other
authors and received criticism concerning
both manner and matter, particularly with
regard to possible omissions. Eventually all
parts of the manuscript were read aloud to
the other authors, and there was much dis-
cussion of questioned points. We feel that
in the main we have reached a truly re-
markable degree of agreement both on the
major and minor principles of ecology,
though some generalizations, emphases, and
conclusions are not shared with equal en-
thusiasm by every author. Fortunately,
these are usually matters of relatively minor
significance.

Many parts of the manuscript were read
critically by persons outside our circle, and
the revised version was again distributed to
the other authors. Finally there was a
period of collation between pairs of authors.
Near the end of the writing each author
was instructed to use his own judgment in
the final polishing of the chapters for which
he prepared the first draft.** Chapters from
various sections were also read to the

* We had originally hoped that many traces
of personal origin of chapters would disappear
during this extended and detailed critical treat-
ment and that final responsibility would rest
entirely with the group. This hope has been
realized in large part, but, as was to be ex-
pected, each author feels decidedly more re-
sponsibility for the selection, organization,
presentation and interpretation of the material
he has himself written than he does for other
chapters, or even for the book as a whole.
Particular responsibility for the different chap-
ters was distributed as follows:

Preface and Introduction ( Chapter 1 ) :
K.P.S. (based on drafts by W.C.A. and T.P.).

Chapters 2, 4 to 16, inclusive, and 23:
W.C.A.

Chapters 3 and 18 to 22, inclusive: T.P.

Chapter 17: W.C.A. and K.P.S.

Chapters 24 and 31 to 35, inclusive: A.E.E.

Chapters 25 to 29, inclusive: O.P.

Chapter 30: K.P.S. and O.P.

General editing of the manuscript: K.P.S.

The four junior authors here acknowledge
the leadership of Dr. Warder Clyde Allee and
their indebtedness to him throughout the
preparation of the present work.

PREFACE

IX

Chicago Ecology Club, and the resulting
discussions were stimulating and profitable.

We take this opportunity to thank many
people for their help in this enterprise. Of
course, final responsibifity for all remaining
errors rests with the authors.

Dr. Theodor Just (Chicago Natural His-
tory Museum) and the late Dr. Chancey
Juday (University of Wisconsin) read all of
Section I, and the latter also criticized the
material on fimnology in Section IV. The
late Dr. F. R. Lilfie, Dr. EHzabeth A. Bee-
man (University of Chicago), and Dr. Ruth
M. Merwin (National Cancer Institute,
Bediesda, Maryland) read Chapter 2, and
the last mentioned checked its bibliography.
Dr. Garrett J. Hardin (Santa Barbara Col-
lege) criticized Chapters 4 to 18, inclusive.
Mr. Peter W. Frank and Mr. Gerson Rosen-
thal (University of Chicago) each read cer-
tain of those chapters. Among others from
the same University, Dr. T. F. W. Barth
(Geology) checked over the paragraphs on
earthquakes. Dr. Ralph W. Gerard (Phys-
iology) and Dr. Clay G Huff (Parasitol-
ogy) gave similar advice and aid concern-
ing other matters in Section H, and Dr.
Charles E. Olmsted (Botany) gave help-
ful botanical aid. Dr. Fritz Haas (Chicago
Natural History Museum) was helpful on
various sections.

Dr. L. C. Birch (University of Sidney)
read Chapters 3 and 18 to 22, inclusive. Of
the staff of Northwestern University, Dr.

William Powers aided with Chapter 28, Dr.
Orrie J. Eigsti read the material on bacteria,
Dr. L. H. Tiffany was consulted with re-
spect to photosynthesis, and Dr. Albert
Wolfson was helpful on the subject of bird
migration (Section IV).

The following men, all from the Univer-
sity of Chicago, helped in the section on
Evolution. Dr. Sewall Wright read the
whole section. Dr. Herluf H. Strandskov
read parts, especially the matters dealing
with population genetics. Dr. Clay G Huff
and Dr. W. H. Tafiaferro criticized and
made suggestions concerning parasitism. Dr.
O. H. Robertson helped similarly with the
treatment of Fneumococcus, as did Dr. E.
J. Kraus with the portion on rusts and with
plant ecology, and Dr. John M. Beal with
botanical names, evolution of chromosomes,
rusts, and at various other places. Dr. Ernst
Mayr (American Museum of Natural
History) read Chapter 32. Mr. Robert F.
Inger was extremely helpful in checking
bibUographic references, and our few ref-
erences to the Russian literature were put
in correct form by Mr. D. D wight Davis
(Chicago Natural History Museum).

The authors are indebted to the Ridge-
way Memorial Fund of the University of
Chicago for the support that made possible
the illustration of the book and to Winifred
Emerson for a critical poUshing of the illus-
trations. We have freely selected, modified,
and redrawn figures from varied somces.

The Authors

CONTENTS

Preface vii 1. Introduction

SECTION I. THE HISTORY OF ECOLOGY

2. Ecological Background and

Growth Before 1900 13

3. First Four Decades of the

Twentieth Century 43

SECTION II. ANALYSIS OF THE ENVIRONMENT

4. The General Environment 73

5. Radiation: A General Introduc-

tion 87

6. Heat 91

7. Light 121

8. Gravity, Pressure, and Sound 129

9. Currents of Air and of Water 140

10. The Substratum 158

11. Physicochemical and Cheaucal

Phases 164

12. Water 177

13. The Atmospheric Gases 189

14. Dissolved Salts as Limiting

Factors 198

15. Combinations of Environmen-

tal Factors 206

16. Ecological Relations of Soil 216

17. BiOTic Factors in Relation to

Individuals 227

SECTION III. POPULATIONS

18. General Properties of Popula-

tions 263

19. Biological Backgrounds for

Population Stltdies 272

20. Certain Demographic Back-

grounds FOR Population Stud-
ies 287

21. The Growth Form of Popula-

tions 305

22. Population Factors and Se-

lected Population Problems 331

23. Animal Aggregations 393

24. The Organization of Insect So-

cieties

419

SECTION IV. THE COMMUNITY

25. Introduction 436

26. Community Organization:

Stratification 441

27. Community Organization:

Metabolism 495

28. Community Organization: Peri-

ODisM 528

29. Community Succession and De-

velopment 562

30. BiOME and Biome-Type in

World DiSTRiBtmoN 580

Xll CONTENTS

SECTION V. ECOLOGY AND EVOLUTION

Introduction 598 33. Adaptation 630

oi T- r> \7 can 34. NATURAL SELECTION 640

31. Ecology and Genetic Variation 599

35. Evolution of Interspecies In-

32. Ecology and Isolation 605 tegration and the Ecosystem 695

INDICES

Bibliography and Author Index 731 Subject Index 803

1. INTRODUCTION

Ecology may be defined broadly as the
science of the interrelation between living
organisms and their environment, including
both the physical and the biotic environ-
ments, and emphasizing interspecies as well
as intraspecies relations. The living or-
ganism may be defined, though somewhat
incompletely, as a physicochemical mech-
anism that is self-regulating and self-
perpetuating, and is in process of equifi-
bration with its environment. The environ-
ment of any organism consists, in final anal-
ysis, of everytliing in the universe external
to that particular organism. Those parts
of the total environment that are evidently
of direct importance to the organism
are regarded as constituting the effective
environment. The relations of any organism
or community of organisms with the envi-
ronment are, in the language ot Raymond
Pearl (p. 266), (1) particular: specific for
every organism; (2) continuous: the organ-
ism Uving in its environment for its total
life; (3) reciprocal: the environment affect-
ing the organism, and vice versa; and (4)
indissoluble: dissociation of an organism
from its environment being impossible. The
organism and groups of organisms are the
essential biological units in ecology, and we
exclude the intraorganismal or cellular en-
virormient except as special cases demand
its examination.

The reciprocal relations require especial
attention. The interaction of the environ-
ment and the organism is obvious in almost
every field of biology. Physiological proc-
esses are correlated primarily or secondar-
ily with environmental fluctuations: energy
for life is derived from the environment;
growth and development show relationship
to environmental factors; environmental
forces and substances impinge upon the
sense organs of animals and the reactive
systems of plants; behavior patterns in large
Dart are responses to environmental pat-

terns; distribution of plants and animals is
determined by variations in the environ-
mental complex; isolation through environ-
mental factors has profoundly influenced
genetic systems of organisms, and the en-
vironment has acted as a selective agent in
deteiTnining the survival of organisms and
populations, thus leading to the evolution-
ary development of hving systems.

in its more scientific aspects, ecology is
intrinsically a difficult subject. In its rela-
tions it depends on many other phases of
biology, and it is built directly, as well as
indirectly, on the physical sciences. The
subsciences of biology and the physical
sciences are in turn dependent upon and af-
fected by ecology. Yet in its close relation-
ship to natural history, ecology is near the
stolon from which all biology has develop-
ed. As such it sometimes seems deceptively
simple, and under many conditions ecology
may really be simple. Almost any good,
precise observation within its extended bor-
ders makes a useful contribution to the
mass of needed ecological information. Its
wide range of subject matter, open to ex-
ploration by diverse techniques, is a major
reason for the lack of ready integration of
the field of ecology as a whole. It is at
any rate obvious that the development of
generaUzations and principles in ecology
and the orientation of its subject matter
with respect to such principles, have been
slow.

Workers in ecology, fike those in any
other broad field, face reproach from more
narrow specialists. Physiologists, for exam-
ple, are hard pressed to meet the rigorous
standards of biophysics or biochemistry,
to say nothing of those of physics or chem-
istry proper. In part this particular diffi-
culty is not directly related to subject mat-
ter, as evidenced by the relative precision
gained by specialists as contrasted with
generaUzers in any field. In part the difii-

INTRODUCTION

culty in biology is associated with the in-
trinsic complexity of the materials to be
analyzed or synthesized.

Biologists working with the social hfe of
insects, or of other animals, are frequently
tempted to regard their own work as more
precise than that done by equally compe-
tent students of human sociology; and those
deahng with human material often feel
compelled to explore subjective psychologi-
cal aspects of sociology that are almost or
completely closed to the student of social
insects.

Much of human sociology is an integral
part of ecology. There are reciprocal in£u-
ences between these two sciences, influ-
ences that are especially apparent in such
practical matters as the development of the
Canal Zone in Panama, with the details and
outreach of the Tennessee Valley Author-
ity, with stream pollution, and with the
whole set of problems centering about the
potential or actual dust bowls of semiarid
regions of the world. Much that is now be-
ing done in such projects is recognized as
ecology.

A major difference between human rela-
tionships and those of other animals is the
role played by the symboHc language of
man, and by ideas, as contrasted with the
restricted use of both among nonhuman
populations. The extent to which animals
other than primates communicate with each
other, and the means employed, are still
matters for investigation. We know much
about the importance of odors as signals,
particularly among such animals as dogs,
ants, and moths. We also know about var-
ious cries, songs, and visual displays that
reveal sexual receptivity, or nonreceptivity,
that faciUtate aggregation or warn of dan-
ger. We have evidence that the complex
activities within the ant colony are integrat-
ed primarily by touch and odor; to regard
such manifestations as language emphasizes
the distinctiveness of human speech. The
demonstration of ideas— particularly of ab-
stract ideas— among the mental processes of
nonhuman animals is still more diflBcult.

We have purposely avoided emphasis on
human sociology, but we hope that in time
a maturing ecology will be properly fused
with that field.

The line between ecology and physiology
is equally difiBcult and perhaps equally im-
possible to draw with exactness. One of the
most helpful distinctions concerns the work-

ing imits in the respective subject matter.
The physiologist seldom gets beyond con-
sidering an individual as his upper limit;
often he is content with some organ or even
with an individual nerve fiber; his research
may focus finally at the molecular level. In
contrast, the ecologist usually regards an
individual organism as his smallest unit, ex-
cept as he needs information about the
functioning of the fiver, pancreas, muscles,
or other organs in order to understand the
general environmental relations of the
whole organism, or of the community. The
kidneys give a remarkably good illustration
of the close correlation that may exist be-
tween an inner organ of the body and the
general environment. For ecology, the
supra-individuafistic units are real entities.
Aggregations, populations, societies, and
various units at or near the community
level present problems rarely recognized by
physiologists working as physiologists. Yet
the problems of this level are real and fie
so near the center of ecology that Shelford
(1929, p. 2) makes the statement that
ecology is the science of animal communi-
ties.

A single Asellus moving upstream in a
small brook has an ecology of its own, even
though it is not at the moment in direct as-
sociation with any organisms other than the
bacteria and other nannoplankton of the
water or those minute forms residing on its
own surface or acting as its parasites. We
have no reason to befieve that this partic-
ular isopod remembers or anticipates con-
tacts with another fiving creature. It is es-
sentially alone, a creature of the moment,
responding to an innate urge to move up-
stream against the current of water. The
positive reaction is not free from environ-
mental influences; it is dependent on such
external relations as the amount of oxygen
and of carbon dioxide present, and on the
ionic content of the surrounding water. The
isopod is also, without knowing it, a mem-
ber of the community of the brook and so
is related to the ground water that feeds
the stream and, to some extent, to the bodv
of water into which the brook flows. At a
different level, the single, isolated isopod
may well have been and may soon become
again a member of an isopod aggregation
with which other animals are also asso-
ciated.

The physical environment impinges di-
rectly on the individual as it does on popu-

INTRODUCTION

lations or on a whole community, and it in-
itiates and diiects the course of action of
innumerable small-scale events. Phenomena
on the largest scale may likewise depend
directly on the physical environment, as ex-
emplified by isostasy, the condition of equi-
Ubrium in which the heavier portions of the
earth's crust sink to form the ocean basins,
while the lighter parts are pushed up as the
continental platforms.

The definition of ecology as the science
of communities may be vahd in its total
implications. The isopod illustration pre-
sents a phase of a much larger problem. In
another example, is the cell, the tissue, or
the organism as a whole the unit? The cell
may itself be broken into parts, and in
genetics we hear much about chromosomes,
chromomeres, and genes. So in ecology
there may be ecological relations of parts of
organisms— the nephiidial system, for exam-
ple—of the whole animal, of populations,
whether aggregated or dispersed, of asso-
ciations and communities, and of biomes. At
whatever level one begins, and whatever
the point of view, one must study all pos-
sible unitary levels before coming to a full
understanding of the ecology of either an
isolated isopod moving slowly upstream in
a small brook, or of the vast biome in which
the brook itself is a minor and almost neg-
Ugible incident.

Close interaction exists between genes
and the general environment, both in devel-
opment and in evolution. A gene may be
helpfully regarded as a reagent in the proc-
ess of development; the environment also
enters intimately into the developmental
processes. Aside from supplying continuity
under suitable conditions, much that is pro-
duced by the gene system can be dupli-
cated by appropriate surroundings, either as
a result of shock furnished by an environ-
mental insult or from the more steady pres-
sure of a steadily continuing physical or
biotic induction. Such subjects are treated
in some detail in any modern work on phy-
siological genetics (Goldschmidt, 1938), in
more specialized books such as Hogben
(1933) or Newman, Freeman and Holzin-
ger (1937), and even in more popular ac-
counts, as in the small book by Dunn and
Dobzhansky (1946).

Animals do not develop without an en-
vironment; contrariwise, even given opti-
mum environment, organisms do not start
to grow without the presence of a spore or

zygote or of a group of cells from a pre-
ceding organism. Both a bearer of heredity
and a suitable environment are necessary for
development. After much discussion, lasting
from the time of Darwin, Galton, and
Weismann, we can now ask fairly exact
questions in this field and expect to find
fairly exact answers. Some pertinent data
are available at various evolutionary levels
such as those of the micro-organisms, the
insects, and man. The relation between
heredity and environment is frequently call-
ed the problem of nature versus nurture.
In its present dress the discussion does not
center about environment versus heredity in
general, but rather concerns the functions
of these two necessary components with
regard to some particular characteristic,
such as the color of the shanks in hens, the
width of the bar in bar-eyed Drosophila,
coat color in certain mammals, or intelli-
gence or stature in man.

Concrete examples may clarify what is
meant by the ecological relations of such
characters. Yellow fat in rabbits or yellow
shanks in hens require a source of yellow
coloring matter, such as is furnished by yel-
low corn or by the xanthophyll from green
foliage or other similar foodstuffs; but, for
yellow to be developed, the enzyme that
breaks down xanthophyll must be absent,
and this lack in the hen or rabbit is asso-
ciated with gene action. Absence of xantho-
phyll from the food yields equally white fat
or white shanks, and one cannot know
whether the absence of yellow is primarily
environmental or genetic, or both, without
more direct knowledge of both the heredity
and the feeding routine. The effect of tem-
perature on the width of the bar in bar-
eyed Drosophila, of heat on the production
of feathers in young frizzle fowl, or of the
absence of iodine in water containing frog
tadpoles fed on an iodine-free diet, all dem-
onstrate significant effects of the environ-
ment on the development of characters
that are also definitely related to the gene
complex (Hogben, 1933).

In man, the best assay of nature in asso-
ciation with or in contrast to nurture has
come from studies of identical twins reared
apart compared with those of others reared
together, and further compared with similar
qualities in fraternal twins. Identical twins
have an identical gene pattern, fraternal
twins do not. A good study of this kind is
that of Newman, Freeman, and Holzinger

INTRODUCTION

(1937), which shows that "physical char-
acters are least affected by the environment,
that intelligence is affected more; educa-
tional achievement still more; and person-
ality or temperament, if our tests are to be
rehed upon, the most."

Reasons for the slow development of
ecology can be foimd in the general state
of nonecological science, in the relative ina-
bihty of ecologists to work with intellectual
and physical tools of precision, and espe-
cially in the scope and iimate complexity of
the subject.

There are few good reasons other than
the convenience ot authors and readers for
not treating ecology as a whole. Plant ecol-
ogists can make a strong case for focussing
on plant relations and largely neglecting
animal hfe, since the plants are primary
producers and play a highly important role
in providing shelter for many types of ani-
mals. Even so, the neglect of animal
activities omits or minimizes such phenom-
ena as grazing and browsing, working of
the soil, seed scattering, and the pollination
of many important flowering plants. Stu-
dents of animal ecology must give due
attention to plants if for no other reason
than that animals Uve in an environment
largely conditioned and controlled by the
plant matrix. Acknowledging the failure of
the present work to develop a unified ecol-
ogy, we fully recognize the need for a
future work on the Principles of Ecology
which will make the logical synthesis of
the two fields.

Plant ecology presents two aspects, vege-
tational and floristic. Animal ecology largely
lacks the vegetational phase so far as land
animals are concerned. It is true that forest
animals differ in general appearance from
those of grasslands, but the differences in
body proportions by no means approach the
contrast in growth forms between grasses
and trees. The general aspect of aquatic
animals stands in marked contrast with that
of land forms, and various convergences
exist among both series that approach what
we understand when a vegetational type is
mentioned. Thus the fishhke form of
whales, seals, walruses, fossil sea reptiles,
tadpoles, certain larvae of lower chordates,
and of the whole galaxy of fishes stands in
distinct contrast wdth typical terrestrial
structures. The sessile animals of coral reefs
and oyster banks approach the terrestrial
vegetational concept even more closely.

Contrary to first impression, the fact that
animal ecology is based primarily on
faunistic considerations tends to simplify its
study, since the student of animal relations
is not so much tempted to pursue the super-
ficial types of inspection that make the
carwindow approach one of the charms and
also one of the pitfalls of plant ecology.

The apphcation of even a well-formu-
lated generahzation to a given situation may
require further research. Thus in the control
of mosquito-borne diseases of man, the
mosquitoes that transmit epidemic yellow
fever behave according to rule. A trained
executive can sit at his desk in New York,
after he has fully learned the principles in-
volved, and give directions which, if faith-
fully carried out, will lead to the control of
the disease. It is not so with the anophehne
mosquitoes that carry malarial parasites.
Each type of malarial vector is a special
case, and, without further knowledge, the
general principles may seem inappHcable to
the given situation. In the southeastern
United States, malaria is transmitted by a
marsh-dwelUng mosquito characteristic of
sluggish water; in Italy, by a form that lives
in the cold running water of the uplands;
in Puerto Rico, by a brackish-water mos-
quito. Under such varied conditions the
needed local detail is of equal value with
knowledge of the underlying general prin-
ciples.

An example of the benefits to be derived
from an approach to ecology through gen-
eral principles is given in the summarizing
paragraph of ocean cinrrents by Sverdrup,
Johnson, and Fleming (1942, p. 399), who
conclude:

"From this brief summary it is evident that
it is virtually impossible to obtain knowledge
of the ocean currents on an entirely empirical
basis. If this were to be accomplished, it would
be necessary to conduct measurements from an-
chored vessels at numerous localities for long
periods and at many depths."

A word is in order about "principles." We
do not wash, nor are we competent, to en-
ter into a philosophical evaluation and
definition of "laws," "concepts," and "prin-
ciples." Ecology proceeds, as does any
empirical science, (1) by the collection of
relevant facts; (2) by the arrangement of
these facts into ordered series according to
their relations and patterns; and (3) by the
development of higher-category knowledge

INTRODUCTION

or principles that synthesize and correlate
the material at hand. Thus the "principles"
we shall attempt to formulate and interre-
late are simply those generalizations in-
ductively derived from the data of ecol-
ogy. We regard the so-called "laws of
nature" as empirical, derived from the facts,
and not the facts from the laws. In this
view, a principle is a means of description
of nature in succinct and compressed form.
This is true in the relatively well-organized
physical sciences, in which the principles
frequently can be reduced by mathemati-
cal statement to the extreme of simplifica-
tion. In the vastly more complex biological
sciences, mathematical formulation of gen-
eralizations is more diflBcult, and possible
only in limited segments of the complex.
The process of inductive generalization is
useful at every stage. The principles de-
rived from the compression of a mass of
data into a science form the main basis for
deductive thinking and for hypotheses
which ask new questions and make possible
new advances, on the one hand by opening
up new fields of inquiry and on the other
hand by progressive correction of the older
generalizations in the light of additional
data.

We subscribe to the general principle of
scientific parsimony ("William of Occam's
razor"), which may be stated as follows:
"Neither more, nor more onerous, causes
are to be assumed than are necessary to ac-
count for the phenomena" (Pearson, 1937,
p. 340). For ecology in particular, the
number of entities should not be unneces-
sarily increased. Furthermore, Morgan's
canon (1894) concerning animal behavior
is essentially a quantitative development of
"Occam's razor" and an application of the
law of parsimony: "In no case may we in-
terpret an action as the outcome of the
exercise of a higher psychical faculty, if it
can be interpreted as the outcome of one
which stands lower in the psychological
scale."

There is an understandable tendency in
any synthesizing discussion to review chiefly
the progress made in recent years or dec-
ades. This is sound practice in many ways,
but one result is that work, often excellent
work, of previous decades or even centu-
ries may be neglected. A false idea of
rapidity of progress is thereby encouraged,
and the concept of the relatively complete
modernity of subject matter tends to be

built up in the thinking of younger readers,
although the minds of authors and editors
may have been entirely free from such a
misconception. We have accordingly made
a serious eflfort throughout this book to sup-
ply historical perspective and regard the
history of ecology and of its antecedent
sciences as an integral and significant part
of our treatment.

Ecological history, like that of zoology in
general, can be summed up briefly as fol-
lows: In the Greek period— either because
such was the case, or because Aristotle did
not cite sources— it was the apparent rule
to study nature directly and to think over
the implications of observations made at
first hand. During the long scholastic period
in the Middle Ages, the influence of which
unhappily lingers on here and there, the
fashion changed to a study of books, or at
least a part of those available. The spirit of
the scientific awakening was at length sum-
marized by the dictum of Louis Agassiz:
"Study nature, not books."* Too often this
became perverted, by practice rather than
by precept, to the study of preserved speci-
mens, and some books. A gradual change
occurred until in the early decades of the
present century the tacit advice became:
Study living and preserved organisms in
the laboratory together with the pertinent
books.

One constant effort of the modern ecolog-
ical movement has been to take the study
of nature again out under the sky. This
could not entirely succeed, in part because
of the difficulties in doing accurate analyti-
cal work in the field. A partial compromise
is attained by our turning to the greenhouse
and breeding cage, where experimentally-
minded ecologists have been met by
workers moving out of orthodox labora-
tories into these substitutes for field condi-
tions. Some ecoloeists have remained stub-
bomlv in the field, where they are being
ioined by a trickle of the more orthodox
indoor students. Laboratory and field ecol-
ogy are interdependent, and both are
essential. At the same time, the check of
knowledge gained directly against printed
accoimts, both as to empirical content and

• An amusing and even paradoxical com-
mentary on this famous aphorism may he
derived from the fact that Agassiz prepared the
first comprehensive bibliography of zoology—
the four volume Bihliosraphia Zooloziae, pub-
lished by the Ray Society (1848-1854).

6

INTRODUCTION

philosophical implications, is being given
more balanced consideration.

The reahty and usefulness of the popu-
lation as an ecological unit were apparent to
us when we outlined the present book, and
our subsequent work has reinforced our
conviction of the importance of the prin-
ciples that center on the population. We
view the population system, whether intra-
species or interspecies, as a biological entity
of fundamental importance. This entity can
be studied with some measure of precision,
and the emergent principles are significant
throughout the field of ecology. The popu-
lation is forged by strong bonds with
autecology through the physiology and be-
havior of individuals; communities are
composed of recognizable population ele-
ments; and evolutionary ecology depends
directly upon population systems, since
selection acts upon populations that evolve
and become adapted to their environments,
to a more important degree than upon in-
dividuals. The study of populations as such,
as operational systems, yields principles
that clarify the nature of group interactions,
interactions that do not exist at the level
of the single organism, and that are too
complex at the community level to be
analyzed in a quantitative way.

The major relations of animals center
around nourishment, reproduction and pro-
tection. The reaction to these needs may be
summarized by the concept of a "drive" to-
wards favorable ecological position. This
usually implies a drive for security of one
kind or another, or of all kinds. The par-
tially mystical idea of a "drive" hides the
nonmvstical one of the survival values fur-
nished by the attainment of nourishment,
protection and sufficient reproduction, or
even by the attempt to secure them.

The situation can be clarified somewhat
by attending to only one of the three fun-
damental needs— protection, for example.
The given animal, or population, may orient
and move actively toward protected places
as a generalized reaction that may become
much more marked in times of particular
stress. Or the individual or population may
wander about, apparently at random, and
come to rest tmder favorable conditions.
Animals may invade a more stable physical
environment such as that furnished by a
pond or a forest, or in winter there may be
a movement down to the forest floor or an
active invasion of its superficial carpet of

leaves and of the soil beneath them. Secu-
rity may be gained by attaining control of
a portion of the environment through the
slow processes of ecological succession
leading toward the estabhshment of an
ecological climax or through the more
active animals moving into natural safe
niches or building their own shelters. A
third mode of progress toward ecological
security, or more assured ecological posi-
tion, is found in societal evolution. These
are all aspects of the tendency toward eco-
logical homeostasis, and this sort of homeo-
stasy is one of the major inclusive principles
of ecology and, with a diflFerent emphasis,
of physiology as well.

The tendency towards homeostasy ex-
tends through the diverse phases of ecology,
whether the subdivisions are based on habi-
tat differences such as those characteristic
of oceanography, of limnology, or of the
land, or of the living habitats of parasites.
Such tendencies are found under primarily
physical relations with nonliving environ-
ments and also when all the relations are
primarily biotic.

The physical universe is indifferent to life
in general and resistant to the influence of
living organisms even in slow-working long-
time trends. For that matter, organisms are
largely indifferent to each other. Dramatic
incidents occur, and there is a strong tend-
ency to record and to overemphasize
these. Animals, under many conditions, and
plants as well, may merely persist; it is then
needful to search out the undramatic rela-
tions that allow them to continue to live
when little or nothing beyond mere exist-
ence is involved. Often only a saving few
individuals survive in a given habitat, and
these may spend much of their time appar-
ently doing nothing at all except remaining
alive. Hibernation, aestivation, "resting"
cysts, and resistant or so-called winter eggs
represent periods of marked quiescence.
The quiet retirement of animals capable of
extreme activity is often a fundamental part
of living. Hens fight and actively establish
social orders based on dominance and sub-
ordinance, yet they spend much more time
in which no activity is evident. Chimpan-
zees exhibit a strong drive for status in a
social group, and yet they too pass only a
small percentage of their time in active so-
cial tension. Outdoor nature is a place
where there is much inactivity. Even in the
teeming tropics an observer frequently has

INTRODUCTION

nothing to do except wait and watch. In
fact, patience is one of the prime prere-
quisites for natnrabstic study of undisturbed
wild life, even when attention is limited to
selected birds or mammals. The essential
impatience of observers is one of the dom-
inant reasons for the growth of experimen-
tation in ecology; but great patience is
required for any adequate long-term pro-
gram of experimentation, the ramifications
of which may seem endless.

Such considerations lead naturally to
thinking about the interrelations between
ecology and animal behavior, since the
active behavior of animals both in field and
laboratory may be striking, and behavior
studies can yield important indications of
current environmental effects. This does not
imply that all studies of animal behavior as
developed at present are directly or even
indirectly ecological (except in a quite re-
mote sense). Students of behavior are much
concerned with psychological problems,
which in turn may lead into physiology and
into philosophy rather than into ecology
proper.

Many of the ecological phases of animal
behavior cluster about the central problems
of distribution, being concerned with the
closely related matter of so-called habitat
selection or, objectively expressed, of modal-
ity. Gradients of important environmental
factors exist in nature both on small and on
large or even gigantic scales. Gradients of
concentration of oxygen, carbon dioxide,
and other chemicals, including food, heat,
moisture, Hght and pressure, to mention no
more, give stimuli to which animals react.
The responses may be fairly direct and ori-
ented, amounting at times to forced move-
ments, or there may be random reactions of
the trial-and-error variety. The results may
either be apparent immediately or they
may be deferred for days, weeks, seasons,
years, centuries or millenia; or finally they
may be discoverable only in the vast per-
spective of geological time. Migrations such
as those of birds and butterflies are fre-
quently large-scale spectacles; in contrast,
important emigrations may be inconspic-
uous events, the effects of which have not
become fully apparent during recorded
history.

Emigrations may have evolutionary as
well as contemporaneous importance. These
time scales sometimes blend, as they
do in illustrations of what is known as the

host-selection principle (p. 615). In theory,
it is only a short step from the host selection
shown by wood-boring beetle larvae that
tend to live in and feed upon a particular
species of tree, to the more crystallized be-
havior shown by solitary wasps that catch,
sting, and oviposit on a particular kind of
caterpillar, grasshopper or spider. (The im-
phed evolution can be explained by modern
assumptions centering about natural selec-
tion.) This brings up also the problem of
search for the right animal to be captured,
stung and parasitized, in which the innate
behavior patterns, commonly and somewhat
roughly called instincts, have real and
far-reacliing ecological implications. (The
interested reader is referred to Tinbergen,
1942, for a behavioristic approach to the
subject. )

Some behavior patterns of higher verte-
brates appear to resemble innate, instinctive
behavior, and yet have been demonstrated
for certain birds to result from a specialized
type of early learning, called "imprinting"
by Lorenz (1935). Imprinting results when
a young animal at an impressionistic age,
when the learning threshold is low, is ex-
posed to a meaningful stimulus or to some
suitable substitute. Normally at such times
the stimulus that becomes imprinted, so to
speak, initiates persisting behavior that may
dominate the animal's activities for the rest
of its life. A common example concerns the
following of an adult of the species, often
the female parent. This behavior results
from a few contacts, or even from a single
contact at the proper age. In the absence of
the parent, the tendency to follow a given
individual may be imprinted by exposure to
some other animal at the crucial time, with
amusing and incongruous results. The tend-
ency is important in the normal building
of family or flock integration; the interest-
ing psychological mechanisms and implica-
tions lead beyond our scope.

Other types of integrations with the bio-
logical or physical environment are also ap-
parent, as are many fundamental questions.
How does an animal find and settle in a
given habitat? How much so-called search
is involved? Is there an element of active
preferential choice, or, more simply, is there
a reaction to the relative absence of dis-
turbing stimuli? To what extent is the
behavior innate, and how much is reestab-
lished each generation? This leads to curi-
osity concerning the possible presence of

8

INTRODUCTION

tradition among nonhuman animals. How
much learning, if any, is involved? To what
extent, if at all, are animals conscious of
their actions or surroundings?

These are troublesome questions con-
cerning which it is difficult to collect exact
and pertinent information, whether from
existing literature, directly from outdoor
nature, or by means of planned experi-
ments. Elton (1933) recognized the exist-
ence of such problems and suggested some
conclusions that depart from current trends
of thought in scientific circles. Apparently
speaking primarily of birds and mammals,
he says (p. 46) :

"Changes in habitat are frequent, and we
do not yet know precisely what relative im-
portance to attach to psychological factors
(new ideas, or broken traditions or accumula-
tive fatigue with old habits) and how much to
organic changes in the form of mutations af-
fecting behaviour. Finally it is of great interest
to inquire whether animals are actually con-
scious of their actions, and whether in this
consciousness there is any element which is at
variance with the usual concepts of animal be-
haviour current among physiologists and also
many ecologists. There is definite evidence that
animals often migrate in response to stimuli
which cannot be called danger signals but
which appear to be unpleasant to them (Elton,
1930). Whether in this behaviour we can dis-
cern feelings akin to aesthetic feelings or
whether they are to be looked upon as me-
chanical aspects of mental balance, cannot be
decided. The whole question of animal be-
haviour in relation to the choice of habitats and
habits in general is of profound importance
both in theoretical science and in practical
economic biology."

These are matters that we cannot yet
solve, but it is important that we should
not continue to ignore their existence. A
major difficulty lies in the absence of an
objective terminology. The use of vaguely
defined terms is associated with the un-
critical humanizing tendencies of many
naturahsts, who in turn give strong avoid-
ing reactions to the carefully objective and
perhaps overcorrected point of view of
critical modem students.

Recognition of community of interests be-
tween the general and comparative phases
of psychology and of ecology calls for com-
mendation of the modem tendency toward
objective terminology in both subjects, as
well as in general biology and other phases
of science. General anthropomorphic con-

cepts and language are to be avoided, ad-
mitting that other considerations such as
clarity and brevity or entrenched usage may
sometimes require exception. It is unfortu-
nate to have to use a Greek or Latin root
meaning "loving," for example, to denote an
ecological relation, when the EngUsh forai
would be objectionable or ridiculous. This
is a language ideal that is frequently diffi-
cult to apply even with conscious and
conscientious eJBEort. There is a severe strain
when one is convinced (a) that the Carte-
sian doctrine is essentially unsound, (b^
that scientific writing should be simple,
clear, and direct, and (c) that even the
words used should not carry partially hid-
den suggestions unsupported by direct
evidence.

A binding principle in ecology, as in
many other phases of biology, deals wdth
the integration of individual units into
larger wholes. Cells of more complex ani-
mals combine into tissues, organs, and sys-
tems, and yet all this complexity develops
from a single cell. Even at the cell level,
certain cells living in close association with
each other— as in lichens, for example— may
not be germinally related. All ecological
communities lack the germinal continuity
characteristic of populations of single spe-
cies and particularly characteristic of co-
lonial animals like sponges or many hy-
droids, or the typical societal colonies of
social bees, wasps, or ants. Interspecific
populations also obviously lack germinal
continuity. Their evolution is traced to a
combination of ecology and genetics that
will be outlined in the section on Evolution.

The relationships between these ecologi-
cal categories may be traced either by the
type method or by the principles treatment
attempted in the present book. Neither ap-
proach is automatically preferable. The
cataloguing of one category after another
gives a readily indexed treatment that
orders the details in a workable manner,
but may conceal the underlying principles.
The approach through principles may con-
fuse the issue so far as facts are concerned
and may be unsatisfactory for those inter-
ested primarily in a catalog of existing data.

The type treatment deals directly with
the ecology of the oceans, one after another,
of bays and gulfs, of the fresh water, and
of the land. The principles treatment draws
evidence now from one and now from an-
other type of habitat, and then passes on

INTRODUCTION

to repeat the process with another principle.
The two approaches continually tend to
become mixed when the documentation of
principles is given in any detail. Recogni-
tion of the existence of a physical environ-
ment as contrasted with a biotic environ-
ment illustrates the principles approach;
even when the physical environment is
broken down into component parts, the
treatment continues to present principles,
when, within the subdivisions such as tem-
perature, light, and moisture, the discussion
centers about principles such as the tem-
perature "laws," Bergmann's rule, and
Corioli's force.

A fresh definition of the community con-
cept is offered in the present work: In
large, the major community may be defined
as a natural assemblage of organisms which,
together with its habitat, has reached a
survival level such that it is relatively
independent of adjacent assemblages of
equal rank; to this extent, given radiant
energy, it is self-sustaining.

This definition places special restrictions
on a term that has often been a useful
catch-all, correctly applicable to any
ecological assemblage ranging from the in-
habitants of a small clod of earth to the
animals and plants living in the northern
evergreen forests of the world. Under the
older usage, "communitv" might refer to a
simple ecological unit illustrated bv a thin
mat of floating algae as well as to the com-
plicated, multistoried tropical rain-forest
(J. R. Carpenter, 1938). A practical solu-
tion seems to be to recognize the usage of
the term "community" both in the restricted
sense indicated by our definition, and in the
extended loose sense. It will occasionally be
necessary, under the conditions, to add or
to imply "s.s." or "s.lat.," "in a strict
sense" or "in a broad sense." We have
wished to avoid further implementation of
the facetious definition of ecology as being
that phase of biology primarily abandoned
to terminology.

There are two fundamental approaches
to ecological communities that are best pre-
sented by considering the two extremes. As
biocoenoses, they may be organized
primarily by the interrelations of the plants
"nd animals as associates; in contrast, the
basic organization may rest on the com-
mon habitat in which the constittient or-
ganisms serve primarily as indicators and
secondarily as associated individuals. Both

types of communities exist in fairly pure
form, and there are closely graded intercon-
nections. The biota of the desert presents
many aspects of a community controlled by
its physical habitat, and the oyster bed is
a classical example of a biotically con-
trolled biocoenosis. Both types present
many different orders of complexity and
size; one of the larger of these, the biome,
requires further mention.

The biome, represented by the northern
coniferous forest in North America, includes
three major plant associations: viz., the
spruce-pine forest of Alaska and northwest-
em Canada; the spruce-balsam fir forest of
northern Canada from the Mackenzie River
through Labrador and southward; and the
pine-hemlock forest of southeastern Canada,
the region around Lake Superior, and
northern Michigan. The climax dominants
of the last two associations are radically
different, but they resemble each other
closely in having a large number of
identical animal constituents that charac-
teristically range through both.

Shelford and Olsen (1935, p. 395) list
the common animals of the coniferous for-
est biome, pointing out that they range
through the three maior plant associations
without conspicuous change. Their analysis
shows the importance of the animals in
definincr biotic units and the weaknesses
inherent in biome concepts based solely on
data concerning plants. The vegetation is
not the sole key to the biome. Furthermore,
the pine-hemlock community has a clear
unity with the transcontinental spruce-bal-
sam fir forest and even with the Alaskan
spruce-pine association. This unity is based
on subclimax stages and on animal con-
stituents some of which may be relatively
unimportant ecologically.

The universality of the biome concept
meets a severe test in the ^eoejraphic frag-
mentation of the major biotic formations.
New Guinea and northern Australia, for
example, tend to be separated by plant
geographers into two areas (Scrivenor et
al., 1943). Contrariwise, most students of
animal distribution unite the two into a
common major zooijeogranhic region. The
concept of the biome, like manv other
ecological generalizations, must be accepted
with proper reservations and adjusted to
the historical prolilems involved.

Ecological formations are not static.
Given time, the advance and retreat of

10

INTRODUCTION

glaciers aflFects the location of the tundra.
Grasslands expand and contract on a vast
geographic scale; deserts wax and wane.
Bodies of water, including whole oceans,
overflow their basins; in another geological
age, the land masses stand high out of
water. These changes follow certain more
or less irregular periodicities that have a
geological time scale. Shorter temporal
progressions also occur. Given sufficient
freedom from man's interference, striking
vegetational changes may occur within the
life of a single human generation. Burnt-
over areas "heal," and, given longer time,
serai successions advance from pioneer
through intermediate stages to the climax
characteristic for the given climate. A com-
munity in this temporal series undergoes
development and maturation before the
succeeding one replaces it. The processes
of biotic development in combination with
those of physiographic succession are refer-
red to as "Community Ontogeny." "Com-
munity Phylogeny" involves the whole
range of continuing adaptational change of
the components of the community. Com-
munity evolution, in a broad sense, has
been made to include several meanings:

1. The development of the climax
through successive biotic changes and
stages— a process comparable to the devel-
opment of the individual.

2. The organic development of the cli-
max when there is a series of underlying
and correlated physiographic changes, suc-
cession in the strict sense.

3. The convergence of community life-
forms, which is implied, so far as plants
are concerned, by speaking of the evolu-
tion of vegetation as contrasted with the
evolution of the species composition of the
community flora. The animal constituents
show the same kind of interrelations in
structures and in physiological adjustments,
and the whole biota can be similarly con-
sidered.

4. The community evolves also as a re-
sult of converging immigration. Thus in the
Chicago area we have elements that have
come from both southeastern and south-
western centers of dispersal, immigrants
from the more northern grasslands and
from the northern forests, relicts from the
glacial age, and regional endemics. The
combination of this third sort of commu-
nity evolution with the convergencies al-
lows us to think of the evolution of the bio-

sociological climax community as a whole
without giving particular consideration to
the evolution of the constituent species.
From this point of view the evolution of
forest or grassland, or other communities,
focusses on their evolution as biotic com-
plexes. Mesozoic and modern forests, for
example, have biotic equivalence, regard-
less of the great differences in the species
and higher groups of both plants and
animals.

5. Such considerations lead to another
aspect of community evolution, namely,
"The phylogeny of the definitive grouping
of species within the community." The sub-
ject is too complex for thorough treatment,
and of necessity we have been essentially
limited to tracing the evolution of pairs of
ecologically related species, or at most to
small groups of species that have appar-
ently evolved under close mutual relation-
ships. This has the advantage of forcing us
to test fundamental interrelations that stand
near the simplest level of community organ-
ization, and it emphasizes our lack of
knowledge of more complicated ecological
phylogenies.

Reconstruction of the cause of evolution
of the biosociological whole requires con-
sideration and integration of all these
aspects. We recognize and can outline the
problem without being able to advance far
toward its solution.

In community relations it is important
to consider the fundamental relations of
protocooperation, disoperation, and, as a
somewhat different category, competition.
These are matters difficult to discuss with
clarity. In part the difficulty lies in the need
to consider both short-run operational
aspects and long-run evolutionary phases.
Aside from such complications, and from
the innate complexities, there is the lack of
sufficient exact and carefully documented
information with which one may test and
modify, and reject or strengthen, tentative
conclusions.

The competition among plants for space
and light and for nutrients is obvious under
many conditions. Such competition is one
of the important relationships that find ex-
pression in the evolution of life forms with
resultant layering. There is also competition
for pollination when, or if, potential pol-
linators are scarce, and for effective mu-
tualism, if one of the mutualistic pair
lacks local abundance. Competition is

INTRODUCTION

11

avoided, at least in part, by tlie evolution of
space and of time separations, or by some
combination of these. Important as compe-
tition may be, it can readily be overstressed;
Clements and Shelford (1939, p. 166)
help to correct this tendency when they
state that "It is desirable to stress again the
fact that competition comprises a relatively
small number of tlie countless coactions
among animals." So far as predation is con-
cerned, tliis conclusion is supported down
to the species level— or to different races
within the species— by the generalization
of Volterra (1931), elaborated by Cause
(1935) and illustrated by Lack (1946),
showing that competition is lessened until
it may become relatively unimportant as a
result of differences in habitats and habits
of predators even when they otherwise
show much similarity. Such a qualification
does not aflFect conclusions concerning
competition between individuals of the
same subspecies unless these, too, come to
develop some slight dissimilarity in individ-
ual habits.

With all these reservations, competition
is a potent factor in animal life, and its re-
sults are not always disoperative. In fact,
there is evidence for what may be called
the biological necessity of predacious types
that eliminate surplus populations by killing
oJSF weaker animals, especially when these
occupy marginal habitats filled beyond their
year-round carrying capacity.

The basic cooperative relations, particu-
larly the more obscure protocooperations,
or biological facilitations, often are difiicult
to demonstrate conclusively under labora-
tory conditions even when using selected
situations and favorable organisms. They
become still more elusive in the field,
especially at the community level, and par-
ticularly for students well grounded in
skepticism. Some of the more apparent
protocooperations under these conditions
include:

1. The role of bacteria in the formation
of soil and in its yearly renewal of fertility.

2. The similar role of bacteria in the
mineral nutrient cycles of the sea and of
fresh-water communities.

3. The full range of subtle interactions
between soil organisms and the soil.

4. The mass effects of organisms on the
toxicity of media.

5. The "rain" of dead organisms from
the surface of the ocean that permits the

development of life in the great fightless
depths of the sea.

6. The protocooperations inherent in the
definition of a dominant organism in the
community as one that receives the full im-
pact of environment and so modifies it that
associated species can five in areas they
could not otherwise invade.

The efiects produced by plants and ani-
mals on their physical, chemical, and biotic
environment that prepare the way for con-
tinuing the community development show
both disoperative and protocooperative as-
pects. The disoperation concerns those
present occupants of the habitat whose
activities are making their own continuance
impossible in that particular place. The
protocooperations come in the preparation
of conditions that will permit the whole
series to move on towards the chmax.

In the community, as well as in its com-
ponent biocoenoses or smaller fragments,
the forces making for ecological facilitation
are, in the long run, generally somewhat
stronger and more widespread than those
tending towards disoperation.

In our ambitious attempt to set forth
ecological principles, it is fitting to empha-
size the unknown elements remaining in the
field. The very existence of some of these
is just beginning to gain recognition.
Others, at the present time, can be outlined
in qualitative terms only; still others, doubt-
less, are as yet wholly unknown. Some few
relations can be given fairly exact mathe-
matical treatment. There is much room for
pure humility among ecologists who are
trying to cope with these loosely foiTnulated
relationships, most of which cannot be ex-
pressed in exact quantitative formulations.

The relations of individuals to tempera-
ture, light, and gravity, and to other
environmental factors, can often be stated
with approximate precision. Population
ecology is quantitative with respect to
description, at least under certain controlled
laboratory conditions, but even students
of this phase of the subject edge away from
prediction except on the basis of statistical
probability based on accumulated data. For
some students this situation produces an
avoiding reaction; for many it constitutes a
challenge; for others of us, less well-
equipped for quantitative studies, it has a
strong primary attraction. We enjoy work-
ing under the necessity of making needed
reservations and keeping in mind the many

12

INTRODUCTION

and varied qualifications that should pre-
vent us from making dogmatic generaliza-
tions.

The inadequacy of the framework of
ecological principles presented in the fol-
lowing chapters is evident; supplementa-
tion and correction are urgent needs for
the advancement of ecology. But it is also
important to point out that it is often im-
possible to find exact and well-chosen data
concerning a given point. The minimum

temperature at which death occurs imme-
diately for any population of a species of
animals is a good illustration. For that mat-
ter, the limits of toleration for all elements
in the physical environment except in gen-
eral terms are unknown for any one species
of animal, even for man. With all our em-
phasis on the need of ecological principles,
it must be emphasized again that in the
formulation of principles, as in testing and
extending them, evidence is basic.

SECTION I. THE HISTORY OF ECOLOGY

2. ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

Carnap (1938) recognized "physics" as a
common name for the nonbiological field of
science and stated that "the whole of the
rest of science may be called biology (in
the large sense)." He immediately saw the
necessity of dividing this wider biology into
two fields, the first of which contains "most
of what is usually called biology, namely,
general biology, botany, and the greater
part of zoology." The second part "deals
with the behavior of individual organisms
and groups of organisms \vithin the en-
vironment; with the dispositions to such
behavior, with such features of processes
in organisms as are relevant to the behavior,
and with certain features of the environ-
ment which are characteristic of and
relevant to the behavior, e.g., objects ob-
served and work done by organisms."

Carnap proceeds to discuss the distinc-
tions between the two phases of biology
primarily from the point of view of human
relations and suggests, among other things,
that the second phase might be made up by
"selecting the processes in an organism from
the point of view of their relevance to
achievements in the environment . . . ."
He continues by saying that "there is no
name in common use for this second field.
. . . The term 'behavioristics' has been
proposed. If it is used, it must be made
clear that the word 'behavior' has here a
greater extension than it had with the ear-
lier behaviorists. Here it is intended to
designate not only the overt behavior which
can be assayed from outside but also inter-
nal behavior (i.e., processes within the
organism); further, dispositions to behavior
which may not be manifested in a special
case; and finally, certain effects upon the
environment."

Carnap distinguishes between such rela-
tions of individual organisms and groups of
organisms and adds that "it seems doubt-

13

ful whether any shaip fine can be drawn
between these two parts." He also states
that such considerations extend to non-
human animals as well as to men.

Thus, late in the 1930's, a philosopher of
high attainments compounded logical neces-
sity with ignorance of the history and pres-
ent development of biological ideas, and
announced as new the discovery of the field
of "bionomics," "ethology," "ecology," or
"relations physiology." This happened at
the University of Chicago, where research
and teaching concerning the relations be-
tween organisms and their environments
had been an active feature of the biological
program since the late 1890's. The iong,
respectable history of this phase of biology
forms the subject matter of the present sec-
tion. Camap's statement is a valuable
introduction to this history, since it demon-
strates anew that ecology fills a natural
niche in biological science. It also gives
warning of the lack of general knowledge
among scholars as to the mass of informa-
tion in this field.

Near the turn of the present century,
W. K. Brooks, founder of the great Johns
Hopkins tradition in biology, expressed
much the same need for an understanding
of the environmental relations of organisms
as that given by Carnap. He stated: "To
study life we must consider three things:
first, the orderly sequence of externa]
nature; second, the living organism and
the changes which take place in it; and
third, that continuous adjustment between
the two sets of phenomena which con-
stitutes life. The physical sciences deal
with the external world, and in the lab-
oratory we study the structure and
activities of organisms by very similar
methods; but if we stop there, neglecting
the relation of the living being to its en-
vironment, our study is not biology or the

14

THE HISTORY OF ECOLOGY

science of life." The idea was already old
when Brooks expressed it.

Now, having placed two shots on this
side of our target, we may try a much
longer range and come up on the historical
development of the basic ideas of ecology
in conventional fashion. The first half of
this historical section will deal with the
beginnings of ecology up to about 1900 and
will be followed by a survey of the rapid
growth of the subject during the present
century.

While the word "ecology" was put to-
gether from Greek roots and is based on
oikos, which means home, the Greeks did
not have a word for it, and it is problemat-
ical to what extent they appreciated the
basic ideas and relationships that the word
now summarizes. In this respect, ecology
does not diflFer essentially from many other
phases of modern biology. The Greeks did
observe the home life of animals after the
relatively unorganized methods of what is
still called natural history, and they were
aware of the necessity for interrelations be-
tween living things and their environment.

Empedocles, about the middle of the
fifth century B.C., said that plants procure
nourishment through pores in stem and
leaves; he obviously realized that plants
have relations with their environment.

Pre-Aristotelian Greeks had developed a
considerable stock of information about
some of the environmental influences in
relation to human health. Hippocrates, so-
called father of medicine, emphasized such
matters. Among the extant writings that
Adams (1849) considers genuine works of
Hippocrates, that "On Airs, Waters and
Places" is strongly environmental in its
medical emphasis. There is a recognition of
the influence of location, exposure, and sea-
son upon health, but Hippocrates also knew
that in order to estimate the effect of a
given season, the nature of the preceding
seasons must also be considered. The first
paragraph of this essay gives his approach
to medicine:

"Whoever wishes to investigate medicine
properly, should proceed thus: in the first
place to consider the seasons of the year, and
what effects each of them produces (for they
are not at all alike, but differ much from
themselves in regard to their changes). Then
the winds, the hot and the cold, especially such
as are common to all countries, and then such

as are peculiar to each locality. We must also
consider the qualities of the waters, for as they
differ from one another in taste and weight,
so also do they differ much in their qualities.
In the same manner, when one comes into a
city to which he is a stranger, he ought to con-
sider its situation, how it lies as to the winds
and the rising of the sun; for its influence is
not the same whether it lies to the north or
the south, to the rising or to the setting sun.
These things one ought to consider most atten-
tively, and concerning tlie waters which the
inhabitants use, whether they be marshy and
soft, or hard, and running from elevated and
rocky situations, and then if saltish and unfit
for cooking; and the ground, whether it be
naked and deficient in water, or wooded and
well watered, and whether it lies in a hollow,
confined situation, or is elevated and cold;
and the mode in which the inhabitants live,
and what are their pursuits, whether they are
fond of drinking and eating to excess, and
given to indolence, or are fond of exercise and
labour, and not given to excess in eating and
drinking."

The applications that follow are not us-
ually impressive in the light of present day
knowledge, but the point of view is mod-
ern. These early teachings are important in
the history of ecology since they give some
inkling of the state of Greek thought before
Aristotle's activities began.

Aristotle (384-322 B.C.) is usually re-
garded as the founder of biological science.
Ramaley (1940) suggested that Aristotle
"hardlv takes a place in ecology, although
he did studv the habits of animals to some
extent." This calls for a look at Aristotle's
writings.* The material given in Section 1
of Book 1 may be outlined in part as fol-
lows:

Animals differ in modes of subsistence.
Aristotle says, in actions, in habits, and in
their parts. They include:

T. Water animals

1. Entirely aquatic

2. Animals that live and feed in water, but

breathe air and bring forth their
young on land.

3. Sea dwellers

4. River dwellers

5. Lake dwellers

6. Marsh dwellers

Elsewhere Aristotle definitelv recognized
amphibious animals.

* D'Arcy Thompson's 1910 translation of
Historia Animalium.

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

15

U. Land animals, which may, however, in-
vade water

Water-"inhaling" animals do not derive sub-
sistence from the land. Some of them live in
water and then change shape and hve on land.
Stationary animals hve only in water, where
they may be (a) attached or sessile; (b)
unattached but motionless.

Means of locomotion of animals: swimming,
walking, flying, wriggling, creeping.

No creature is able to move solely by flying
as fish move by swimming.

Flocks of birds differ in power.

Some birds are present at all times; others
are seasonal.

Some are gregarious; others are solitary.

Some gregarious animals are social.

Some birds are gregarious, but none with
crooked talons have that habit.

Many fish are gregarious.

Social animals have a common object in
view.

Some social animals have a ruler; some do
not.

Animals may have a fixed home or be
nomadic.

Diets differ: they may be (c) carnivorous,
(b) graminivorous, (c) omnivorous, or
(d) special, e.g., honey.

Some animals have dwellings; some do not.

Some are nocturnal, others diurnal.

Some are tame, some wild; some wild ani-
mals are easily tamed, e.g., the elephant.

Domesticated animals all have wild relatives.

Some emit sounds; others are mute.

All animals without exception exercise their
power of singing or chattering chiefly in
connection with intercourse of the sexes.

Some five in fields; others on mountains;
some frequent abodes of men.

Some are salacious, e.g., the cock; others are
inclined to chastity.

Some marine animals hve in open sea, some
near shore, some on rocks.

Animals differ in character:

(a) Good-natured, sluggish

(b) Quick-tempered, ferocious

(c) Intelhgent, timid

(d) Mean, treacherous

(e) Noble, courageous

(/) Thoroughbred, vdld, treacherous
(g) Crafty, mischievous
(h) Spirited, affectionate, fawning
( i ) Easy-tempered
(/■) Jealous, self-conceited
Many animals have memory.

Aristotle's observations on the breeding
behavior of animals are scattered through
his writings on zoology, which, in general,
are not so well organized as might appear
from the foregoing outlme. They are not
yet ecology. They do constitute good nat-

ural history, for the first major attempt, and
they represent a part of the stuti from
which ecology has developed. It may be
remembered that natural history contains
elements of other phases of biology, of
anatomy and taxonomy, for example, as
well as much of ecological importance.

i^amaley (1940) regards Theophrastus as
the first ecologist in history. Theophrastus
was a student and friend of Aristotle's
and succeeded him as leader of the Athen-
ian Lyceum. Ramaley says that Theophras-
tus wrote sensibly of the communities in
which plants are associated, of the relations
of plants to each other and to their nonhv-
ing environment. According to Greene
(1909, p. 125), Theophrastus definitely
forecast the natural associations of plants
in particular places. He distinguished (1)
marine aquatics, (2) marine httoral plants,
(3) plants of deep fresh water, (4) those
of shallow lake shores, (5) plants of wet
banks of streams, and (6) of marshes. He
wrote of trees that grow on exposed, sunny
mountain slopes, of those that flourish only
on northern exposures, and also of those
limited to the more frigid summits.

As has been shown, Aristotle gave a
somewhat similar classification of animals
in relation to their habitats. In fact, Zeller
(1931, p. 202) states that the extant writ-
ings of Theophrastus on plants follow
Aristotle in their leading ideas. Theophras-
tus did found plant systematics, wrote on
plant geography, and developed a sort of
plant physiology. He also knew enough
about color changes in animals to show
that he had some grasp of the color adap-
tation of animals to their environment.

Even the best of the Greeks did not have
all their facts straight and showed tenden-
cies toward accepting travelers' tales un-
critically, which some modems have at last
outgrown. They used anthropomorphisms
with plants and animals ahke about on a
level with those found in "nature study"
today. Aristotle, great as he was, appar-
ently was no greater genius than are our
best modern thinkers, and perhaps not less
great, either. It may be added that Aristotle
was probably no stronger in sheer mental
abihty than the best of the ancients who
lived 2500 years before him, though there
were more facts accumulated by his time
with which he could deal. We judge a
man or a group of men historically by the
end product they leave behind, and a good

16

THE HISTORY OF ECOLOGY

lasting end product, even in afiFairs of the
intellect, does not necessarily trace back to
the work of one brilliant man.

Certain rule-of-thumb ecological knowl-
edge was evidently widespread among the
Hebrews of 2000 years ago, though they
were not notably a scientific people. The
"parable of the sower," for example, shows
that the relation between habitat and yield
was well understood, though not in these
words.

The Romans used widely distributed folk
knowledge in creating the science of agri-
culture. In their hands, this grew primarily
from hunting and fishing, enriched by early
experience with plant and animal hus-
bandry. Roman agriculture was fertilized by
the writings of the Greeks and put into
practice with their own common sense. It
was based on empirical ecological observa-
tions and was frankly economic in outlook.

Pliny the Elder (A.D. 23-79), one of the
best of the Roman writers of the period,
owes his reputation to his Natural History,
which was the starting point of modern
faunal study. Pliny's account tends to be a
confused jumble of compiled notes without
logical organization. Nordenskiold (p. 53)
defends Pliny against overharsh critics who
accuse him of being a soulless compiler, be-
cause, "more honest than Aristotle, he
quotes his sources." Like Aristotle, Phny
used an ecological system of classification.
Among his categories we find the recogni-
tion of terrestrial, aquatic, and flying
animals.

Ramaley (1940) also recognizes the good
in Pliny's work. He quotes with approval
the following: "A soil that is adorned by
tall and graceful trees is not always a
favorable one except of course for those
trees. What tree is taller than the fir? Yet
what other plant could exist in the same
spot? Nor are verdant pastures so many
proofs of richness of soil. What is there that
enjoys greater renown than the pastures of
Germany? But they are a mere thin layer
of earth with sand underneath." Here we
have a suggestion, not only of plant indica-
tors, but also of some of the pitfalls in their
use.

After the Roman spark of interest there
were few signs of activity in what we now
call ecology. The foundation sciences of
geography and climatology were unde-
veloped. Even chemistry and physics could
not yet lay the groundwork for physiology.

so that ecology had to wait. For a thou-
sand years there was stagnation. When
Greek writings again became popular, they
were all too slavishly accepted as ultimate
authority.

The Greek spirit of inquiiy was redis-
covered in the Renaissance. AJbertus Mag-
nus (1193±-1280) wrote, like Theophras-
tus, of plants of streamsides and marshes
and of the relation between the habitat of
a tree and the quaUty of its wood. While
there were some signs of scholarly growth
from within Europe, yet the development
of ecology, as of other phases of biology,
stood still or even regressed until the geo-
graphic experiences of Marco Polo and of
the Portuguese and the catalyzing discovery
of America forced biologists to turn from
authority to the study of the thing itself.
The interest in new animals and plants,
their habits, and their possible usefulness,
thus helped to bring on the reawakening
of science, especially as regards the fore-
runners of ecology.

The writings of Gesner (1516-1565) and
Aldrovandi (1522-1605) mark the begin-
ning of this movement, which was forced
by the accumulation of greater knowledge
of local and exotic animals. Greene (1909)
writes with high appreciation of the Ger-
man herbahst, Cordus, who lived briefly
about this time (1515-1544). Concerning
the bearing of his work on ecology, Greene
says (p. 310) : "We have already been
learning that even from most primitive
times every botanist was an ecologist; at
least to the extent of observing and record-
ing the special environment which every
kind of wild plant ajffects, and sometimes
to the mentioning of some of its associate
species. Valerius Cordus, being well-skilled
in both chemistry and mineralogy, goes be-
yond all his predecessors in that he names
the petrography of a plant's habitat or
otherwise indicates the constituency of the
soil in which it is to be looked for."

Robert Boyle (1627-1691) is sometimes
referred to as the first of the modern
chemists. His biological observations were
incidental. In 1670 he published the earhest
experiments upon the effect of low atmos-
pheric pressures on animals. The forms
tested comprised mice and young kittens,
various birds, including a duck and a
duckling, snakes, frogs, and different
invertebrates, among them several kinds of
insects. The point of view from which he

ECOLOGICAL, BACKGROUND AND GROWTH BEFORE 1900

17

made his experiments is shown in the fol-
lowing passage (p. 2012) :

"We put a full-grown Duck (being not then
able to procure a fitter) into a Receiver, where-
of she fill'd, by our guess, a third part or some-
what more but was not able to stand in any
easy posture in it; then pumping out the Air,
though she seemed at first (which yet I am not
too confident of upon a single tryal, ) to have
continued somewhat longer than a Hen in her
condition would have done; yet within the
short space of one minute she appeared much
discomposed and between that and the second
minute, her struggling and convulsive motions
increased so much that, her head also hanging
carelessly down, she seemed to be just at the
point of death; from which we presently res-
cued her by letting the Air in upon her: So
that, this Duck being reduced in our Receiver
to a gasping condition within less than two
minutes it did not appear that, notwithstanding
the peculiar contrivance of nature to enable
these water-Birds to continue without respira-
tion for some time under water, this Duck was
able to hold out considerably longer than a
Hen, or other Bird not-Aquatick might have
done."

Boyle was impressed by the resistance
of cold-blooded animals in his vacua. He
experimented with recently bom kittens:
"Being desirous to try, whether Animals,
that had lately been accustomed to live
without any, or without a full Respiration,
would not be more difficultly or slowly
killed by the want of Air . . . and found
that: These tryals may deserve to be pros-
ecuted with further ones, to be made not
only with such Kittens, but with other very
young Animals of different kinds; for by
what has been related it appears, that those
Animals continued three times longer in the
Exhausted Receiver, than other Animals of
that bigness would probably have done."

These quotations show that the approach
to Boyle's experimentation was distinctly
ecological in the present usage of a word
unknowTi to him and that his experiments
were well conducted and not overinter-
preted. His main technical weakness lay in
failure to record for many of his experi-
ments any indication of the degree of re-
duction of air pressure in his self-styled
"Vacuo Bovliano."

Reaumur (1683-1757) has a place near
the beginning of the 2;reat modern tradition
of natural history. His most notable work,
"Memoires pour servir a I'histoire des in-
sectes." filled six large volumes. He was

concerned with the conditions of Ufe of
insects, as well as with their structure, and
he experimented with their habits of Ufe,
including leaf-mining, gall formation, and,
more especially, the community hfe of so-
cial insects. He studied parasitism among
the Hymenoptera. He made observations on
shell formation in mollusks, movement of
primitive animals, and the digestion of food.
Reaumur was a man of much influence in
his own day, and his work is still held in
high esteem, as witness the appearance in
1926 of one of his hitherto unpublished
manuscripts, translated and annotated by
William M. Wheeler.

The modern aspect of ecology did not
begin to take form until early in the eight-
eenth century. Linnaeus (1707-1778) and
Buff on (1707-1788), each in his character-
istic style, made notable contributions. Nor-
denskiold (p. 215), with some truth and
pardonable patiiotism, proclaims that in ad-
dition to founding modern systematics,
Linnaeus originated all that is now called
"phenological, ecological, and geographic
zoology and botany" by his descriptions of
the influence of external conditions.

Of Buff on, Lankester (1889) said that he
"alone among the greater writers of the
three past centuries emphasized that view
of living things which we call 'bionomics.'
Buffon deliberately opposed himself to the
mere exposition of the structural resem-
blances and differences of animals, and,
disregarding classification, devoted his
treatise on natural history to a consideration
of the habits of animals and their adapta-
tions to their surroundings. . . . Buffon
is the only writer who can be accorded his-
toric rank in this study." Buffon's great
principle of environmental induction is still
an important rallying point in dynamic bi-
ology. This should not be confused, as
apparently it is at times, with Lamarck's
principle of the inheritance of eflFects of use
and disuse.

ENVIRONMENTAL PHYSIOLOGY:
RANGE AND ADJUSTMENT

We now know that there are two types
of environmental effects that may be dis-
tinguished conveniently as examples of (a)
developmental, maintenance and/or tolera-
tion phvsiologv, and (b) response physiol-
ogy. The line between them is not necessar-
ily sharp, nor are they mutually exclusive.

In addition to his work on the natural

18

THE HISTORY OF ECOLOGY

history of insects, Reaumur was a pioneer
in developmental physiology. Interestingly
enough, he laid the foundation for the mass
of modern work on the summation of tem-
perature when (1735) he found that the
sum of the mean daily temperatures of air
in the shade made a constant for any given
phenological period. Abbe in a book com-
piled in 1891 and finally published in 1905
quotes a translation from Reaumur as fol-
lows: "It would be interesting to continue
such comparisons between temperature and
the epoch of ripening and to push the study
even further, comparing the sum of the
degrees of heat for one year with the
similar sums of temperature for many other
years; it would be interesting to make com-
parisons of the sums that are effective
during any given year in warm countries
with the effective sums in cold and tem-
perate climates, or to compare among
themselves the sums for the same months
in different countries."

Reaumur expanded this statement else-
where into the suggestion that, since the
same grain is harvested in different cli-
mates, a comparison should be made of the
same temperatures for the months during
which the cereals accomplish the greater
part of their growth and maturity in warm
countries like Spain and Africa, in tem-
perate countries Hke France, and in cold
countries like those of the extreme north.
Here we have the background for the geo-
graphic application of temperature summa-
tion that underlies, in theory at least,
certain modern work such as the life zone
concept of Merriam and the "bioclimatic
law" of Hopkins.

Gasparin in 1844, in commenting on
Reaumur's ideas on this subject, recognized
in them the germ of all work on the quan-
tity of heat necessary to mature different
kinds of plants. According to Abbe, Adan-
son, soon after Reaumur, disregarded sub-
freezing temperatures and took only the
sums of those above freezing. More than
three-quarters of a century later Boussin-
gault in 1837 in his Rural Economii com-
puted the total heat required to ripen grain
essentially according to Adanson's sugges-
tion. His data indicate that the required
number of dav degrees increases as the
latitude decreases.

Quetelet (1846) added the idea of a
threshold of awakening from winter dor-
mancy; even so, in his summary (cf. Abbe,

1905, p. 188) Quetelet used the sum of
temperatures, or the sum of the squares of
temperatures above freezing for his basic
data. Alphonse de CandoUe, by 1865, knew
that if the time in days required for seed
germination is multiphed by the accumulat-
ed degrees centigrade, the results are more
consistent if the minimum germinating tem-
perature for the species, rather than freez-
ing of water, is taken as the base.

It remained to work out the physiological
zero for different plants. Gasparin (1844)
adopted 5° C. as the beginning of "effective
temperature." By 1852 (fide Abbe) he had
recognized that these early preoccupations
with temperature were faulty in that the
effect of other meteorological conditions
was also important in phenological affairs.
He suggested that rainfall, sunshine, and
related meteorological data should also be
considered in such analyses.

Candolle (1865) found that, contrary
to the opinion of certain workers, some
seeds will germinate at 0° C. and possibly
at even lower temperature if the water can
be kept liquid. He knew about minimum,
maximum, and optimum germinating tem-
peratures and emphasized the difference
between effective and ineffective tempera-
tures.

Abbe summarizes these and many other
records of the measurement of environmen-
tal factors and their effects on plants.
Among other matters, he reviews the modi-
fication of Boussingault's day degrees by
Tisserand (1875), who used hours of light
between sunrise and sunset multiplied by
the mean temperature to give "sunshine-
hour degrees." The data indicate that,
for the maturation of spring wheat and
barley, this mixed summation appears to
decrease as the latitude increases.

Abbe also traces the development of in-
formation concerning the effect of light on
germination and growth of plants from
that of Edwards and Colin in 1834 through
the cautious conclusion of Pauchon (1880)
that light favors germination when the
seeds are below their optimum germinating
temperature. Abbe discusses the invention
by Arago before 1850 of thermometer cou-
ples composed of black-bulb and colorless
bulb pairs to measure total insolation, which
Marie-Davy improved. By 1867, Roscoe
knew from measurements in Europe and
Brazil that, unlike heat, the chemical action
of light reaches its maximum effect at noon

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

19

The measurement of the evaporating
power of the air with a Piche evaporimeter
had been recorded in the Montsouris An-
niiaire for 1888. Knowledge of other effects
of wind is much older. The relation of wind
to the dispersion of spores had attracted at-
tention, and certain of the relations to vege-
tation were also known. For example, Woll-
ny (1891, vol. 14, p. 176) records that the
catch of living spores on suitable glass
plates in forests is about one-third of that
found in the open country.

Interrelations between living organisms
were also being studied. Cordus, the Ger-
man herbalist, in his Historia Plantarum,
published posthumously in 1561, had de-
scribed the tubercles on lupine roots. It is a
far cry from this initial description to the
experiments on nitrogen fixation that flour-
ished in the 1880's. By the end of that
decade, much of the basis for present day
knowledge of the symbiotic functioning of
root tubercles had been experimentally out-
lined (see Abbe, 1905, p. 136 ff).

It is perhaps pardonable to pause in the
midst of this historical survey to point out
a fact that is steadily becoming more and
more evident. When Brooks was writing the
passage referred to earlier in this chapter,
or when, to anticipate. Warming was study-
ing the vegetation of the Danish dunes in
the early 1890's, there already existed a
rich literature concerning the relations of
organisms to their environment. Having
made this point, it is unnecessary to trace
out each detailed advance. We do need to
turn to the zoological developments of the
nineteenth century to find how far general
knowledge about the environmental rela-
tions of animals had progressed by the end
of that period.

The work of tracing the history of ecol-
ogy is made easier by the books of Daven-
port and Semper. Davenport brought to-
gether much ecological information in his
Experimental Morvholo^i/ (1897-1899, 2
vols., 508 pp.) and documented his writing
in modem st^'le. The excellent review by
Semper (1879 to 1881), called Animal
Life, covers a part of the same literature.
Both these men had a hand in the rise
of self-conscious ecology, a topic that will
be considered in due time.

The advances in animal ecoloijv during
this period can be more soundlv evaluated
if the history of plant physiology is also
considered. This is summarized by Sachs

(1882) and Pfeffer (1900-1906). The
more distinctly ecological discussion by
Klebs (1896) of the conditions of existence
as they affect the reproduction of algae and
fungi is also significant.

It had been suggested before the 1890's
that respiration of anaerobic bacteria and
of other parasitic organisms resulted from
the breaking down of oxygen-containing
compounds present in the nutritive medium
(cf. Loew, 1891, p. 760). Much earlier,
Kiihne (1864) had shown experimentally
that protoplasmic movement in the ameba
is slowed down in the absence of oxygen,
while subsequently it was found that the
presence of increased amounts of carbon
dioxide immobilizes quickly, but kills slowly
(Demoor, 1894).

The preliminary information concerning
acclimatization to poisons had been worked
out both with man (Binz and Schulz, 1879)
and other animals (Ehrlich, 1891). Obser-
vations on many organisms had yielded the
generalization that an organism which pro-
duces an albuminoid poison is resistant to
that poison. Thus Fayrer (1872) reported
that snakes were not killed by injections of
their own poison; modern studies show that
such immunity is only relative (Keegan
and Andrews, 1942).

Determinations by Bezold as early as
1857 showed that the amount of water ordi-
narily present in body tissues varies with
different species. By 1896 it was known
that seeds do not germinate if they contain
only 10 to 15 per cent of water and that
certain animals can revive after being desic-
cated. Leeuwenhoek mentioned in a letter
written in 1702 that when dry stuff from a
gutter was put in water, organisms ap-
peared, and Hall (1922) states that Baker
in 1764 had revived nematodes after they
had been in a dried state for twenty-seven
years. Spallanzani, in the late eighteenth
century, similarly revived dried rotifers.
Preyer (1891) coined the modern term
"anabiosis" to apply to apparent death, and
Davenport believed (1897), but admittedly
could not prove, that anabiosis could re-
sult from acclimatization rather than selec-
tion. Semper (1881, p. 174) doubted whe-
ther, after the protoplasm was actually and
truly desiccated, revival could take place,
though he knew that eggs of the phyllopod
crustacean Aviis could be kept in mud for
vears and still hatch out if properlv mois-
tened. Other cases of recovery after ex-

20

THE fflSTORY OF ECOLOGY

tended drying were known. For example,
Cooke (1895) summarized instances that
show the tenacity of life of desert snails.
One of the most spectacular concerns two
specimens of Helix desertorum that were
glued to appropriate supports and exhibited
in the British Museum from March 26,
1846, to about March 15, 1850, when one
revived and fed after being placed in water.

Bachmetjew (1907) cites a fairly rich
literature which grew during the latter
half of the nineteenth century dealing with
the eflFect of humidity upon the develop-
ment of insects and insect populations and
upon such other matters as body form and
color.

By 1890 many of the essential relations
of osmosis had been worked out for plant
cells by PfeflFer (1877) and De Vries
(1884). It had been known for an even
longer time that the ameba shrinks in a
weak saline solution and swells on return
to fresh water (Kiihne, 1864). In the late
1870's Schmankewitsch reported that if the
fresh-water flagellate Anisonema acinus is
cultivated for many generations in water to
which sea salt is added gradually, its struc-
ture is modified, and Griiber (1889)
changed the marine form of the heliozoan
Actinophrys sol to the more vacuolated
fresh-water form, and vice versa.

Davenport (1897) could make the gen-
eralization that the capacity for resistance
to stronger salt solutions seems to be closely
correlated with the conditions of the medi-
um in which the organism has been reared;
he cited a series of observations dating
back to those of Beudant (1816) and show-
ing that mollusks living in the diluted sea
water of littoral regions, such as Ostrea or
Mytiliis, could resist the ill eflFects of expo-
sure to fresh water better than mollusks
from the open sea. Beudant also showed
experimentally that fresh-water and marine
organisms could go far towards becoming
accustomed gradually to the opposite type
of medium, or, in more general language,
that by varying the density of the culture
medium slowly, we may, with time, vary
the resistance of individuals. Such experi-
ments were much extended during the
nineteenth century as, for example, by Pla-
teau (1871) on the fresh- water isopod
Asellus and bv others on representatives
of almost all the principal animal groups.
Schmankewitsch's oft-quoted experiments
(1875) in which he transformed the brine

shrimp Artemia salina to the so-called A.
milhaiiseni and back by rearing it in differ-
ent concentrations of salt water are prob-
ably the most dramatic of these otherwise
half- forgotten experiments. A consideration
of the relation of mineral nutrients, espe-
cially those of the soil, to the growth of
plants led to the strong emphasis that
Liebig (1840) placed on what is now
known as Liebig's "law of the minimum"
(seep. 198).

Experimental analysis of the effect of
hght extended throughout this period. Ed-
wards (1824) stated that tadpoles would
not develop well in the dark. Others in the
fifties and sixties found no effect of light
or darkness on the rate of growth, while
Yung (1878) claimed that tadpoles grew
more rapidly in length in the light. Wood
(1867) reported a positive influence of re-
flected light on the color of butterfly
chrysalids.

Modem work on the effect of wavelength
of light on animal development apparently
began with that of Beclard (1858), and the
foundation for present knowledge concern-
ing the relation between wavelength and
photosynthesis was laid by Draper (1844),
Sachs (1864), and Pfeffer (1871). For
plants that contain chlorophyll, it became
known that, within limits, the rate of assim-
ilation decreases as light intensity decreases
(Reinke, 1883, 1884). For plants and other
organisms, the most diverse upper limits of
intensity were known by 1896. Experimen-
tation on the lethal effect of light on bac-
teria dates back to Montegazza, according
to Nickles (1865), and was first studied
with thoroughness by Downes and Blunt
(1877, 1878), who found that the blue end
of the spectrum was actively bactericidal,
but that red was not similarly effective.

Organisms are normally subjected to a
diurnal period of darkness and of light.
Smith (1933) says that the first mention
in literature of the influence of the length
of day on plants is found in the writings
of Linnaeus, in 1739. Linnaeus thought,
however, that the rapid growth and speedv
maturity of arctic plants result from heat
rather than from the light supplied by the
lengthened davs. Davenport (1897, p. 421)
records that Trew in 1727 had studied
the effect of alternation of light and dark-
ness on the rate of growth in plants. Once
opened, the subject attracted attention, but
it was not until the work of Sachs (1872)
that a continuous curve of plant growth

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

21

was obtained, demonstrating clearly that
growth increases during tlie night, has a
maximum about daybreak, and then falls to
a minimum about sunset. Garner (1936)
traced the development of photoperiodicity.
Moleschott (1855) reported that the frog,
Rana escidenta, produces carbon dioxide
more rapidly in light than in darkness; and
Bidder and Schmidt (1852) had found that
starving cats show a diurnal rhythmicity in
loss of weight, with least rapid loss during
the night. It would be interesting to know
if temperature changes were properly
controlled.

Schiifer (1907) was the first person in
the present century to present evidence that
length of day is a factor in bird migration.
He traces the idea back to a Swedish poet,
Runeberg, who was reported in 1874 to
have thought that "it is the longing after
hght, and that alone, that diaws the birds
southward" in the autumn, and that they re-
turn to the long days of the Scandinavian
summer for the same reason. The views of
Runeberg did not pass unchallenged, for
Newton (1874) objected that since both
autumn and spring migrations are initiated
before the respective equinoxes, the birds
in both instances are journeying toward
increasingly shortened days.

Apparently without knowing about Rune-
berg's ideas, Seebohm (1888) wrote con-
cerning the autumnal migration: "The an-
cestors of the Charadriidae were probably
not in search of warmth for the climate of
the Polar Basin was in those remote ages
mild enough: nor in search of jood, which
was probably abundant all the year round;
but in search of light during the two or
three months when the sun never rose
above the horizon." Schiifer comments on
the fact that Seebohm apparently did not
realize that birds might return to the arctic
region on account of the lengthened days
to be found there.

The custom of providing domestic fowls
with added Hght in order to increase egg
production is said to be traceable to Spain
in 1802. The practice was introduced into
North America in 1895. The effects of the
increased length of the light period on the
egg production of hens becomes evident
in ten to twelve days' exposure. The same
practice is now applied in the raising of
fowls for food.

Many observers, from Spallanzani (1787)
and Saussure (1796) down to Brues
(1939), have been interested in collecting

data on animals and plants of thermal
waters.

Dutrochet (1837), for plants, and Klihne
(1859), for animals, head a long fine ol
distinguished workers who agree that, with-
in hmits, an increase of heat accelerates
protoplasmic movement. Semper (1881, p.
129) could cite sound data to show that
an increase in temperature strikingly in-
creases the rate of development of many
animals and concluded, accurately enough:
"Many other examples might be added . . .
all providing the same effect of a rising
temperature; but, unfortunately, so far as 1
know, none give an exactly determined
tlrermal curve for particular species . . ."
The first such curve to be pubHshed ap
pears to have been that by Lilhe and
Knowlton (1897).

Modern interest in the degree of heat re-
quired to produce death dates to Spallan-
zani (1787). Edwards (1824), Dutrochet
(1837), and Bert (1876) are among those
who investigated it. Unfortunately, experi-
mental conditions were not carefully con-
trolled and standardized. Even so, the work
of this period fairly well fixed the ideas that
prevail today and supplies much of our
present information on this subject. In gen-
eral, this early work showed that while
certain flagellates were not killed, under
the conditions used, until about 50° C, and
while for many groups 45° C, or there-
abouts, represents a common death point,
the majority of the metazoa are killed below
40° C. or even below 35° C.

Temporary cold rigor and death point as
a result of low temperature similarly at-
tracted attention, particularly from 1860 to
1890. The information was sufiBcient to al-
low Davenport (1897) to make the sound
generalization that there is no fatal minimal
temperature for desiccated protoplasm. At
the other extreme, according to Doyere
(1842, p. 29), rotifers and tardigrades,
which in water are killed before the tem-
peratmre reaches 50° C, after drying may
be heated to 120° C. and still survive. This
supplies further evidence of the increased
resistance of dried protoplasm. Semper
(1881, p. Ill) cited as a recent discovery
that hibernating mammals have a consider-
ably lowered temperature, which Horvath
had found to reach 2° C. in the ground
squirrel, CiteUus citeUus.

Experiments on acclimatization to high
temperatures were also carried on in the
later decades of the nineteenth century.

22

THE HISTORY OF ECOLOGY

Those of Dallinger (1887), still cited ex-
tensively, covered several years, during
vvlrich time he slowly acchmated a popula-
tion of flagellates to heat. At the beginning
they started to die if raised to 23^ C;
finally they were Hving at 70° C. At this
point the experiment was terminated by
an accident; neither the nature of this
event nor DaUinger's emotions at the time
are revealed in tiie original reports.

Davenport's conclusions, based on knowl-
edge available in 1896, have a distinctly
modern sound. In general terms, not in ex-
act quotation, he says (1897, p. 277)
that when dynamic conditions vary quan-
titatively, a quantitative variation in metab-
ohsm will follow such that metabohsm be-
gins to slow down as limiting conditions
are approached. And finally: "A vital phe-
nomenon occurring in a given protoplasmic
mass can be reproduced only when the
dynamical conditions are reproduced, and
the structural hmiting conditions are in no
wise closely approached."

Semper's earher Animal Life (1881) is
less fully documented and hence is some-
what less helpful in strict chronology. His
book has the distinct advantage of being
written from much more nearly the modern
ecological point of view than was Daven-
port's. A brief review of some of his points
will increase our knowledge of, and respect
for, the ecological information available at
the close of the 1870's.

Semper knew of monophagy in the strict
modern sense among both carnivores and
herbivores. He also knew that monophagy
is often closely connected with the occur-
rence of special organs or structural rela-
tions, or with special adjustments in the
Life history. He clearly foreshadowed the
modern conception of "key-industry" ani-
mals, and he worked out in principle what
has come to be called the "pyramid of num-
bers" (p. 52).

Protective color changes in animals have
long been a matter of interest. Semper
(p. 91) reports that Stark in 1830 recorded
observations on color changes in several
different kinds of fishes; Shaw in 1838 was
perhaps the first to conclude that fishes
that can change color are apparently pro-
tected thereby from predators. Lister
(1858) found by experimentation that a
connection exists between eyes and chroma-
tophores in frogs, a relationship later inde-
pendently confirmed by Pouchet (1876),

who experimentally demonstrated that the
connection existed through the sympathetic
nervous system. Except in the growth of
detailed knowledge and the formulation of
the ratio hypothesis to explain background
matcliing (Keeble and Gamble, 1904), the
next important advance in the matter of
knowledge about cliiomatophore activity
came with the relatively recent insight into
the role of hormones and of neural hmnors
in the ecological relations of animals ca-
pable of color change to fit their environ-
ment.

Semper strongly doubted the significance
of the classification of animals according
to the temperature zones in which they five
in "fortuitous community." He thought that
the well-being of animals that five in asso-
ciation depends far more essentially on the
variations and extremes of temperature than
on the absolute degree of heat to which
they may be simultaneously exposed at any
given time. Hence he found the cUstinction
that Mobius had made between stenother
mal and eurythermal to be as important as
we now hold it to be.

In a much more speciaUzed field, Semper
anticipated the modern human preoccupa-
tion with "Lebensraum" and extended the
earher experiments of Hogg (1854) to
show that the fresh-water isopod Asellus
and the pond snail Lijmnaea would be
stunted if grown in too small a volume of
water. He failed to find an adequate ex-
planation experimentally and invented the
hypothesis of the presence of an unknown,
but necessary, substance, which was present
in the water, probably in a minute quan-
tity. Since a certain quantity would be
needed, it follows that below a minimum
volume, growth would be retarded. While
we know much more now than when
Semper was experimenting, this problem is
still essentially unsolved; the present knowl-
edge about the importance of vitamins and
other trace substances lends significance to
Semper's guess.

Semper was a morphologist, uninterested
in ecological relations before he went to
the Philippines on his great expedition.
Close contact with coral reefs in particular,
and with the wealth of life in general, ap-
pears to have changed his approach to bi-
ology. This is a dramatic, though not an
isolated, example. The effect of similar per-
sonal experience with varied and, to them,
exotic aspects of nature, during their voy-

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

23

ages on the "Beagle" and the "Rattlesnake,"
respectively, exerted strong formative influ-
ences upon Charles Darwin and T. H. Hux-
ley. Many others have had and continue
to have their biological thinking channel-
ized and intensified by direct observations
on the unaccustomed richness of the eco-
logical relationships of plant and animal
life of the tropics.

Milne-Edwards (1857) pubhshed a basic
contribution on the processes and organs
of respiration in animals. In the next two
decades, knowledge of the respiration of
aquatic animals was advanced decidedly.
In this connection, the work of Bert (1870)
and Fritz Miiller was available to Semper.

Bert (1878) emphasized the interrela-
tions between barometric pressure and oxy-
gen tension. He knew that the eflFect of
lowered or increased atmospheric pressures
can be obviated by adjusting the final par-
tial pressure of oxygen to that to which
the animals are acclimated. Fairly large
changes from this pressure are normally
harmful. Animals with closed, or nearly
closed, internal reservoirs of air show me-
chanical eflFects from variations such as
might be expected from a general knowl-
edge of the phvsical principles involved.
Bert also knew about the internal release
of nitrogen in decompression. It is an item
of more than passing interest that a trans-
lation of this thousand-page monograph was
published in 1943.

The importance of the evaporating power
of the air on animal distribution was well
recognized by 1880. There was also a con-
siderable body of knowledge concerning
mechanisms that allow gill-breathing ani-
mals such as crabs, and fishes such as Peri-
ophthalmus, to invade the land, sometimes
for extended periods of time. Forel's ob-
servations on the reinvasion of deep water
by the air-breathing Lymnaeidae were also
on record.

The ecologically-minded zoologist of the
1870's was also interested in the influence
of water in motion upon such matters as
the clinging power of mollusks, erosion of
shells, form of coral reefs and the relation
of currents of water (or air) to the distri-
bution of species. The importance of the
substrate was recognized, and many natural
history aspects of reciprocal reactions of
living organisms upon each other were
given much attention, especially the rela-
tions of sexes and various sorts of symbi-

osis, including commensahsm, mutualism,
and parasitism. Semper was also quite
aware of the relationship between his data
and the Darwinian theory of evolution. In
this he seems to have been in advance of
some of the more self-conscious ecologists
who followed him.

RESPONSE PHYSIOLOGY*

Ecological aspects of response physiology
are mainly concerned with phases of be-
havior. The attention centers on the behav-
ior of animals, since their reactions are
much more marked than are those of plants.
The responses of organisms are important
in ecology because they are frequently ini-
tiated primarily by the environment and
in turn react upon it. Since vocalization,
which may be easily and sometimes pre-
cisely interpretable in communication from
man to man, is not equally revealing among
other animals, the most sensitive clue to the
eflFect being produced by an environment
is frequently gained from the response
physiology of the reacting animals.

The history of this aspect of ecologv also
traces back to Aristotle, who recorded a
somewhat systematic account of the be-
ha\ior of many sorts of organisms. His ob-
sers'ations, despite their defects, exerted
an influence in this phase of developing
ecological knowledge which, with the pos-
sible exception of that of Reaumur (1683-
1757), was hardly equalled before the time
of Charles Darwin.

Wallace in Malaya and South America,
Hudson in the Argentine, Bates on the
Amazon, Belt in Nicaragua, and many
others made sturdy contributions to our
knowledge of the behavior of little-known
animals, which they observed on expedi-
tions or in out-of-the-wav places. Espinas'
consideration of social animals (1877) was
based on records or observations concern-
ing native as well as exotic forms. Brehm's
Tierlehen in its successive editions was the
outstanding natural history of the period
as BuflFon's Hisfoire Naturelle had been a
centurv earlier. Romanes made good obser-
vations, not onlv on the behavior of Cehiis
monkevs, but also on jellyfishes, starfishes
and sea urchins. Preyer experimented on

• The interested student is referred to
Holmes (1916) and Warden, Jenkins, and
Warner (1935) for the history of the study of
animal behavior.

24

THE HISTORY OF ECOLOGY

the behavior of starfish. Darwin contributed
his classic and essentially ecological study
on the earthworm; although, as usual, his
observations were exact, his long-range
conclusions on earthworms appear to have
been erroneous (cf. Keith, 1942). Fabre.
Lubbock, the Peckhams. and many others
reported penetrating field observations of
insect behavior.

In animal behavior, as in self-conscious
ecology and other phases of biology, the
decade and a half centering about 1900
showed a remarkable outburst of impor-
tant biological work which, while firmly
grounded historically, was still unusually
original. A mature contribution came from
Whitman (1898) in his Woods Hole lec-
ture on "Animal Behavior" in which he
demonstrated a naturalist's sensitivity re-
garding the necessity for full acquaintance
with the normal behavior of animals before
experimenting on them. He insisted, on the
basis of pertinent original observations on
the behavior of a leech, of Necturiis, and of
pigeons, that often the origin and signifi-
cance of a given behavior pattern antedate
individual acquisitions and are a part of
the problem of the origin and history of
organization itself, as well as reveal adjust-
ment between the animal and its normal
environment. Whitman's work on animal
behavior, though many of his results were
too long left unpublished (cf. Whitman,
1919), still influences current programs for
the analysis of ecological and other aspects
of behavior.*

The late 1890's and the early years of
the present century were enlivened by the
controversy that developed between the
forced movement, nonadaptive explana-
tions of animal behavior of Loeb and his
school and the adherents of the more com-
pUcated "trial and error" adaptational sys-
tem of Jennings. Happily we can now see
that the views are largely complementary,
and they have already been knit, notably
by Kiihn, along with other elements, into
a comprehensive system of orientational be-
havior (cf. Fraenkel and Gunn, 1940).

By 1897, Davenport, in his Experimental

• Whitman was himself an able naturalist.
He brought C. B. Davenport to the recently
founded University of Chicago, in part to
foster field studies, and he had much to do
with the early development of C. C. Adams
and V. E. Shelford.

Morphology, which reviewed a much wider
field, was able to summarize a hterature
in response physiology almost as extensive
as in developmental and toleration physiol-
ogy. The topics he treated historically in-
clude chemotaxis, hydrotaxis, tonotaxis,
thigmotaxis (stereotaxis), rheotaxis, geo-
taxis, electrotaxis, phototaxis, photopathy.
and thermotaxis. Much of the literature
cited is from the decades immediately pre-
ceding publication, but Davenport calls
attention to early work, such as that of
Trembley (1744, p. 66) that Hydra viridis
moved toward the light even when the
lighted slit is turned toward cooler air.

Some of the ecological queries that such
studies helped to answer are:

1. Do animals have definite reactions
that enable them to find the habitat suit-
able to their ecological tolerances?

2. Are animal reactions adaptive?

3. Is a given behavior pattern innate or
conditioned ( learned ) ?

4. Do any animals other than man seem
to be conscious of their behavior? if so, to
what extent? Is there a choice of habitats?
Do animals show preferences?

RELATION OF POPULATIONS TO
ENVIRONMENT

General biologists, and even ecologists,
who have read thus far, may ask: Is this
the history of ecology? Without referring
to the discussion of the rise of self-conscious
ecology, which will be considered when
the background is adequately prepared, an
answer may be quoted from an early eco-
logical summary. Adams (1913) said:

"There are also so many degrees and kinds
of work that go by the name ecological, which
may or may not be, and so many also which
are truly ecological but which do not pass
under that name, that it is necessary that the
student shall be able to see through its di-
verse guises and recognize its essential char-
acter. Whenever the question arises as to the
ecological character of a fact, inference, or
conclusion, its ecological validity may be
tested in the following way: Do the facts, in-
ferences, or conclusions show a response to
the inorganic or organic environment:

"1. As an individual of a species or kind of
animal?

"2. As a group of taxonomically related
animals?

"3. As an association of interacting animals?"

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

25

According to Adams, any of these responses
might properly be considered ecological.

rhe treatment of developmental, tolera-
tion, and response physiology may be tested
by the first of these queries. The present
section is written about the second; the
third point will be considered later. At the
turn of the century, the present discussion
would have centered about the history of
the ecology of species as distinct from that
of individuals. Now, in the 1940's, it is con-
cerned with populations. The difference is
not great, since current definitions of a
species are in terms of natural populations
or groups of populations.

The study of populations is not so far
removed from developmental, toleration,
and response physiology as at first appears.
Even the mathematical theory of popula-
tions is built around a framework of facts
or assumptions concerning animal behavior
(cf. Thompson, 1939). The primary biolog-
ical functions of a population include the
birth, nutrition, growth, reproduction, and
death of its members. As organisms or
populations grow, they draw their food
from outside themselves and may efiEectively
diminish the surrounding food supply.

Malthus (1798), an early student of
populations, calculated that while numbers
of organisms may increase in geometrical
progression, their food supply may never
increase faster than shown by an arithmet-
ical progression; a resulting discrepancy fre-
quently develops between the population to
be fed and the available food. Malthus
identified the drive for coitus with that for
reproduction, and at first thought both were
inexorable in man, as in other organisms.
As a result, there arises, he said, a violent
competition, which leads to a struggle for
existence (his phrase) until population in-
crease is finally controlled by catastrophe
or, in man (1803 edition), by purposive re-
straint from procreation.

As ecologists, we may happily avoid the
bitter controversy that sprang up almost
immediately about the matter of human
birth control and focus our attention on
the more general imphcations of the Essay
on Population. The ideas were not entirely
new, and much of the earUer history can
be found in the discussion of pre-Malthus-
ian doctrines of population by Stangeland
(1904). Machiavelfi, 275 years before
Malthus, had realized the danger that hu-

man populations may increase beyond the
means of subsistence in Umited areas and
that such an increase would then be
checked by want and disease. Botero pre-
sented a similar thesis in 1590. Hale
(1677), Buffon (1751), Franklin (1751),
Wallace (1761), and Bruckner (1767),
among others, anticipated Malthus. In fact.
Hale stated that the increase in human
population tends to occur in geometrical
ratio, which is one of the important proposi-
tions of Malthus. Yet it was Malthus who
focussed attention on the problem and so
set the stage for all demographic studies in
sociology and for the controversy about the
"struggle for existence" in biology.

Darwin (1859) found one of the bases
for his theory of natural selection in the
reasoning of Malthus, and A. R. Wallace
was also influenced by it when independ-
ently arriving at nearly the same evolu-
tionary ideas (Darwin and Wallace, 1858).
Twenty-four years before the pubhcation of
the Origin of Species, Quetelet, the Belgian
statistician, assumed (1835) that resistance
to the growth of a population increases in
proportion to the square of the rate of
population growth, much as the resistance
to a projectile increases with the square of
its speed. Quetelet speaks of a population
as though it were an entity.

Verhulst, a student and a colleague of
Quetelet's, in 1838 published a short essay
entitled "Notice sur la loi que la population
suit dans son accroissement," in which he
cited the ideas of "le celebre Malthus" and
those of Quetelet and proceeded to develop
briefly an equation describing the course of
population increases in proportion to popu-
lation density. His equation plotted into the
now well-known S-shaped population curve
with upper and lower asymptotes, which he
called the logistic curve. In his original
paper, Verhulst gave certain tests of good-
ness of fit of this curve against data for a
few human populations of western Europe.
Verhulst died in 1849 at the age of forty-
five. His work on populations attracted little
attention. Miner (1933) found only one
reference to it in "modem times" before the
rediscovery of the logistic curve by Pearl
and Reed in 1920; thus population studies
were long dominated by the cruder and
partially erroneous ideas of Malthus.

There seems to have been a general inter-
est in human populations in the early dec-

26

THE HISTORY OF ECOLOGY

ades of the niiieteentli century. Doubleday
(1841), stimulated by his skepticism con-
cerning the validity of the population
theory of Malthus, brought forth his "true
law of population." He said in part (p. 6) :

"The great general law then, wliich, as it
seems, really regulates tlie increase or decrease
both of vegetable and of animal life, is tiiis,
that whenever a species or genus is endangered,
a corresponding effort is invariably made by
nature for its preservation and continuance, by
an increase of fecundity or fertility; and that
this takes place whenever such danger arises
from a diminution of nourishment or food, so
that consequently the state of depletion . . .
is favorable to fertility; and that, on the other
hand, . . . the state of repletion, is unfavor-
able to fertihty, in the ratio of intensity of
each state, and this [holds] probably through-
out nature universally, in the vegetable as well
as in the animal world . . . ."

Doubleday was mainly concerned with
human phenomena. He accurately detected
the fact that the well-to-do and rich repro-
duce less rapidly than the poor, and inac-
curately thought that this human situation
and similar phenomena in plants and
animals were wholly expUcable in terms of
the effects of overrich mineral nutrients on
plants and overfeeding with domestic ani-
mals, including man.

The next contribution, that of WiUiam
Farr, did not grow out of the same set of
considerations that had intrigued Malthus,
Quetelet, Verhulst, and Doubleday. Farr
was especially concerned with mortahty. In
1843 he discovered that, within limits in
England, there was a relation between the
density of the human population and the
death rate such that mortahty increased as
the sixth root of density. Farr returned to
the problem in 1875 and tested his earlier
discovery against population and mortahty
data from all districts of England and
Wales for the years 1861 to 1870, finding
that when the districts were listed in the
order of their mortahty, the latter always
increases with the density, but less rapidly.
In general terms, Farr's rule states that if
the death rate is represented by R and the
density of the population per unit area by
D, then R = ^D"*, where c and m are con-
stants.

Brownlee (1915) rehabihtated this rule
by showing that the statistics used by Farr,
which came from the decade 1861 to 1870,
compared favorably with those from the

decade 1891 to 1900. The only correction
needed arose from tire improvement of san-
itation in the intervenmg years.

It is easy to jump ahead of our chiono-
logical story. In 1852 Herbert Spencer pub-
hshed an outhne of "A Theory of Popula-
tion, Deduced from the General Law of
Animal Fertihty," which he later incor-
porated in his Principles of Biology (1867)
and expanded to make a whole section of
that work. The essence of his later state-
ment is:

"Individuation and Genesis are necessarily
antagonistic. Grouping under the word Indi-
viduation all processes by which individual life
is completed and maintained, and enlarging
the meaning of the word Genesis so as to in-
clude all processes aiding the formation and
perfecting of new individuals; we see the two
are fundamentally opposed. Assuming other
things to remain the same— assuming that en-
vironing conditions as to cUmate, food, enemies,
etc., continue constant; then, inevitably, every
higher degree of individual evolution is fol-
lowed by a lower degree of race multiplication,
and vice versa. Progress in bulk, complexity,
or activity involves retrogress in fertihty; and
progress in fertihty involves retrogress in bulk,
complexity, or activity."

We sympathize with Doubleday, who
complained (1853, p. xxix) about an earher
version of this idea: "The author will now
venture a few brief remarks on positions of
a very erudite review of the 'True Law of
Population' . . . pubhshed . . . under the
name of 'Herbert Spencer.' It is not easy to
evolve the exact doctrine of the reviewer
from the load of learned diction ..."

Stated simply, Spencer's ideas were that
when the amount of energy is hmited, the
greater the proportion used in the growth
of nutritive aspects of the individual, the
less there is left for reproduction. Double-
day found this suggestion entirely unac-
ceptable.

Darwin took over without criticism the
whole of the Malthusian doctrine as regards
the geometric ratio of population growth
and the resulting struggle for existence. He
documented these ideas extensively with
data from nonhuman as well as from human
populations. The use he made of them is
well and generally known. In the Origin of
Species he also clearly recognized that
populations exist as units. Thus the evolu-
tion of instincts of neuter insects can be
explained on the ground that the colonies

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

27

(populations) are selected as units. As with
many other phases of biology, Darwin's
work gave direction to population studies
without containing much that was strictly
concerned with this particular field.

Farr, as we have seen, returned in .1875
to his discussion of problems related to the
human population of England as revealed
by the accumulated vital statistics. He saw
clearly that a decrease in death rates and
a resulting increase in longevity do not
necessarily lead to an increase in popula-
tion, since, as he cogently remarks, the
associated birth rate may fall to an equiva-
lent extent. He knew that in man, as in
other organisms, the possibility of popula-
tion increase in geometrical ratio exists; but
(and here Malthus had erred) so also may
the means of man's subsistence. Not only
had the population of the United States of
America doubled itself every tAventy-five
years for a century and a half, but the
means of human subsistence had also in-
creased in geometric ratio and at an even
greater rate. This must frequently hold true,
since the plants or animals on which man
feeds can increase (or decrease) even more
rapidly than longer-lived, slow-breeding
man. Restated in terms of the pyramid of
numbers, which Farr did not do, this can
be turned into another general principle.

A close consideration of the ideas of
Malthus concerning population growth and
control, and of Darwin concerning evolu-
tion, would seem to require oscillations in
the populations of what would now be
called key-industry animals and in those of
the carnivores that feed upon them. Spen-
cer (1863) wrote about this "rhythm in
number of each tribe of animals and plants"
in approximately modem terms. We have
recently been reminded by Elton (1942)
that knowledge of mouse plagues, which
represent an outstanding oscillation in
nature, dates back to early Hebrew history
and that such plagues were well known to
Aristotle, Theophrastus, Pliny, and others of
the classical period. They were obser\'ed
somewhat critically during the last decades
of the nineteenth century, the formative
years for much of modern ecology.

Knowledge concerning populations had
another line of ancestry in the biometri-
cians, Galton, Weldon, and Karl Pearson.
Aside from Weldon's work (1898) on the
relation of the survival of crabs in Plymouth
Harbour (England) to the width of the

carapace, and a few similar papers, these
men contributed disappointingly little
directly to the knowledge or theory of
populations. It remained for an American
disciple, Raymond Pearl, to make the transi-
tion from biometry to population studies
that somewhat approximates the ecological
approach to the subject. Like his redis-
covery (with Reed, 1920) of Verhulst's
logistic curve and his eflFective use of that
curve as a quantitative expression of poten-
tial rate of increase and of environmental
resistance, these developments by Pearl
came too late to aflFect the early rise of
ecology. Their modern aspects and their re-
lations to other phases of present day ecol-
ogy will be treated later (p. 46).

ECONOMIC BIOLOGY

Many population studies have a strong
economic tiend, and the pressure of eco-
nomic problems not only accelerated the de-
velopment of an adequate basis for modern
ecology, but continues to stimulate eco-
logical development today. Three broad
economic interests of man— fisheries, agricul-
ture, and certain aspects of medicine— are
closely related to ecology. The need for
more precise information concerning food
fishes and the conditions of their existence
has been one of the potent drives in the
study of the ecolog)' of aquatic habitats.
The relation between ecology and agricul-
ture is even more obvious; many of the
environmental relations of plants were stud-
ied in the eighteenth and nineteenth
centuries, as well as in earlier and more
recent times, because of their direct bearing
on agricultural problems. The data re-
viewed by Abbe (1905) were discovered
primarily because of their immediate
economic application, and Abbe's compre-
hensive review was itself similarly moti-
vated.

On the animal side, an important element
in the background of ecology came from
work with insects in relation to man-growTi
crops and to the control of diseases of do-
mestic animals and of man. Precise
summaries of the history of these develop-
ments will be found in books devoted to
economic and to medical entomology,
especially those on the history of entomol-
ogv, notably Howard (1930) and Essig
(1931). The treatment here wdll be sug-
gestive rather than comprehensive.

The regulation of population size of

28

THE HISTORY OF ECOLOGY

noxious insects is a primary problem
which has long been attacked. One ecolog-
ical method uses the natural controls of
trouble-making insects. Fungus diseases at-
tracted attention at an early date; Forbes,
(1895) traced the history of knowledge of
such diseases of insects in Europe and
America and described in detail additional
experiments designed to stop the inroads
made by the chinch bug, Blissus leucop-
terus, upon farm crops in Illinois. As early
as 1880 Thomas had observed a relation
between temperature and rainfall and the
development of excessive populations of
chinch bugs.

Another phase of insect control, distinctly
ecological in approach and in general im-
plications, comes from the use of predatory
species and insect parasites to attack de-
structive species. Sweetman (1936) has
summarized the history of such eflForts. It
appears that Forskal (1775) gave the first
written account of this usage when he de-
scribed the introduction of colonies of
predatory ants from the nearby mountains
into Arabian palm orchards to attack other
ants that were feeding on the date palms.

Sweetman (1936) notes that Erasmus
Darwin wrote about the possibilities of bio-
logical control in 1800. In 1840 in France
large numbers of native carabid beetles
were placed on poplar trees to destroy
caterpillars of the gipsy moth. The interna-

tional transfer of parasites to prey on
introduced insect pests was suggested by
Fitch in 1854 and was put into effect by
Planchon and Riley in 1873. Other early
experiments of this nature in the 1870's and
1880's were almost forgotten in the success
achieved, largely as a result of the work of
C. V. Riley, by the importation of a
coccinelhd beetle from Australia into Cali-
fornia in 1889 to control the cottony-cush-
ion scale.

Oscillations between insect pests and
their parasites were demonstrated independ-
ently by Howard (1897) and Marchal
(1897) for different species. Two other
workers, (Bellevoye and Laurent, 1897)
provided the outline of a mathematical
theory of the biological control of popula-
tion size. They set up a fairly simple equa-
tion to show how such a state, now called
a steady state, would be maintained.

Growth of knowledge about the interre-
lations of organisms with respect to
mammalian disease also proceeded at a
rapid pace in the closing decades of the
last century. Herms (1939) records that
Josiah Nott of New Orleans published an
essay on the origin of yellow fever in 1848
in which he expressed the belief "that mos-
quitoes give rise to both malaria and yellow
fever." This was a fortunate guess. Carlos
Finlay of Cuba set forth a similar theory
for yellow fever about 1880 and conducted

Table 1. Important Diseases Known before 1900 to Be Insect-Borne (Data Extracted Chiefly

from Herms, 1939)

Disease

Causative Organism

Principle Vector

Discoverer* of Vector

(or Name Closely

Associated)

Filariasis

Wuchereria. bancrofli

(Filaria)
Babesia higemina

(Protozoan)
Trypanosoma hrucei

(Protozoan)
Plasmodium

(Protozoan)
Bacillus (Pasteurella)

pestns
A filtorahlo virus

Culcx faiigans

(Mosquito)
Boophihis anmdaius

(Tick)
Glossina morsilans

(Tsetse fly)
Anopheline mosquitoes

Xenopsylla

(Rat flea)
Aedes aegypti

(Mosquito)

Patrick Manson, 1878

Texas cattle fever . .
Nagana

Theobald Smith, F. L

Kilbourne, 1893
David Bruce, 189.5

Malaria

Ronald Ross, 1897

Bubonic plague. . . .
Yellow fever

P. S. Simond, 1898
Walter Reed, 1900

* Names of other men closely connected with these discoveries, or some of them, can be
found in Herms' text and are not repeated here, even though their omission may do an injustice
to worthy workers.

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

29

experiments on the subject. King (1883)
gave nineteen reasons why mosquitoes
should be considered as possible vectors of
malaria. King knew about Finlay's work,
but he deserves credit for extending it to
malaria at a time when even certain ento-
mologists well acquainted with mosquitoes
rejected the idea.

The relations that had been established
by 1900 are summarized in Table 1. We
have taken the liberty of bringing informa-
tion concerning the causative organisms
and insect vectors up to date rather than
give here the more imperfect statements of
1900.

Medical entomology was in a state of
rapid growth at the end of the period cov-
ered by the present chapter, and scholarly
consolidation of the field had already be-
gun; this was shown by the appearance of
the first comprehensive, critical and histori-
cal study of the known disease-carrying
activities of arthropods, that by Nuttall
(1899). The medical masterpiece by Smith
and Kilbourne (1893) deserves independent
mention, not only because of its medical
significance, but also because of its careful
and critical use of the techniques of field
experimentation.

Forbes, an alert student of the literature
of the subjects with which he dealt as
well as with natural phenomena themselves,
may well have had many of these develop-
ments in applied entomology in mind when
he wrote the following orienting paragraph
(1895) as an introduction to his discussion
of the diseases of the chinch bug:

"... Another division of biological science,
little known to the general public by its name
as yet, and but lately (distinguished as a
separate subject, ... is now commonly called
oecology. It is the science of the relations of
living animals and plants to each other as liv-
ing things and to their surroundings generally.
It deals with tlie ways in wliich heat and light,
moisture and drouth, soil and climate, and
food and competitors and parasites and pre-
dacious enemies, and a long list of agencies
additional, act upon living things, and the
ways in which these living things react in turn;
it includes, in short, the whole system of life
as exhibited in the interactions between the
plant or animal and the environment, living
and without life. It is a very comprehensive,
complicated, and important subject; how com-
prehensive and important we see at once when
we learn that the whole Darwinian doctrine
belongs to it on the one hand, and that all
agriculture depends upon it on the other. It

covers, indeed, the whole field of active life
and all forms of matter and energy as affecting
living things in any way."

EVOLUTION:
STRUGGLE AND COOPERATION

The history of the growth of knowledge
of organic evolution has been told fre-
quently and well. We need only call
attention to the twin facts (a) that the his-
tory of the rise of evolution in its modern
biological connotation repeats much of the
history of ecology in that many of the same
men were involved, and {b) that the sub-
ject matter of each of these two aspects of
biology strongly overlaps.

The nearer we approach modern times
and modern preoccupations, the greater is
the divergence in men as well as in matter.
Although shadowy ideas of evolution, and
even forerunners of the theory of natural
selection, are much older (cf. Zirkle, 1941),
for the purposes of this sketch we may well
begin with Buffon, the great theoretical
biologist of the eighteenth century. We get
a glimpse of the essence of his evolutionary
ideas from the following quotation from
his Histoire Naturelle (Paris, 1749 ff.:
translation quoted from Dendy, 1914) :

"If we again consider each species in differ-
ent climates we shall find obvious varieties both
as regards size and form; all are influenced
more or less strongly by the climate. These
changes only take place slowly and impercep-
tibly; the great workman of Nature is Time:
he walks always with even strides, uniform
and regular, he does nothing by leaps; but
by degrees, by gradations, by succession, he
does everything; and these changes, at first
imperceptible, little by little become evident,
and express themselves at length in results
about which we cannot be mistaken."

Buffon's main contribution to evolution-
ary biology was the idea that the environ-
ment can permanently affect the life of
organisms by the process now called en-
vironmental induction. Buffon influenced
Erasmus Darwin's ideas, and also those of
Lamarck. Although he anticipated Malthus
in understanding the implications of popu-
lation pressure, and while he had a clear
appreciation of the struggle for existence,
Buffon was not a consistent thinker, and he
may be as truly classified with Cuvier as
a catastrophist as with Lamarck and Eras-
mus Darwin as a forerunner of modern
evolutionary views.

30

THE HISTORY OF ECOLOGY

Lamarck's contiibutions are more widely
known as a result of the publicity, mainly
adverse, given to his now generally aban-
doned tlieory of evolution through the
inheritance of characters acquired by use
and disuse or by a more direct effect of the
environment. Lamarck summed up his con-
clusions in the Histoire Naturelle des Ani-
maux sans Vertebres (Paris, 1815; cf.
Dendy, 1914, p. 382). Lamarck's Philo-
sophie Zoologique (1809) is better known.
He placed the effects of needs and of re-
sulting habits of animals, together with
their manner of life and the conditions
under which their ancestors have Hved, in
the forefront of his explanation of the
bodily form and general qualities of a given
animal.

Darwin's (and Wallace's) theory of evo-
lution is based on principles equally
ecological though radically different.
Among the important ones we may recog-
nize Malthusian overpopulation and the
resulting struggle for existence with ensuing
natiiral selection. Except for the fundamen-
tal part, which is concerned with the
nonenvironmental origin of many, probably
of the majority, of heritable variations, the
remainder of the factors involved in Dar-
win's theory are now recognized as being
clearly ecological in nature. The exception
just noted is even more important than Dar-
vvin thought, since he was not altogether
free from Lamarckian enviromnentaUsm.
The ecological substratum of Darwin's and
of Wallace's thinking is brought into
clearer hght when we recall the extent to
which each was influenced by zoogeo-
graphic considerations.

The supporting theory of geographic iso-
lation (Wagner, 1868; Gulick, 1888, 1905)
also grew out of zoogeographic studies and
has even more of an ecological bent than
does general Darwinian theory.

It would be interesting, and perhaps not
without value, to consider briefly the rea-
sons for the failure of some early ecologists
to recognize and insist upon the close con-
nection between their newly vivified subject
and the important generalizations of evolu-
tionary theory. Perhaps, however, such a
discussion can be dismissed with the sug-
gestion that a part of the psychology
involved is not wholly unlike that of a
vigorous adolescent in establishing his inde-
pendence from actively possessive parents.

From a certain viewpoint, there are two

main approaches to the phenomena oi
ecology and of biology in general, and each
yields its element of truth. The more usual
approach has been by way of the
individuaUstic, egocentric position of the
neo-Darwinians that Darwin himself empha-
sized. This approach is usually developed
about some phase of person-to-person com-
petition, and hence the word "competition"
has wrongly come to be wholly associated
with the harmful interactions of organisms
that yield results which are the opposite of
cooperations, and may be called disopera-
tions. The history of the use of this
approach is almost identical with much of
that of evolutionary theory since Darwin's
time. Opposed to the individuaUstic empha-
sis, there is the concern with group-cen-
tered, more or less altruistic tendencies,
such as have frequently been considered
under the heading of cooperation, which
careful students nowadays consider as en-
tirely nonconscious proto-cooperation in all
lower forms. The word itself in this connec-
tion should imply merely that the
interactions under consideration are more
beneficial than harmful for individuals or
group units.

The germ of the idea of natural coopera-
tion, along with that of natural selection,
can be traced to the biologically absurd
poetry of Empedocles (p. 14). Thereafter
the idea was kept somewhat alive, often in
barely recognizable form, by the succession
of thinkers from Aristotle to Herbert Spen-
cer and others who saw human society as a
natural outgrowth from the hfe of other
animals. They were opposed by an equally
impressive succession of men who thought
of society as an artifact. A fairly exhaustive
history of this phase of the subject is given
by Espinas (1877).

More positive philosophical emphasis on
the nonegocentric interpretation of nature
began with Anthony Cooper, third earl of
Shaftesbury, who about 1700 recognized
that racial drives exist that can be explained
only by their advantage to the group. Adam
Smith emphasized the same qualities in his
Theory of Moral Seiitiments (1759) under
the heading of "sympathy" or "fellow feel-
ing"; his more famous Inquiry into the
Wealth of Nations" (1776) is completely
based on the opposed force of self-interest^
and he did not publicly reconcile the two.
Later, Feuerbach (1846-1890) emphasized
the same idea under the heading of "love,"

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

31

and Comte (1830) called it "altruism."
Such developments are reviewed sympathet-
ically by Lange (1865). It may be added
that Spencer argued both sides of the rela-
tion between egoism and altruism. In his
Principles of Ethics (1893, p. 201) he
said: "If we define altruism as being all
action which, in the normal course of
things, benefits others instead of benefiting
self, then, from the dawn of life, altruism
has been no less essential than egoism.
Though primariily it is dependent on ego-
ism, yet secondarily egoism is dependent on
it."

With the growing perception in the last
few decades of the significance of cooper-
ative forces in nature, there has been a
reawakening of interest in Darwin's attitude
on the subject. As wdth other aspects of
evolutionary biology, Darwin was more
broadminded than many of his followers.
His recognition that insect castes can be
explained on the basis of natural selection
of the whole interacting insect social group
shows an appreciation of one distinctly
nonegoistic aspect of social hving. Weis-
mann (1893), in his controversy with Her-
bert Spencer over the importance of ac-
quired characters, forcefully elaborated this
point so far as the "all-sufficiency of natural
selection" is concerned. Weismann did not
grasp the more general implications that the
phenomena he discussed indicate a general
cooperative tendency in nature. He did see
clearly that cooperation between the parts
of organized wholes— whether the wholes
are individual animals, as in the evolving
proportions of the Irish stag, or are social
entities, as with the evolving neuters of an
ant colony— could come about by natural
selection of germinal variations. It is an
interesting question whether Darwin him-
self went further.

Much can be and has been made of
Darwin's statement in the Origin of Species
regarding the struggle for existence in
which he says (Murray's library edition, p.
46):

"I use this term in a large and metaphorical
sense, including dependence of one being on
another, and including (which is more impor-
tant) not only the life of the individual, but
success in leaving progeny . . . The mistle-
toe is dependent on the apple and a few other
trees, but can only in a far-fetched sense be
said to struggle with these trees, for, if too
many of these parasites grow on the same

tree, it languishes and dies ... In these sev-
eral senses, which pass into each other, I use
for convenience sake the general term of
Struggle for Existence."

Perhaps it would be the fairest possible
treatment to follow Geddes and Thompson
(1911, p. 167), who were friendly observ-
ers of Darwin and Darwinism and of the
point of view now under discussion.

"Darwin's characteristic fundamental idea of
the intricacy of inter-relations in the web of
life, lies below the idea of the struggle for
existence, and therefore below the idea of nat-
ural selection. Unless we appreciate the
fundamental natural history fact of the web of
life, we cannot rightly understand how slight
differences can be of critical moment in deter-
mining survival. The entanglements are so
intricate that a slight variation may be of sur-
vival-value to its possessor,"

Our italics indicate a suspicion that even
Geddes and Thompson were much con-
cerned with the success of the individual,
an individual enmeshed, to be sure, in a
recognized and important web of life. Again
in the same book (p. 174), in speaking of
family and group selection, which they list
as one of several kinds of selection, they
summarize the matter thus:

"Though Darwin did not wholly overlook
this (indeed in at least one notable passage
he expresses it) there is no doubt that the gen-
eral tone and treatment of Darwinism . . .
has been deeply coloured by the acute individ-
ualism of Darwin's and the preceding age.
We may therefore restate the concluding thesis
of our own 'Evolution of Sex' (1889) since
elaborated in various ways by Drummond, by
Kropotkin and others. It is that the general
progress both of the plant and the animal
world, and notably the great uplifts, must be
viewed not simply as individual but very
largely in terms of sex and parenthood, of
family and association; and hence of gregarious
flocks and herds, of co-operative packs, of
evolving tribes, and thus ultimately of civilized
societies . . . above all therefore, of the city.
Huxley's tragic vision of 'nature as a gladiatorial
show' and consequently of ethical life and
progress as merely superimposed by man, as
therefore an interference with the normal order
of Natvire, is still far too dominant among us."

Representative of T. H. Huxley's atti-
tude. Caiman (1939) writes:

"When Huxley wrote that among animals
and among primitive men, 'Life was a continual
free fight, and beyond the limited and tem-
porary relations of the family, the Hobbesian

32

THE HISTORY OF ECOLOGY

war of each against all was the normal state
of existence,' he was, not for the first time,
overstating the case."

In the Descent of Man Darwin gave nat-
uralistic examples of mutual aid. His whole
thesis that man is descended from other
animals requires that he should recognize
that man's altruistic drives should have
their precursors among his nearer ancestors
and would probably be recognizable among
his closer living relatives.

That the individualistic emphasis was
common in British scientific circles during
Darwin's later life and that group-centered
interpretations were novel is shown by the
following quotation from Nature (21: 285,
Jan. 22, 1880) :

"We notice an important communication
which was made by Prof. Kessler at the annual
meeting of the St. Petersburg Society of
Naturalists on January 8, [1880] on the 'Law
of Mutual Help,' as one of the chief agents in
the development and progress of organisms.
Prof. Kessler, although an able follower of
Darwinism, thinks that the struggle for exist-
ence would be insufficient to explain the
progress in organic life, if another law, that of
sociability and of mutual help did not power-
fully work for the improvement of the organ-
isms and for strengthening the species . . . ."

Espinas' (1877) great work, which pre-
ceded Kessler's lecture, emphasizes the
naturalness of the cooperative social drives;
Darwin-like, he implemented his conclu-
sions by pertinent observations drawn from
many aspects of natural history and from
various levels of the animal kingdom. He
had little immediate influence upon the
thinking of the biologists, although more
recently many have come to recognize the
value of his work. Thus Deegener (1918)
and Wheeler (1923 and later) give evi-
dence of having been influenced by his
ideas and by the evidence he collected.

Forbes (1887) recognized the existence
of cooperative interests even in apparently
opposed forces in the ecological community.
He said:

"It is a self-evident proposition that a species
cannot maintain itself continuously, year after
year, unless its birth-rate at least equals its
death-rate. If it is preyed upon by another
species, it must produce regularly an excess of
individuals for destruction, or else it must cer-
tainly dwindle and disappear. On the other

hand, the dependent species evidently must
not appropriate, on an average, any more than
the surplus and excess of individuals upon
which it preys, for if it does so, it will continu-
ously diminish its own food supply, and thus
indirectly, but surely, exterminate itself. The
interests of both parties will therefore be best
served by an adjustment of their respective
rates of multiplication, such that the species
devoured shall furnish an excess of numbers
to supply the wants of the devourer, and that
the latter shall confine its appropriations to the
excess thus furnished. We thus see there is
really a close community of interest [sic] be-
tween these two seemingly deadly foes."

Kropotkin's writings (1902) on mutual
aid are still quoted, perhaps more fre-
quently by less critical students, and, to-
gether with the teachings of Geddes and
Thompson, serve to round out the develop-
ments in this aspect of ecology at the turn
of the century. Needless to say, the new
century opened with the emphasis still
centered upon the individual and his prob-
lems rather than upon the group, whether
as a community, a more closely knit bio-
coenosis, a population, or a mere aggrega-
tion of organisms.

Despite the development of the coopera-
tive idea by Delage and Goldsmith (1912),
Reinheimer (1913), and Patten (1916), the
turn toward present day emphasis on the
importance of natural cooperation did not
come until about the beginning of the
1920's; this development will be traced in
the following chapter.

During the second half of the nineteenth
century considerable attention was given to
the phenomenon of symbiosis, more, it
seems, as an oddity in an egocentric world
than as an indication of any general under-
lying biological principle. The writings of
Van Beneden and of Oskar Hertwig illus-
trate the point. Later, in the first decades of
the present century, Kammerer and
Deegener, among others, saw the more
general implications of widespread sym-
biosis.

THE NATURALISTS

Ecologists have not usually been greatly
concerned with biological theory. Converse-
ly, they have kept their feet planted, as
firmly as the often slippery substratum
would permit, on the soil or in the mud
and water of field experience. This tend-

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

33

ency is by no means new, but stems rather
from the long line of excellent naturalists,
whether travelers or stay-at-homes, who
contributed much to the background of
the subject. This is not the place to set
forth the needed history of natural history;
combined with what has aheady been said
on the subject, the barest outline must suf-
fice. Basic as is their service to ecology, we
must pass over the host of taxonomists of
the latter half of the nineteenth century,
except as they contributed directly to eco-
logical observation.

The contributions of the Greek, Roman,
and earlier natxiraHsts of northern Europe
have already been mentioned. The writings
of many others have been or will be dis-
cussed in other connections. We want to call
attention to such observations as those fur-
nished by Martin (1698), who gave an
early description of the breeding and some-
thing of the populations of the sea birds of
St. Kilda in the Outer Hebrides, and to
those of White (1789), who described the
natural history of his native village of
Selborne.

The varied contributions of explorers
and collectors hke Bates, Belt, and Hum-
boldt, and of observers hke Fabre, Forel,
and the Peckhams, to name no more, are
not limited merely to the background of
modern ecology; then: observations often
emerge into the foreground.

Wallace's Island Life and Malay Archi-
pelago, Bates' Naturalist on the Amazons,
Belt's Naturalist in Nicaragua, Fabre's
fascinating accounts of the habits of insects
of the countryside in France, Audubon's
recently reprinted Birds of North America
and Brehm's From North Pole to Equator,
with his greatly expanded Tierlehen— again
to name no more— are still desirable reading
for any alert animal ecologist.

Louis Agassiz, the many-sided naturalist,
played an important role in laying the
foundation on which ecology was later
built. In 1846, when he was almost forty
years old, Agassiz came to America from
his native Switzerland with an established
reputation based on teaching and on much
scholarly work with fossil and Living fishes
and on his study of glaciers. His later
scientific work was also of high quahty. In
America, Agassiz had an extraordinary
career as a naturahst both at home and on
expeditions. His influence as a lecturer and
above all as a teacher revivified the study of

nature in this country and made naturalists
more respected members of many com-
munities. He taught the men who in turn
trained the pioneer American ecologists. His
final success was with a summer seaside
laboratory on Penikese Island off Woods
Hole, Massachusetts, established in 1873,
the year after Anton Dolirn completed the
first building of the zoological station at
Naples. Agassiz at the Penikese laboratory
exerted an influence on American biology
out of all proportion to the length of the
short summer session in this, the last year
of his Hfe."

The naturaUsts of the later decades of
the nineteenth century rounded out certain
phases of ecology or of allied subjects in
approximately their present form. Thus the
zoogeographical regions of the world, out-
fined on the basis of the taxonomic relation-
ships of animals, and the smaller faunal
areas of North America and Europe re-
main on the maps much as the nineteenth
century naturafists left them. Though often
used, especially by nonecologists, the limits
of Merriam's fife zones have undergone only
sHght change since early in the present cen-
tury, and, moisture considerations aside
(see p. 114), they appear in modern works
much as Merriam outlined them in the
1890's. The whole vast field of tlie recipro-
cal relations between flowers and pollina-
tion by insects was largely estabhshed in
its present form by the eighteenth and
nineteenth century naturafists (cf. MiiUer,
1883; Knuth, 1898-1905).

Fortunately for ecology, robust work in
natural history still continues in the twen-
tieth century and will be discussed in the
next chapter.

• Many marine biological laboratories have
arisen as a direct or indirect result of the last-
ing success of Dohrn's "Stazione Zoologica" at
Naples and of the influence of Agassiz's
meteoric venture at Penikese. The Marine Bio-
logical Laboratory at Woods Hole is the direct
descendant of the latter. We wish to record
our judgment that many of these laboratories,
despite their favorable locations, have not as
yet had an important direct influence on the
development of ecological science. The more
recently established "Oceanographic Institu-
tion," also at Woods Hole, is becoming an
exception in its relation to the marine ecology
of the future. The much more humble labora-
tories scattered about the fresh waters of
Europe and the United States have been more
consistently important in ecological research.

34

THE HISTORY OF ECOLOGY

THE COMMUNITY CONCEPT

Recognition of the existence of com-
munities of living organisms in nature is
not new. As shown earlier in this chapter,
the idea dates back to the classical Greeks.
In the modern period, according to Braun-
Blanquet (1932), Heer (1835), Lecoq
(1854), Sendtner (1854), and Kerner
(1863), all sought to understand the basic
causes of the interrelations of certain
plants, and Kerner "brought even to the
laymen an understanding of the principal
plant communities of Austria-Hungary to
the environment."

Clements (1905) traced recognition of
the plant formation to Grisebach (1838),
who recognized it as the fundamental fea-
ture of vegetation. Earlier writers, Cle-
ments continues, "notably Linne (1737,
1751), Biberg (1749), and Hedenberg
(1754), had perceived this relation more
or less clearly, but failed to reduce it to a
definite guiding principle." Clements adds
that the acceptance of the "formation" as
a unit of vegetation took place slowly, but
this point of view came to be more and
more prevalent as a result of the work of
Kerner (1863), and a half-dozen others, in-
cluding Warming (1889)." Clements and
Shelf ord (1939) state that "the idea of the
plant community in general extends back-
ward for nearly two centuries," and, as re-
gards the biotic community, "Post (1868)
recognized that the organic world should
be dealt with in its entirety, but seems to
have had no definite idea of the community
as a unit."

Darwin's recognition of the web of life
concept has akeady been mentioned. His
famous illustration of the relationship be-
tween the number of cats and the amount
of clover seed in an English community
illustrates his understanding of possible in-
tracommunity relationships. Saint-Hilaire
(1859) foreshadowed the concept, and
Haeckel (1869), in his classical definition
of "Oecology," also vaguely recognized the
existence of communities.

Edward Forbes (1843-1844), in study-
ing the animal distribution in British wa-
ters and the Aegean Sea, discovered "prov-
inces of Depth" which "are distinguished

• Warming's bibliography in the 1909 edi-
tion of his Oecology of Plants does not list a
title for 1889 among his thirteen publications
between 1869 and 1894, inclusive.

from each other by the associations of
the species they severally include. Cer-
tain species in each are found in no other;
several are found in one region which do
not range into the next above, whilst they
extend to that below, or vice versa. Certain
species have their maximum of develop-
ment in each zone, being most prolific in
individuals in that zone in which is their
maximum, and of which they may be re-
garded as especially characteristic. Mingled
with the true natives of every zone are
stragglers, owing their presence to the sec-
ondary influences which modify distribu-
tion."

Forbes clearly recognized the dynamic
aspect of the interrelations between organ-
isms and their environment. He stated liis
conclusions as follows (1843, p. 173):

"The eight regions in depth are the bcene
of incessant change. The death of the indi-
viduals of the several species inhabiting them,
the continual accession, deposition and some-
times washing away of sediment and coarser
deposits, the action of the secondary influences
and the changes of elevation which appear to
be periodically taking place in the eastern
Mediterranean, are ever modifying their char-
acter. As each region shallows or deepens, its
animal inhabitants must vary in specific asso-
ciations, for the depression which may cause
one species to dwindle and die will cause
another to multiply. The animals themselves,
too, by their over-multiplication, appear to be
the cause of their own specific destruction. As
the influence of the nature of the sea-bottom
determines in a great measiure the species pres-
ent on that bottom, the multiplication of
individuals dependent on the rapid reproduc-
tion of successive generations of MoUusca, etc.,
will of itself change the ground and render it
unfit for the continuation of life in that lo-
cality until a new layer of sedimentary matter,
uncharged with living organic contents, depos-
ited on the bed formed by the exuviae of the
exhausted species, forms a fresh soil for simi-
lar or other animals to thrive, attain their
maximum, and from the same cause die oflF."

This is an early, perhaps the first, state-
ment of ecological dynamics, a subject much
emphasized in recent decades (see p. 563).
Elsewhere, Forbes (1844) regarded self-
produced, local destruction of a species as
a kind of "rotation of crops" and shows
clearly that he was more concerned with
the alternation of fossihferous and nonfos-
siHferous geological strata than with the
processes that we now know are connected

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

35

with the biotic control of some important
phases of ecological succession.

The subdivision of the Httoial region of
the ocean into faunal provinces, as Dana
(1852, 1853), Packard (1863), and VerriU
(1866) have done for Atlantic coastal
waters, is based primarily on the observed
distribution of species and groups of species
and secondarily on physical factors such as
temperature and geographic features such
as capes. From the most southern Floridian,
through the Carohnian, Virginian, and
Acadian, to the most northern Syrtensian
province, the geographic faunas of these
naturalists suggest the biomes (biotic for-
mations) of more recent workers (cf. Shel-
ford et al., 1935). If proposed today, they
might be designated by biological terms to
suggest their taxonomic composition, rather
than by geographic names that suggest
their distribution.

We now know that this is the historical
background against which to view the re-
markable work of VerriU and Smith (1874)
which, despite the praise given by Adams
(1913), did not receive the recognition or
have the influence among ecologists that
it merited. They found "three quite distinct
assemblages of animal hfe, which are de-
pendent upon and Umited by definite physi-
cal conditions of the waters which they
inhabit." These three primary groupings
were: (1) the animals of the bays and
sounds; (2) those of the estuaries and other
brackish waters; and (3) those of the cold
waters of the ocean shores and outer chan-
nels.

In each of these assemblages, VerriU and
Smith recognized that certain kinds of ani-
mals are restricted to particular localities
because of their relation to the character of
the bottom or of the shore. "Thus," they
say, "there will be species, or even large
groups of species, which inhabit only rocky
shores; . . . others that prefer the clean
gravelly bottoms where the water is several
fathoms deep." These may be still further
divided. The mud, for example, has differ-
ent characteristics in different places, and
"the different kinds are often inhabited by
different groups of animals." In describing
the animals that Uve in these habitats, they
report: "It has not been found desirable to
mention, in this part of the report [the gen-
eral discussion], all the species found in
each, but only those that appear to be most
abundant and important." They also knew

that the population during the day differed
from that found at night in the same spot
and that there were seasonal changes as
weU.

This somewhat extended report of VerriU
and Smith's work indicates correctly that
they were impressed with the organization
of communities upon the basis of their rela-
tion with their physical habitat rather than
as a result of interrelations between constit-
uent organisms. The latter were not im-
known to them, for, among other instances,
they state that "SheUs of oysters provide
suitable attachment for various shells, bry-
ozoans, ascidians, hydroids, sponges, etc.,
which could not otherwise maintain their
existence on muddy bottoms, while other
kinds of animals such as crabs, annehds,
etc., find shelter between the sheUs or in
their interstices." Thus VerriU and Smith
saw certain of the interrelationships that
exist on an oyster bank.

A few years later Mobius (1877) wrote
of these in greater detail; his much-quoted
passage wiU be repeated here (from the
1883 translation) both because of its his-
torical significance and because of its dis-
tinctly modem tone.

"Every oyster-bed is thus, to a certain de-
gree, a community of living beings, a coUection
of species and a massing of individuals, which
find here everything necessary for their growth
and continuance, such as suitable soil, sulficient
food, the requisite percentage of salt, and a
temperature favorable to their development.
Each species which lives here is represented
by the greatest number of individuals which
can grow to matiu-ity subject to the conditions
which surround them, for among all species
the number of individuals which arrive at ma-
turity at each breeding period is much smaller
than the number of germs produced at that
time. The total number of individuals of all
the species living together in any region is the
sum of the survivors of aU the germs which
have been produced at all past breeding or
brood periods; and this sum of matured germs
represents a certain quantum of Hfe which
enters into a certain number of individuals, and
which, as does all life, gains permanence by
means of transmission. Science possesses, as yet,
no word by which such a community of living
beings may be designated; no word for a com-
munity where the sum of species and individ-
uals, beings mutually limited and selected un-
der the average external conditions of life,
have, by means of transmission continued in
possession of a certain definite territory. I pro-
pose the word Biocoenosis for such a com-

36

THE HISTORY OF ECOLOGY

munity. Any change in any of the relative fac-
tors of a bioconose produces changes in other
factors of the same. If, at any time, one of the
external conditions of hfe should deviate for a
long tune from the ordinary mean, the entire
bioconose, or community, would be trans-
formed. It would also be transformed, if the
number of individuals of a particular species
increased or diminished through the instru-
mentahty of man, or if one species entirely
disappeared from, or a new species entered
into, the community."

S. A. Forbes (1887) apparently took over
and expanded the ideas of Mbbius. The
quotation already given (p. 32) shows that
Forbes recognized a "close community of
interest" even between predators and prey
in a community. Warming (1895) saw the
unity of plant communities as a result of
his study of the vegetation of Danish dunes.
Braun-Blanquet, disregarding the zoological
studies we have just reviewed, ranks Warm-
ing's work as the most important landmark
in the development of community ecology
since that of Heer. In one important re-
spect, this estimate is just: modern com-
munity studies have mainly been stimulated
by Warming's findings rather than by those
of his zoological predecessors, Edward
Forbes, Verrill, Mobius, and S. A. Forbes.

Communities may be integrated by the
requirements imposed by a uniform, cir-
cumscribed habitat as well as by the mutual
uiteractions between organisms such as
those that characterize a biocoenosis. The
two kinds of integration do not necessarily
yield similar results. Caves furnish one of
the striking examples of a unity imposed by
the habitat. Interest in cave hfe was strong
in the Darwinian period of the last century.
Attention was focussed particularly on the
origin and evolution of cave faunas. This
involved a consideration of adaptations,
especially those of sense organs, the migra-
tion of preadapted animals into caves, the
degeneration of eyes and other features, and
the conditions of existence to be met there.
Food habits of cave animals, including what
we now call food chains, and ultimate
sources of food were also studied. Absolon
in Europe, and Packard and Eigenmann in
America, engaged in such investigations.
The summaries of progress to date and
bibhographies by Packard (1888, 1894)
indicate that a fair knowledge of the gen-
eral relations of cave animals had been
attained by the closing years of the nine-
teenth century. Active work along the

same Hues continued into the new century
(see p. 48) and will be critically dis-
cussed in the section on Evolution.

Quantitative studies of the plants and
animals of a given community appear to
date from the work Hensen began in 1882,
the results of which were published in the
latter part of 1887. Hensen was primarily
interested in two questions: (1) What
quantities of hving plankton organisms does
the sea contain in a given area at a certain
time? And (2) how does the quantity of
plankton vary from place to place and
from time to time? He attempted to find
answers to these questions by collecting
plankton quantitatively by means of small-
meshed nets drawn through a known vol-
ume of water.

A large and critical Hterature soon de-
veloped, much too voluminous and compU-
cated for us to review thoroughly. An early
summary is given by Johnstone (1908),
and some of the more important papers
are fisted by Adams (1913) in his excel-
lent annotated bibhographies.

Hensen's work at once stirred up con-
troversy. Haeckel (1890) doubted the
vafidity of Hensen's conclusions in a mem-
oir done in his usual attractive style, to
which Hensen (1891) repfied effectively.
Kofoid (1897), though also engaging in
quantitative studies, dissented from Hen-
sen's conclusions, and Lohmann (1901)
undertook to show that Kofoid had not
understood the nature of the method he
criticized. Kofoid (1903) gave an excellent
and detailed report on a quantitative study
of the plankton of the Ilfinois River. In
fact, quantitative as well as quafitative
plankton studies flourished to such an ex-
tent that Shelford used to warn his classes
in the early years of the present century
that ecology was not a synonym for plank-
ton study.

Quantitative methods were soon appfied
to the investigation of communities of the
inshore bottom of the ocean by Petersen
(1893 and later) and to those of the land
by Pound and Clements (1898), Dahl
(1898), and others.

HYDROBIOLOGY

Discussion of the rise of self-conscious
ecology will be delayed only for a brief
further consideration of the development
of hydrobiology or, more exactly, of its

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

37

components, oceanography and limnology.
These subjects are concerned with all mat-
ters that apply closely to oceans, bays, gulfs,
and seas on the one hand, and to inland
waters, especially lakes, ponds, and streams
of fresh water on the other. Forel (1892)
called oceanography and limnology sister
subjects, and such they remain, with a
close family resemblance, but without hav-
ing fused into a unified science.

In so far as oceanography and hmnology
deal with organisms in relation to their
aquatic environment, or with bodies of
water as an environment of living things,
they are a part of ecology. In so far as
these subjects are concerned with physical
or chemical features such as depth, waves,
currents or types of bottom, or with the
chemical composition of the water, as items
of interest in themselves, they have a rela-
tion to ecology similar to that of soil science
or physiography on land or of meteorology
for the world in general.

The history of the earhest knowledge
concerning animal life in water coincides
with much of the early development of
biology in general, and its relation to the
early history of ecology has already been
traced (p. 14 ff). Attention was focussed on
the larger aquatic animals, especially on
the fishes of relatively shallow waters. The
gradual accumulation of information re-
garding these animals in relation to their
surroundings came mainly from the expand-
ing lore of the fisherman. Larger aspects of
oceanography, and to some extent of lim-
nology, too, were developed from the needs
of navigation.

Study of the smaller organisms in water
dates from Leeuwenhoek's improvement of
the microscope (1632-1723). He himself
discovered rotifers and Protista. During the
century and more immediately after Leeu-
wenhoek a motley assortment of men with
diverse backgrounds devoted themselves to
the study of the taxonomy and natural his-
tory of small aquatic organisms. Many of
these students of aquatic microscopy seem
to have been curious about the Infusoria,
much as we are today about aquatic
bacteria.

This exploratory period reached a note-
worthy stage in the work of Ehrenberg,
who, among his other contributions, began
a transition to aspects of microbiology more
closely related to modem interests. Murray
(1895, p. 77) says of him:

"In 1836 Ehrenberg produced his first
works." His name will remain inseparably
connected with the discoveries relating to the
microscopic organisms of the sea. . . . One
salient point may be dwelt on, viz., the con-
nection he established between certain classes
of living microscopic organisms and the part
they played in geological times. . . . His ob-
servations exercised a great influence on the
study of micro-organisms, whose role in nature
is in an inverse ratio to their size."

Johannes Miiller started the next ad-
vance when, about 1845, he began to use
a tow net to obtain samples of small marine
organisms from the North Sea. It remained
for Lilljeborg and Sars to recognize for the
first time the existence of a pelagic fauna.
Needham and Lloyd (1916) make the fol-
lowing comment concerning this discovery:

"Lilljeborg and Sars . . . found a whole
fauna and flora, mostly microscopic— a well
adjusted society of organisms, vidth its produc-
ing class of synthetic [sic] plant forms and its
consuming class of animals; and among the
animals, all the usual social groups, herbivors
and camivors, parasites and scavengers. Later,
this assemblage of minute free-swimming or-
ganisms was named plancton. After its discov-
ery the seas could no longer be regarded as
'barren wastes of water;' for they had been
found teeming with life."

Lohmann (1912, p. 22) states that dur-
ing the 1840's Ehrenberg, the Enghsh bot-
anist Hooker, and the Danish naturalist
Orstedt, taken together, recognized the
role of diatoms and desmids in the nutrition
of marine animals. They also found that
these plants and the radiolarian protozoans
are important in the formation of deposits
on the ocean floor (cf. Coker, 1947)

Lamport (1910) cites numerous papers
by each of these pioneers, the earliest of
which was published by Lilljeborg in 1853.
Hensen (1887) proposed the modem term
"plankton" for this assemblage of floating
organisms; his development of quantitative
plankton studies has already been discussed
(p. 36).

OCEANOGRAPHYt

According to Edward Forbes (1844), the
naturalist's dredge is a modification of the

* Ehrenberg had actually published in 1830
and 1832.

f More detailed discussion of the history of
oceanography is given by Murray (1895),
Murray and Hjort (1912), Herdman (1923),
and Coker (1947).

38

THE HISTORY OF ECOLOGY

fisherman's oyster dredge and was first used
in biological research by the Italians, Mar-
siU and Donati, and after them by Soldani,
about the middle of the eighteenth century.
These men "sought to explain the arrange-
ment and disposition of organic remains in
the strata of their country by an examina-
tion of the distribution of Hving beings on
the bed of the Adriatic Sea." The dredge
was introduced in more northern waters by
a Dane, O. F. Miiller, in 1799 as a means
for general exploration of the sea bottom
(Herdman, 1923).

Reports on the presence of animals in the
bottom deposits of the deeper waters of
the ocean appear to date from the records
of Sir John Ross (1819), who reported on
four deep-sea "soundings" made during his
voyage to Baffin's Bay in 1817-18. Samples
were obtained with a device of his own
invention that brought up a quantity of the
bottom deposits. Worms were taken at
depths of 6000 feet, and both worms and
other forms were secured from depths of
2700 feet and more. He also found a star-
fish attached to his line at least 2400 feet
below the surface. A few years later Risso
(1826) described a "bathybial" fish fauna
that extended to 350 fathoms (2100 feet)
in the Gulf of Genoa. Such information did
not become widely distributed, since the
announcement by James Clark Ross (1847)
of animals taken at a depth of 2400 feet
and even at 6000 feet during his Antarctic
expedition of 1839-40 was hailed as a new
and important discovery.

In 1839 the British Association for the
Advancement of Science appointed a com-
mittee to encourage dredging operations.
Edward Forbes was a leading spirit. His
"provinces of depth" have already been
outlined (p. 34)). Among the other con-
clusions given by Forbes (1844), the fol-
lowing are pertinent here:

"The number of species is much less in the
lower zones than in the upper. Vegetables dis-
appear below a certain depth, and the diminu-
tion in the number of animal species indicates
a zero not far distant. . . .

"The greater part of the sea is far deeper
than the point zero; consequently, the greater
part of deposits forming, will be void of or-
ganic remains.

"Animals having the greatest ranges in depth
have usually a great geographical, or else a
great geological range, or both."

The conclusion concerning the existence
of a depth zero of Hfe became a matter of
controversy. Often the zero point was lo-
cated at about 300 fathoms (1800 feet),
and, as we have akeady seen, it was dis-
credited as a generahzation for animal hfe
before it was first announced. This did not
prevent the matter from becoming a focal
point for exploration of the deeper waters
of the oceans. Mistaken observations or in-
terpretations, if not overweighted with au-
thority, may be stimulating. A dramatic
history of scientific progress could be writ-
ten in terms of known human errors and
their final correction. The existence of a
universal azoic zone was not disproved until
the dredgings of the Challenger expedition
(1873-76) brought up bottom-dwelling ani-
mals from the greatest depths reached. For
plankton, as we shall see, the doctrine
lingered still longer.

Many factors contributed to a strong
movement for oceanographic research from
the 1830's to 1900 and beyond. This was
the great era of oceanographic expeditions,
motivated in part by the kind of general
scientific curiosity that provides support for
astronomical observatories. A recurrent
specific curiosity that runs through much
of the history we are tracing focusses on the
relation between present day submarine de-
posits and the fossiliferous strata in ter-
restrial rocks. These more abstract interests
were reenforced by the need for practical
information in connection \vith laying and
maintaining transoceanic cables, by the
continued and gro\ving interest in fisheries,
and in the problems concerned with naviga-
tion. After certain initial success, there was
added the drive of strong nationalistic com-
petition, shared by most of the great mari-
time nations.

Among the most prominent of the natu-
ralists closely connected with expeditions
wholly or in part concerned with oceanog-
raphv, we mav name Charles Darwin on
the Beasle (1831-36). J- D- Dana on the
Porpoise (1836-39), Joseph Hooker with
the Erehus and Terror (1839-43) and
T. H. Huxley on the Rattlesnake (1846-
50). This incomplete list serves to call at-
tenb'on to the high quality of men who,
early in their scientific careers, were ex-
posed to the opportunities for work and re-
flection afforded by such expeditions. Ex-
perience gained on these voyages left a

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

39

strong mark on the later thinking of these
men, and their own high (juality exerted a
profound influence on the further develop-
ment of biological oceanography.

M. F. Maury, an important pioneer in
oceanic research, especially as concerns the
meteorological problems of navigation, was
also interested in marine biology. He pub-
lished the first bathymetrical map of the
North Atlantic in the 1854 edition of his
book. Explanations and Sailing Directions to
Accompany the Wind and Current Charts.
In this map he drew contour lines for 1000,
2000, 3000 and 4000 fathoms. He cor-
rectly thought that most of the bottom de-
posits away from land came from the skele-
tons of animals that five near the sea sur-
face, but was mistaken in thinking that the
conditions in the deep sea made hfe im-
possible in ocean depths. A paragraph from
his writings will give some of his reasoning
(1858, p. 174):

"Does any portion of the shells which
Brooke's sounding rod brings up from the bot-
tom of the deep sea live there; or are they all
the remains of those that lived near the surface
in the light and heat of sun, and were buried
at the bottom of the deep after death? . . .
The facts, as far as they go, seem to favor the
one conjecture nearly as well as the other.
Under these circumstances I am inclined,
however, to the anti-biotic hypothesis, and
chiefly because it would seem to conform bet-
ter with the Mosaic account of creation. The
sun and the moon were set in the firmament
before the waters were commanded to bring
forth the living creature; and hence we infer
that light and heat are necessary to the creation
and preservation of marine life, and since the
light and heat of the sun cannot reach to the
bottom of the deep sea, my own conclusion, in
the absence of positive evidence upon the sub-
ject, has been the habitat of these mites of
things hauled up from the bottom of the great
deep is at and near the surface. On the con-
trary, others maintain, and perhaps with equal
reason, the biotic side of the question. Profes-
sor Ehrenberg, of Berhn, is of this latter class."

Maury then gives an exchange of letters be-
tween Ehrenberg and himself in which the
pros and cons of the matter are stated
fairly and without heat.

G. C. WalUch, naturahst on the Bulldog,
summarized the opposite point of view in
1862 in statements that Murray thought
sufficiently significant to quote in the his-
torical pages of his summary for the Chal-

lenger reports (1895, p. 95). He begins the
list with the assertion that

"The conditions prevailing at great depths,
although differing materially from those which
pre\ail near the surface of the ocean, are not
incompatible with the maintenance of animal
life"

and concludes that

"The discovery of even a single species liv-
ing normally at great depths warrants the in-
ference that the deep sea has its own special
fauna, and that it has always had it in ages
past, and hence that many fossiliferous strata,
heretofore regarded as having been deposited
in comparatively shallow water, have been de-
posited at great depths "

Herdman (1923) devoted separate chap-
ters to the following men as founders of
oceanography: Edward Forbes, Wyville
Thompson, John Murray, Louis and Alex-
ander Agassiz, Albert Honore Charles,
Prince of Monaco, and Anton Dohrn of the
Zoological Station at Naples. The work of
Edward Forbes has already been discussed,
and Murray has been repeatedly mentioned.

Thompson was the active leader of the
Challenger expedition (1873-76), the ob-
ject of which was the scientific exploration
of the sea with regard to physical, chem-
ical, geological, and biological conditions.
The scientific results were pubHshed in fifty
large quarto volumes, prepared mainly un-
der the editorship of John Murray, himself
one of the naturahsts of the expedition. The
reports were written by notable speciahsts;
Murray later singled out the work of
Haeckel on the Radiolaria as being espe-
cially outstanding. It is difficult even yet to
evaluate the full importance of the contri-
butions made by this great voyage of
oceanographic exploration. The reports re-
main a half-forgotten mine of information.

Among his many other activities, Louis
Agassiz made dredgings and soundings off
the coast of Florida and came to some sig-
nificant conclusions on the permanence of
the ocean basins. This matter is still the
center of a warm controversy, and a quo-
tation from Agassiz (1869, p. 368) is help-
ful in giving historical perspective:

"From what I have seen of the deep-sea
bottom, I am already led to infer that among
the rocks forming the bulk of the stratified
crust of our globe, from the oldest to the
youngest formation, there are probably none

40

THE HISTORY OF ECOLOGY

which have been formed in very deep vi^aters.
If this be so, we shall have to admit that the
areas now respectively occupied by our conti-
nents, as circumscribed by the two hundred
fathom curve or thereabout, and the oceans,
at greater depth, have from the beginning
retained their relative outhne and position;
the continents having at aU times been areas
of gradual upheaval with comparatively shght
oscillations of rise and subsidence, and the
oceans at aU times areas of gradual depres-
sion with equally shght oscillations."

Alexander Agassiz, son of Louis, is much
more closely identified with oceanographic
expeditions and with oceanography in gen-
eral than is his more famous father. His
work is associated with the cruises of the
Blake and the Albatross. His active con-
nection with oceanography extended from
1877 to 1905 and included both general
exploration by dredges and nets and much
study of the coral reef problem. The con-
clusions reached by Alexander Agassiz con-
cerning the origin of coral reefs were di-
rectly opposed to the subsidence theory of
Charles Darwin. After a great deal of
search, the younger Agassiz could not find
an atoll or barrier reef the formation of
which, he thought, could be adequately
explained by Darwin's subsidence theory.
He also concluded as a result of extensive
dredging that the benthic animals of the
Caribbean Sea are more closely related to
the deep-sea animals of the Gulf of Panama
than to those of the deep Atlantic, a con-
clusion that has stood the test of time to
date. His book (1888) deserves especial
mention.

As a result of working with a tow net
that could be opened and closed under
water at any depth, Alexander Agassiz
modified somewhat the old idea of an azoic
depth zone. He thought that there were
practically no plankton organisms in the
vast intermediate waters of the ocean below
a depth of about 200 fathoms until one
came near the bottom. Murray and others
disagreed, and on this note of friendly dif-
ference of opinion the nineteenth century
closed with the azoic zone problem consid-
erably modified, but still alive. We may
properly overstep the time limit for the
present chapter and bring this particular
matter down to 1934 by a quotation from
Krogh (p. 430) :

"The number and total mass of organisms
decreases very rapidly with the depth. This has

been estabhshed again and again both for net-
plankton and for nannoplankton organisms and
is well illustrated by the figures given by
Hentschel for the number of nannoplankton
organisms present in 1 liter of ocean water in
the area 0-10° S and 10-20° W in the At-
lantic: Surface, 10,100; 50m., 9400; 100m.,
2700; 400m., 260; 1000m., 90; 2000m., 50;
3000m., 18; 5000m., 15."

While the plankton population is much re-
duced, there is no completely azoic region
indicated by these data.

Herdman (1923, p. Ill) quotes John
Murray's estimate of Alexander Agassiz's
influence on oceanography as follows:

"If we can say that we now know the
physical and biological conditions of the great
ocean basins in their broad general outhne— and
I believe we can do so— the present state of our
knowledge is due to the combined work and
observations of a great many men belonging to
many nationalities, but most probably more
to the work and inspiration of Alexander
Agassiz than to any other single man."

This estimate, which has the approval of
two excellent students of the subject, may
help rescue the son from the comparative
obscurity produced by the shadow of his
father. Alexander Agassiz's last studies and
his last expedition in the Albatross came in
the early years of the present century;
hence we have reached the end of the
period to be covered in the present chapter.
The ecological problems of the ocean had
been outlined before 1900, and many of
them were well advanced toward solution.
With some notable exceptions, such as
Mobius' recognition of the oyster bed as a
biocoenosis, the possible ecological impHca-
tions of these studies had not been
emphasized.

LIMNOLOGY*

The development of limnology lagged be-
hind that of oceanography, as shown by
the fact that Forel (1892), in the first vol-
ume of his monograph on Le Leman (Lake
Geneva, Switzerland), defined hmnology
as the oceanography of lakes. Despite much
good work on the taxonomy and natural
history of fresh-water organisms, it re-
mained for P. E. Miiller (1870), a Dane,
to recognize the existence of a pelagic
planktonic fauna in lakes, such as Lilljeborg

• Short historical sketches of limnology are
given by Lampert (1910) and Welch (1935).

ECOLOGICAL BACKGROUND AND GROWTH BEFORE 1900

41

and Sars had found in the Baltic Sea (p.
37), This advance was based on a trip to
the Swiss lakes in 1868. Beginning analyses
of physical conditions in lakes preceded
Miiller's announcement. Simony was a
pioneer in such studies; as early as 1850
he had reported in some detail concerning
thermal stratification in lakes.

Forel is regarded as the founder of lim-
nology, not because his work was chronolog-
ically first, but because of its long-contin-
ued significance. His paper of 1869 dealing
with the bottom fauna of Le Leman, though
not his initial publication, set the stage for
his Hfe work. His prolonged study of Swiss
lakes reached a peak with the appearance
of the three successive volumes of his mon-
ograph Le Leman (1892, 1895, 1904).
Forel's generalizations, in the form of the
first comprehensive discussion of limnol-
ogy, were published just after the close of
the period covered by the present chapter
and are specifically noted in the following
one (p. 47).

The contributions of Forel include the
first demonstration of a deep-water com-
munity in lakes, the setting up of the first
complete limnological plan for the study
of a lake, and, what is more important, its
practical realization. Welch, in the index
to his 1935 textbook of limnology, cites
the work of only three men more frequent-
ly than that of Forel: Juday, Birge, and
Shelford, in that order.

Lampert's summary (1910, p. 13) of
Forel's historical status in limnology gives
some interesting comparisons. In free
translation he says; Without reducing the
merit of the lesser investigators, who like
Forel recognized the significance of sys-
tematic fresh-water research and of whom
especially Weismann [August Weismann of
germ plasm fame], Du Plessis-Gouret, and
Fritsch must be mentioned, we may still
date the beginnings of limnology as a
science from Forel's 1869 paper.

Weismann's contributions to limnology
began in 1877. Du Plessis-Gouret, who had
already published jointly with Forel, wrote
in 1885 of the profundal fauna of Swiss
lakes, and Anton Fritsch, among other con-
tributions, established in 1888 the first
fresh-water biological station. This was a
portable laboratory with at first some 12
square meters of floor space. The laboratory
was set up on the shores of three different
lakes in the Bohemian Forest before 1899."

The dredging operations in Lake Su-
perior, made by S. I. Smith in 1871 and
reported at length in 1874, deserve men-
tion. He apphed to Lake Superior many
of the methods used by Verrill and Smith
in their work on the invertebrate life of
Vineyard Sound (p. 35) and reports,
among other data, a table showing the
bathymetrical distribution of the species
taken. This promising opening of limnolog-
ical studies on the Great Lakes has not
yet been adequately developed.

Limnological work, once begun, flour-
ished greatly in Europe and on the smaller
lakes and rivers in the United States. Such
investigations were in full swing in the
last decade of the nineteenth century.
Early quantitative studies in this field
have already been discussed. Kofoid's in-
vestigations of plankton in the IlHnois River
(1903) were carried on from 1894 to 1899
and again deserve mention.

A number of comprehensive bibliogra-
phies of limnological work have appeared,
two notable ones before 1900. Lampert's
first edition of his Das Leben der Binnen-
gewdsser (1899) contained a fairly com-
prehensive bibliography. In the same year
there appeared a workman-like review by
H. B. Ward of advances during the years
from 1893 to 1898. This review contains
a bibliography of thirty-eight closely
printed pages of citations to work pubhshed
during this brief interval. Its pages remind
us that the relict fauna of Tanganyika and
of Baikal were being studied, as were also
problems concerning the origin and dis-
persal of fresh-water animals. Cave life was
receiving attention, and Ward states (1899,
p. 332) that "Lorenzi, Packard, and Len-
denfeld have given summaries of our
knowledge regarding cave animals with
frequent references both morphological and
ecological \sic'\ to the freshwater fauna of
such localities."

From this bibliography of Ward's we find
that the veterans were busy during the
half-decade under consideration. They are
represented by men like Sars and Forel.
Many of the stalwarts of twentieth century
limnology had also begun work. Birge was

" Fritsch's "portable laboratory" was made in
eighty sections so that it could be dismantled
in an hour and a half^ moved to another lake
and set up again in two and a half hours. It
weighed about 1000 kg. (personal communica-
tion from Chancey Juday).

42

THE HISTORY OF ECOLOGY

writing about Cladocera, about limnetic
Crustacea of Lake Mendota, and about
the relation of areas of inland lakes and
the temperature of the water. Juday, who
had not yet established his productive
scientific partnership with Birge, reported
in 1896 on the plankton of Turkey Lake
in Indiana. Reighard of Michigan; Wes-
enberg-Lund, student of Danish lakes;
Zschokke, who studied Alpine lakes of
Switzerland; and Apstein, prominent for his
work on the plankton of the Holstein lakes,
are all cited by Ward. Zacharias, founder
of the enduring biological station at Plon,
Germany, was especially prolific during
these years of the 1890's, while Whipple,
and Ward himself, contributed extensively.
The development of limnology, far from
being at the end of a period, was in full
and active growth in 1900. Limnology had
already made direct contact with ecology,
notably in Forbes' essay The Lake as a
Microcosm. Although the subjects had by
no means fused, the development of mod-
em, self-conscious ecology owes much to
the groundwork laid by the pioneers in lim-
nology and oceanography, that is, to the
sound development of knowledge concern-
ing hydrobiology before 1900.

THE RISE OF SELF-CONSCIOUS ECOLOGY

The foregoing pages give in some detail
samples of the substrata on which self-
conscious ecology developed. Certain of the
persons mentioned were directly important
in the early growth of the subject in the
strict sense; many were not. It is customary
to begin the schematized textbook sketches
of the history of ecology with the \vritings
of Buffon, who lived from 1707 to 1788 and
emphasized, among many other interests,
the interrelations of organisms. Saint Hilaire
(1859) clearly outlined the scope of such
relationships under the name of "ethology,"
which he conceived of as including "the
study of the relations of the organism with-
in the family and society in the aggregate
and in the community." John Stuart Mill
(1848) in his Lo^ic antedated St. Hilaire
in using the word "ethology," by which he
meant the science of human character. It
has been argued that since the character
of an organism is revealed only through its
reaction to the environment, there is no
essential difference between human and
other aspects of "ethology."

Haeckel (1869) coined the term "Oekol-
ogie," from which the modern "ecology"
has been derived. He defined the content
of his Oekologie as "comprising the relation
of the animal to its organic as well as its in-
organic environment, particularly its friend-
ly or hostile relations to those animals or
plants with which it comes in contact."
Semper (1881) distinguished between the
physiology of organs and that of organisms;
the latter is concerned, he says, with the
"reciprocal relations which adjust the bal-
ance between the existence of any species
and the natural, external conditions of its
existence, in the widest sense of the term."

Lankester (1889) under the term bio-
nomics included a miscellany that contained
the lore of the hunter and herdsman, the
science of breeding, and the study of or-
ganic adaptation. A few other terms have
been suggested for these or related phases
of biology, but none is important, except
the tendency, which still continues, to des-
ignate much of ecology as "biology." We
read of the "biology" of a snail or of a "bio-
logical" survey, when the treatment is
mainly ecological. This usage is to be
deplored.

Subdivisions of the subject matter of
ecology began at an early date. Schroter
and Kirchner (1896, 1902) recognized the
ecological relations of the individual as
"autecology" and those of communities of
organisms as "synecology." As stated
earlier, Forbes (1895) formulated a defini-
tion of ecology and pointed out that eco-
nomic entomology is simply applied
ecology.

This, then, brings ecology and its fore-
runners approximately up to 1900. It is
clear that the field was ripe for further
development, a development that has pro-
ceeded with quickening pace. The situation
at that time is correctly summed up by
Pearse (1939) as follows: "At the begin-
ning of the twentieth century ecology was a
young, but an established, science, and
such eminent ecologists as Wasmann
(1901), Dahl (1901) and Wheeler (1902)
were discussing whether Saint-Hilaire's eth-
ology or Haeckel's ecology should be used
to designate the science of relations of or-
ganisms to environments."

Ecology was even more firmly established
as a special field of botany, for Cowles
(1901) began his important report on
physiographic ecology with the statement

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

43

that: "Within the last few years the sub-
ject of ecology has come to find a place
of more or less importance wherever botany
is studied in its general aspects." Cowles
indirectly documents his point by his Uter-
ature citations for 1896 to 1900.

The end of the nineteenth century is a
convenient, though not a logical, division
between the early history of ecology and
its more recent development. Unlike tlie
modern subject of genetics, which has de-
veloped mainly from the spectacular redis-
covery of Mendehan heredity in 1900, we
can now see that for ecology the years
connecting the centuries mark a time of
relatively smooth progress. Ecologists of the
early 1900's gave praise to Semper for his
recognition of the physiology of organisms
in relation to "natural conditions of exist-
ence," and researches in this field proceeded
steadily. Work on ecological aspects of ani-
mal behavior was active. Population studies

were moving at an increasing rate. Evolu-
tionary thought was in gradual transition,
with the theory of natural selection,
known by some even then to be largely eco-
logical, still holding the attention of biol-
ogists. Ideas concerning natural coopera-
tion were growing. Natural history had
passed its peak of activity in university
circles, but was directly and broadly re-
lated to the preceding years. The same is
true for oceanography; the related subject
of Hmnology was in the midst of a notable
advance. In self-conscious ecology, the
community concept had been clearly ex-
pressed, and there was active research in
animal and particularly plant ecology.
Scientific attention in general was focussed
on nonecological phases of biology, and
the science of ecology, now well and firmly
rooted, could continue to develop outside
the distorting influences often accompany-
ing high popularity.

3. FIRST FOUR DECADES OF THE TWENTIETH CENTURY

INTRODUCTION

At this point let us take stock of what
has already been said of the historical ante-
cedents and background of ecology. We
have covered in considerable detail some
2200 years of ecological history. From the
viewpoint of ecology, four general chron-
ological periods have been recognized:
(1) the contributions of the Greeks and
Romans; (2) the subsequent thousand or
so years of stagnation; (3) the develop-
ments of the sixteenth, seventeenth and
eighteenth centiu-ies that led into (4) the
nineteenth century studies. It has been sug-
gested that since the Renaissance the major
contributions to the growth of ecology oc-
curred along four channels: developmental
physiology, response physiology, relation
of species to their environment, and organic
evolution.

Enough of a background has been pre-
sented to show that ecology had multiple
origins. It was descended neither from a
single idea nor from isolated facts. The task
now confronting us is that of showing how
"modern" or twentieth century animal ecol-
ogy has come into being and how it is
practiced today. There are many ways of
approaching this problem. For our purposes

it seems best to adopt a chronological treat-
ment based roughly on the first four dec-
ades of the twentieth century. It will be
necessary, particularly in discussing the
later decades, to appreciate that even pres-
ent day ecology is not so clearly delimited
as are, for example, modern genetics or
many other biological disciplines. This
means that we are compelled to discuss and
consider certain borderfine fields. The point
is emphasized by examining the "Ecology"
section of a recent (1940) issue of Biolog-
ical Abstracts; the following subheadings
are listed: "General Animal Ecology;"
"General Plant Ecology;" "Hydiobiology"
(Oceanography, Limnology); "Ecology of
Wildhfe Management— Aquatic and Terres-
trial," and "BiocHmatology, Biometeorol-

ogy-"

It is advisable to discuss briefly certain
aspects of the history of plant ecology dur-
ing the twentieth century before attention
is focussed on animal ecology. Plant ecol-
ogy got off to a faster start at the turn of
the century. Thus, as will be shown later,
it had a great impact on the thinking and
research of certain pioneer animal ecolo-
gists. The development of plant ecology
has been reviewed by Conard (1939).

Our responsibihty is not to linger on

44

THE HISTORY OF ECOLOGY

plant ecology per se, but to appraise this
field as it has provided fact and catalyst for
zoological developments. Specific relation-
ships will be pointed out further on, but
these generalizations emerge:

1. The investigations of early plant ecol-
ogists were favored somewhat by the fact
that plants are essentially fixed geograph-
ically and not greatly subject to rapid
dispersal.

2. Plant ecology, naturally enough, de-
veloped regionally according to the local
resources that could be exploited and
studied.

3. Plant ecology gave an early and sig-
nificant orientation to animal ecology in
several ways: (a) It stressed the fact that
communities or complex natural popula-
tions exist over the face of the earth and
are subject to analysis. This gave a telling
impetus to animal synecology. (b) It crys-
tallized certain comprehensive ecological
concepts such as succession and thus sent
animal ecologists out into the field to see
if animals also furnished data to support
the concept, (c) It developed certain tech-
niques of field study that could be used
with but minor mocification by the zoolo-
gist, (d) It emphasized in an ecological
sense the fact that plants stand in an im-
portant relation to animals in terms of nutri-
tion, breeding, and shelter niches. And,
perhaps most important, (e) it gave psy-
chological stimulus around the turn of the
century by showing the zoologist that first-
rate botanists were investigating ecological
problems and getting results. In short, the
animal ecologist owes much to the plant
ecologist in a historical sense, and, on land,
he is still dependent on plant ecology for
much of his zoogeographic description.

Our task now is to discuss the growth of
twentieth century animal ecology. We find
that by dividing the years from 1900 to
1940 into their four component decades,
we can consider each of these decades both
as a unit and as an interrelated part of the
whole pattern. This is not a completely
arbitrary treatment. A case can be made
for the point that, during this span, ten
years seemed to be about the actual interval
for certain types of work to materialize and
certain ideas to be synthesized Thus, there
is nothing really difi^erent between, say,
the years 1910 and 1911 or 1930 and 1931,
but there does appear to be a real histor-
ical difiEerence between 1900 and 1910 or

1911 and 1920 in terms of the development
of animal ecology.

Our treatment varies somewhat according
to the individuaUty of the decade in ques-
tion, but in general we hope to ask, and
so far as possible to answer, the following
four questions for each:

1. What were the research focal points?

2. Who were some of the leaders in the
research fields discussed?

3. What was the historical impact of
the work of these men?

4. What grew out of the decade that
seemed significant?

The reader should keep in mind that the
absence of a favored name or citation in the
following pages does not necessarily signify
that it has been overlooked or deemed un-
important. It may mean just that, or, con-
trariwise, it may mean merely that there
is not enough space for its inclusion. It is
necessary to emphasize that in dealing
with the foregoing questions we are
sampling historical data, and that our
sample is not a random one, but is selected.
Accordingly, our cases are subject to bias,
as, for example, our overemphasis on Amer-
ican historical illustrations. From one point
of view this is poor technique with obvious
limitations. But from another aspect it is
sound, since it does permit us to present
our notions of what is significant and there-
by evaluate ecological history as we see it.
With these preliminaries we turn to the first
decade of the twentieth century.

1900-1910

During this period of ecological growth,
ecological investigations seem to have fallen
into the following categories: response phys-
iology, developmental and toleration physi-
ology, natural history, hydrobiology, suc-
cession, and general synecology. These did
not originate de novo with the turn of the
century. Most of them had antecedents in
earlier work, as we have shown.

Response physiology, or ecological as-
pects of behavior, was studied actively dur-
ing this period. Davenport's "Experimental
Morphology," the second edition of which
appeared in 1908, was still shaping ideas
and new researches. This was the period
when "trial and error" behavior was much
in the scientific headlines. Jenning's classic
Behavior of the Lower Organisms (1906)
had a firm impact on ecological thinking.
It showed that environmental stimuli, even

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

45

if of subtle character, could control an ani-
mal's orientation and pattern of movement.
Also, it had a si2;nificant influence on the
ihinkinci; of biologists generally. The first
edition of The Animal Mind by Washburn
(1908), to be followed by several further
editions, laid certain foundations for the
study of animal behavior.

There was much writing during the dec-
ade on the behavior of a single species,
"^his IS well typified by the study by Ray-
mond Pearl, whose excellent and original
monograph on the behavior of Planaria
summarized the state of things at that time
iT- these words (1903. p. 511) :

"... Aside from the researches of a few
investigators on a small number of forms, we
have little detailed knowledge of the behaviour
of lower organisms. It is coming to be realised,
too, that knowledge of what an animal does is
just as important in the general study of life
phenomena as a knowledge of how it is con-
structed, or how it develops."

There are also some writings on social
behavior. Wheeler's classical book on "Ants"
appeared in 1910 (reprinted. 1926) and,
through its emphasis on ant behavior, did
much to stimulate behavior studies on the
social insects and to provoke comparisons,
sometimes invidious, between insect and
human responses. A paper by Craig (1908)
on pigeons suggested that the vocalization
of these birds had some function in the
social control of the flock.

The field of developmental physiology
was equally active. It also received impetus
from Davenport's summary. Mention of sev-
eral studies wall suffice to show the nature
of the research of this period. Among
others, the works of Bachmetjew (1901,
1901a, 1907) stand out. He not only sum-
marized a wealth of literature, but pre-
sented as well many original observations
and interpretations. Bachmetjew was con-
cerned largely with the effect of light and
temperature on various phases of the de-
velopment and distribution of insects. Prob-
ably one of his more significant contribu-
tions was his summary of the eflFect of low
temperature on insect protoplasm. Chap-
man (1931, p. 61) states this in concise
form as follows: "The insect may be
cooled below the freezing point without
being injured. The freezing point may be
past, and the insect may exist in an under-
cooled condition. When it does freeze, the

heat of crystallization will be equal to the
undercooling temperature, and the body
temperature will rebound to the freezing
point. Cooling will again proceed; and
when the insect reaches the undercooling
point the second time, death follows, ac-
cording to Bachmet Jew's conception." More
modem views do not completely agree
with this interpretation, but in 1901 it was
an important pronouncement with cogent
ecological implications. Bachmetjew also
discoursed on light and temperature in rela-
tion to zoogeography.

Branching ofi^ from developmental physi-
ology is a phase of research that some ecol-
ogists designate "toleration physiology" or
"toleration ecoloey." In such work the con-
cern lies with the limits of toleration for
organisms exposed to various intensities of
environmental factors or combinations of
thesf factors. During the decade 1900 to
19 K'' there were some studies of this type,
and an example or so mav be cited. Pack-
ard (1905, p. 33) published a paper on
the efi^ect of low oxygen tension on sur-
vival of certain marine fishes and inverte-
brates of the Woods Hole (Massachusetts)
area. In addition, he showed that if the
blood alkalinity of Ftinduhis heteroclitus
was increased, there was a corresponding
increase in the tolerance of the fish to lack
of oxygen. Contrariwise, increasing the
acidity of the blood made the fish less tol-
erant of low oxygen tensions. Bachmetjew
(1907) recognized this general problem for
insects and published a list of extremes of
temperature that various insects have been
known to tolerate. Another such list ap-
peared in Davenport (1908).

Natural history has always been inextri-
cably interwoven with ecology. In fact,
ecology has been called "scientific natural
history." Much of the content of ecology
is natural history, and the ecologist usually
experiences a certain pleasure in observing
and recording the "history of nature." But
natural history is not a closely definable
entity. It may range from superficial and
even misleading nature study, to excellent,
precise investigation. Earlier in this section
we saw how this phase of ecology contrib-
uted to the rise of the science.

During the decade 1900 to 1910 ex-
amples can be chosen that rrm the gamut
of type. There were books such as that of
Chapman (1900), designed largely for the
nature student and amateur. It is hard to

46

THE HISTORY OF ECOLOGY

evaluate the influence of works like this on
ecological development. Then there were
books such as Seton's Life Histories of
Northern Animals (1909). These contrib-
uted much that was useful to the ecologist.
Seton's book combined a wealth of infor-
mation about life histories and habits with
an extensive bibliography. Von Neumayer
(1906) published his two volume compen-
dium on exploration. Adams (1913 p. 63)
says of this study: "A very important work,
particularly for the traveling naturalist.
Chapters by specialists, valuable references
on collecting natural history specimens, and
other phases of scientific exploration are in-
cluded." More technical natural history
studies of this period are typified by the
papers of Reighard (1903), Andrews
(1904), and Forbes (1907).

Reighard's paper, on the "Natural His-
tory of Amia calva," published as a tribute
to the Harvard zoologist Edward Laurens
Mark, is an excellent case in point. This
author, who worked for four seasons in the
millponds of the Huron River, records a
wealth of careful observation about this
fish. He discusses such aspects as second-
ary sexual characters, habits not peculiar to
the breeding season, nest-building, guarding
of the empty nests by males, guarding the
eggs, protective colors of males, history of
the eggs and young in the nest, history of
the young outside the nest, and the be-
havior of the male while with the school. In
a historical chapter one need hardly make
the point that sound data such as these,
multiplied many times to include many dif-
ferent animals, are of profound importance
both during the decade of their publication
and for years afterwards as well. Reighard's
paper on Amia drives the point home!

The paper of E. A. Andrews on the
breeding habits of Cambarus affinis was as
thorough a study of an arthropod as that
just described was for a vertebrate. An-
drews covered much the same sort of
observation as did Rei8;hard. In addition,
he added some simple biometric linear
measurements of the whole animal and cer-
tain of its parts that did much to embellish
his work. Biometrv was already making its
influence felt on ecologv and natural his-
torv. In England, Karl Pearson was in the
midst of his dynamic career, and in Amer-
ica. Ravmond Pearl and C. B. Davenport,
to be followed soon bv J. A. Harris, were
applying statistical methods to many kinds

of data. Studies such as these had the
vital importance of forcing ecologists to
think in a more analytical fashion about
group characteristics.

The latter point is even better made by
looking at two 1907 papers of S. A. Forbes.
This excellent naturalist of ^Vheeler's "com
and saleratus" belt did much for ecology
starting with his estimable essay. The Lake
as a Microcosm (p. 36). In the 1907 stud-
ies Forbes discussed the local distribution
of Illinois fishes and the bird population of
Illinois in autumn. In both papers the
reader detects careful observation, apprecia-
tion of the natural history of the forms
studied and an insistence that numbers
as well as names should be listed. In the
fish paper Forbes (1907a) develops what
he calls the "Coefficient of Association," de-
signed to show the frequency with which
one species is found associated with another
in nature. This statement took the following
form:

C.A. =

ad

be

where a equals the total number of collec-
tions to be used in the computations; b, the
number of collections containing the more
abundant of two species to be compared
with another; c, the number of collections
containing the less abundant of these
species, and d, the number of collections
each of which actually contains both species
together. Despite the fact that Forbes'
coefficient is imperfect and is not used by
modem workers, it did serve the important
function of stating a real problem and sug-
gesting a solution.

In a verv real sense, htfdrobioJogy (both
oceanography and limnology) has devel-
oped as a subscience in its owti right. How-
ever, since we shall be referring continually
in this book to ecological principles derived
from the data of hydrobiology, and since its
early historical development is one and the
same with ecology proper, we must examine
its contribution to our historical analysis.
During the decade 1900 to 1910 many in-
vestigations of aquatic ecology were
published. We shall sample a few repre-
sentative studies.

From the marine aspect Johnstone's book
(1908) and the papers of Ostenfeld (1908)
and Sumner (1910) are characteristic.
Johnstone's book was a competent summary
of modem oceanography. In the first part

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

47

he discussed the North Atlantic ocean,
types of life in the sea, including notes on
fishes and fishing; in the second part he
stressed the quantitative method as applied
to plankton census and productivity; and in
the last part he dealt with the "metabolism
of the sea"— food relationships, bacteria, and
nitrogen circulation. Even to a modern
worker the book is a sound contribution. It
is safe to assume that its impact on aquatic
ecology was considerable.

Ostenfeld's paper was important, since it
showed clearly "... the controlling rela-
tion of marine vegetation upon animal
associations and particularly the fish of the
coast of Denmark" (Adams, 1913, p. 89).
Work of this type indubitably helped to
draw together plant and animal ecology.
Sumner's paper is an excellent example of
a certain type of field study. The bottom
fauna and flora of an area around Woods
Hole, Massachusetts (namely. Buzzards
Bay and Vineyard Sound), were studied in
relation to temperature, character of bot-
tom, depth, saUnity, and density. The local
distribution of each species was carefully
determined and mapped. Conclusions were
drawn as to wliich factors were most im-
portant in shaping the observed distribu-
tions. In addition, the author formulated
some opinion about the geographical origin
of the fauna of the region.

Fresh-water ecologists or hmnologists also
were making rapid strides during the first
decade of the twentieth century. This pe-
riod prospered under the influence of F. A.
Forel (1841-1912), a professor in the Uni-
versity of Lausanne, who has been called
the "founder of modern limnology." In 1901
Forel published his Handbuch der Seen-
kunde. Allgemeine Limnologie. The impor-
tance of this volume is well indicated by
Welch (1935, p. 5) in these words: "This
book is the first general presentation of
limnology from the modern standpoint. In
fact, it might well be termed the first text-
book of limnology. In brief, hmnology is in-
debted to Forel for the first knowledge con-
cerning the profundal fauna of fresh-water
lakes, for the first program for limnological
investigations of such waters, and for the
execution of such a program, resulting in
'Le Leman,' which was long a model for
subsequent work."

A first-rate paper by Kofoid (1903) on
the plankton of the Illinois river was a de-
tailed, meticulous study with a definitely

ecological point of view. In 1904 E. A.
Birge published a paper in which he clearly
demonstrated thermal stratification in in-
land lakes and formulated a standard
method of expressing it. In a historical dis-
cussion one is tempted to pause over the
names of Birge and his colleague Juday to
pay tribute to their cogent contributions to
aquatic biology. Another book that seems
to have been important during this decade
was that by Knauthe (1907).

There is some point in dealing specifically
with ecological succession. This was the era
when plant ecologists were interested in the
phenomenon. The animal ecologist was
starting his investigations on succession, to
be followed actively in the next ten years.
Cowles published his "Sand Dunes" paper
in 1899. This stimulated the zoologists
V. E. Shelford and C. C. Adams, who were
ecologically inclined from their association
with Davenport at the University of
Chicago, to examine the concept from a
zoological aspect. In 1907 Shelford reported
on the succession of tiger beetles {Cicin-
dela) in the same dunes region where Cow-
les had studied. He " . . . traced the rela-
tion of Cicindela to the succession of plant
communities. The distribution of eight spe-
cies of tiger beetles was in close correspond-
ence with the zoned habitats and communi-
ties, and the conclusion was reached that a
similar harmony existed with respect to the
fauna in general" (Clements and Shelford,
1939, p. 8).

Adams' 1909 paper shows even more
respect for the concept of succession
than does Shelford's. It starts with this in-
teresting quotation from John Stuart Mill:

"Of all truths relating to phenomena, the
most valuable to us are those which relate to
their order of succession. On a knowledge of
these is founded every reasonable anticipation
of future facts, and whatever power we pos-
sess of influencing those facts to our
advantage."

Adams reviews much of the background for
ecological succession current at that time.
He discusses general principles as well as
specific avian illustrations. From his studies
of the latter he reaches this conclusion (p.
134):

"... Bird succession means a change from
the dominance of certain species or associa-
tions to that of others. Thus in the beginning
a slight change in abundance of a species may

48

THE HISTORY OF ECOLOGY

be noted, with a corresponding decrease in
another; and this proportion may continue to
change until the intruder becomes dominant
and the rival form may disappear entirely. The
process of change, as a rule, is not limited to
a single species, but usually involves several or
all of the members of the association, as when
a dune invades a swamp and tlie swamp birds
are completely replaced by those frequenting
the sand dunes."

Later we shall have more to say of the im-
pact of succession on the rise of ecology.

The term "synecology" apparently was
coined by the botanists Scliroter and Kirch-
ner in 1902 from the Greek prefix syn,
meaning "together." Since that time ecolo-
gists have used synecology in a general
sense to imply the association of individuals
in contradistinction to the ecology of an iso-
lated organism ("autecology").* There
have been attempts to define the term with
more precision. Thus there is the definition
of Turesson "the ecology of communities;"
of Riibel, "the relation between the commu-
nity and its habitat;" of Braun-Blanquet,
"the study of the dependence of commimi-
ties upon one another and upon the environ-
ment," and of the Third International Bo-
tanical Congress, "the study of conditions
of the environment and adaptation of spe-
cies taken in association." For our present
purposes we shall use synecology in a broad
fashion only and select several early twen-
tieth century studies that depict the state
of the science at that time. Obviously, many
of the papers aheady reviewed are syne-
cological in part, but a few cases per se are
in order.

In 1903 Davenport published a paper on
the ecology of a Cold Spring Harbor (New
York) sand spit. This was a sofid study that
stressed the local distribution of animals
with respect to local habitat zones. The spit
was divided into two areas, the periodically
submerged zone and the beach zone, and
the fauna of these two was studied. Daven-
port stressed those adaptations of the fauna
particularly adjusted to these two niches.
Another representative study was that of
Ruthven (1906) on an ecological survey of
the Porcupine Mountains in Michigan. This
was interesting in that the author placed
the faunas in a framework of biotic associa-
tions and, as Adams puts it, "treated them

• Autecology is frequently used to mean the
environmental relations of a single species in-
stead of a single individual. It is not so used in
this book.

from the dynamic and genetic standpoint."
The monograph of Eigenmann (1909) on
"Cave Vertebrates of America" deserves
mention here. Although this work has not
stood the test of time so far as its interpre-
tations are concerned, it did serve a real
function in placing on record many data
on the adjustment between cave forms and
their habitats and the phylogenetic regres-
sion associated with that adjustment.

Under the heading of quantitative syn-
ecology the 1907 note of McAfee deserves
mention primarily because it illustrates the
use of the quadrat method for sampling
surface fiora and fauna. McAfee presented
in some detail census data of four square
feet of forest and meadow floor at several
times of the year. The data are then enu-
merated relative to species, and an attempt
is made to show how the nutritional re-
sources of the floor are utihzed by the bird
population. The latter is important because
it stresses the community as a whole rather
than isolated habitat niches.

The ingenious Forbes in 1909 had a
novel idea and approach. He studied the
Indian corn plant in relation to its insect
infestation. Using as his biological focus the
fact that corn is both introduced and under
"the constant supervision of a guardian and
the services of a nurse," he develops the
argument that this species is ecologically
maladjusted and vulnerable to a dispropor-
tionate amount of insect competition. His
analysis of this corn-insect nexus is an inter-
esting study in synecology.

This concludes our treatment of the 1900
to 1910 period. We shall return briefly to
this decade later when we try to draw some
generalizations.

One other point must be raised. The
reader may ask with justification : Why have
there not been reviewed works on evolution
as they contribute to ecological growth?"

• One book that appeared during the decade
and focussed attention on evolutionary pro-
cesses was Darwin and Modern Science, edited
by A. C. Seward (1909). This volume con-
tained twenty-nine essays written by eminent
contributors in commemoration of the fiftieth
anniversary of the publication of The Origin of
Species. Certain of these essays were distinctly
ecological and should be mentioned: "The Se-
lection Theory," by August Weismann; "Geo-
graphical Distribution of Animals," by Hans
Gadow; "Experimental Study of the Influence
of Environment on Animals," by Jacques Loeb;
and "The Value of Colour in the Struggle foi
Life," by E. B. Poulton.

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

49

The answer is that, during this period, the
growth of ecology and evolution were so
inextricably woven together that it seems
artificial to separate the two. Many of the
studies we have mentioned in foregoing
pages contain data, conclusions, or concepts
that bear on evolution or speciation. In
other words, certain ecologists of these
times had a lively interest in such matters.
This is as it should be, and it epitomizes the
viewpoint of this book and its authors."

1911-1920

As we survey the second decade of the
twentieth century from the viewpoint of
ecological history, these items impress us:

1. There was no major readjustment of
focus between this decade and the first.

2. Not much theoretical synthesis of the
material of ecology was attempted.

3. More work was done in the sense that
there were more investigators.

4. Technical advances in other fields-
physics, chemistry, physiography, climatol-
ogy, physiology, biometry, and so on— reflect
upon ecological research largely through
refinement of methods and mensuration.

5. Some books (both text and reference)
of use to the ecologist were published.

6. The British Ecological Society and The
Ecological Society of America were founded
in 1913 and 1916, respectively, to aid ecolo-
gists and their enterprises.

In short, this seems to be primarily a dec-
ade of sure, gradual growth without much
reorientation.

Since the literature of this decade is
more extensive than that of the 1900 to
1910 era, there is a temptation to devote
more space to it. This we cannot do. We
can only sample as before and trust that
our samples are sufficiently representative
to be meaningful.

Some of the books that appeared should
be mentioned. Books are valuable in a his-
torical survey because they indicate what
was considered important at the time and
how the subject matter was studied. Two
physiological texts were published that
ecologists found useful: Piitter's Vergleich-

• For the sake of accuracy, however, it
should be mentioned that certain ecologists
were veering away from an evolutionary view-
point in the first decade. A good example,
perhaps, was V. E. Shelford, who, during that
period, was crystallizing his ideas on "physio-
logical animal geography" in contradistinction
to historical or faunal animal geography.

ende Physiologie (1911) and Bayhss
Principles of General Physiology (second
edition, 1918). In 1913 C. C. Adams pub-
Hshed his Guide to the Study of Animal
Ecology. This served the useful purpose of
classifying the diverse literature of ecology
and outhned a reading program for stu-
dents. Probably the most valuable book of
the decade was Shelford's Animal Com-
munities in Temperate America (1913).
Here was a summary of much original field
research organized around a number of
habitats within a restricted area (Chicago).
The author gave due weight to physi-
ography, the nonbiotic and the biotic envi-
ronment, and to the quantitative enumera-
tion of animals. Although it is out of date
in some respects, teachers and students to
this day turn to it for ecological guidance.
It was reprinted without essential alteration
in 1937.

Several books on hydrobiology appeared
and served a real need. Murray and Hjort's
The Depths of the Ocean (1912) became
rapidly a standard treatise on oceanography,
and the compendium Fresh-Water Biology
(1918), edited by Ward and Whipple, fa-
cilitated the study of limnology, particularly
through its emphasis on taxonomy. In 1916
Needham and Lloyd published The Life of
Inland Waters, "an elementary textbook of
freshwater biology" that served a useful
purpose in field zoology and beginning ecol-
ogy courses. In 1913 L. J. Henderson
published The Fitness of the Environment.
While not an ecological study in the re-
stricted sense, this book was a provocative
statement on the relation of the environ-
ment to its organism. It forced ecologists to
think in new and somewhat theoretical
terms and thereby exerted a healthy influ-
ence both on them and on the development
of their subject. We shall return specifically
to this book in a later section (p. 76).

In 1915 Jordan and Kellogg brought forth
their Evolution and Animal Life, which con-
tained many correlations between ecology
and evolution and thus deserves mention in
this place. In the preface the authors state:
"... the writers have tried to give a lu-
cid elementary account, in limited space, of
the processes of evolution as they are so far
understood." The chapters with particular
ecological flavor are "Natural Selection and
Struggle for Existence;" "Geographic Isola
tion and Species-Forming;" "Geographical
Distribution;" "Adaptations;" "Mutual Aid
and Communal Life among Animals;" and

50

THE HISTORY OF ECOLOGY

"Color and Pattern in Animals." This was a
useful book which, in the second decade,
emphasized the close connection between
ecology and organic evolution.

These, then, are some of the books that
ecologists were reading during the decade
1911 to 1920. Of course there were others,
but the ones mentioned should suffice as a
sample. It is our task now to survey briefly
certain specific papers as we did in the
preceding section. We use the same head-
ings as before: viz., natural history; re-
sponse, developmental and toleration physi-
ology; hydrobiology; succession; and syne-
cology. In addition, we shall have a word
to say about the growth of quantitative
methods.

Since ecology is always based in the
final analysis on natural history, we find
that subject constantly present and to be
accounted for. During the decade 1911 to
1920 many first-rate natural history papers
were published. These ranged from such
popularized reports as Brunner's Tracks and
Tracking (1912), which was an "illustrated
guide for the identification of mammal and
bird tracks or footprints," to such compre-
hensive studies as those of Herrick (1911),
Belding and Lane (1911), Needham
(1920), and Pearse and Achtenberg
(1920).

Response physiology was an active phase
of ecology during the second decade. While
the investigations ranged considerably in
type, there was a drive towards expressing
animal behavior in as precise terms as
possible. Frequently, this led the study into
experimentation as distinguished from
uncontrolled observation. The ecological
contributions were made largely through
knowledge acquired of the way a single
environmental factor induced an organismic
response. Review of several studies will
clarify these points.

A paper that was interesting from both
the behavioristic and ecological points of
view was that of Severin and Severin
(1911) on death feigning in two aquatic
bugs, Belostoma and Nepa. These investi-
gators were concerned with three aspects of
the problem: careful description of the
death-feigning attitudes, environmental fac-
tors inducing death feigning, and the
possible significance of this response when
expressed in terms of survival value. For
example, it was found that while Belostoma
assumed either of two attitudes, Nepa

"froze" in the position it held at the time
the stimulus was presented. The authors
noted that dryness decreases and moisture
increases the duration of the death feint in
Belostoma and that high air temperature
shortens the duration for both species. Their
general conclusion about the character of
the response is that "... the death feint
in arthropods is simply a non-intelligent
instinctive act" (p. 39).

Dawson (1911), in "The Biology of
Physa," approached this topic with a be-
havior emphasis, but reported much that
was ecological, particularly in two sections
of the paper: "The Relation of Physa to Its
Natural Environment; Including a Compre-
hensive Analysis of the Habits of Physa in
the Ann Arbor Region," and "The Food and
Feeding Activities of Physa." The section
on "Psychic Phenomena" contains an inter-
esting and ecologically pertinent discussion
of the "source of stimuli received by Physa
in field habitats." Present day ecological
work would profit by careful analyses of
the latter type! In 1911 S. O. Mast pub-
lished Light and the Behavior of Organisms.
This was a valuable stimulus to compara-
tive psychology, and it also synthesized
much that was instiTictive to the ecologist.
Also during the decade Jacques Loeb
(1918) published his well-known and
polemic book on a mechanistic interpreta-
tion of behavior. Forced Movements, Tro-
pisms and Animal Conduct.

Developmental physiology underwent
more specialization during the decade. It
also linked itself closely with embryology.
Nevertheless, many papers were published
that contributed to the growth of ecology.
LeFevre and Curtis (1912) reported at
length on the reproduction of fresh-water
mussels. Much of their work had distinct
ecological and parasitological emphasis.
Thus they discussed the development of the
embryonic mussels in the gills ("marsupi-
um") of the mother. They studied breeding
seasons and recognized "summer breeders"
and "winter breeders." They described the
development and behavior of the glochidia,
including the parasitization of the fish by
these larvae. Finally, they dealt with the
establishment of the young mussel on the
bottom and its subsequent maturation.

During this decade there was a growing
focus, later to reach fuller clarity, on the
effect of the physical environment upon
developmental rates. Usually, either tern-

FmST FOUR DECADES OF THE TWENTIETH CENTURY

51

perature or humidity was the variable
studied. Headlee's 1917 paper is a represen-
tative example. In this he analyzed the
eflFect of humidity on duration of metamor-
phosis in the bean weevil, Bruchus ohtectus.
For a paper published during the decade,
but dealing with temperature rather than
with humidity, the reader is referred to
Krafka (1920).

Earlier, we called attention to the pubh-
cation in 1918 of the second edition of
Bayliss' Physiologi/. This magnificient vol-
ume immediately became a source book for
physiologically minded ecologists (as it did
for many other biologists) and did much
for the field. It was useful especially in the
area of developmental physiology.

Not many publications were concerned
directly with toleration physiology between
1911 and 1920, although this phase was
touched on incidentally in numerous places.
A good example of this approach per se is
the paper of Shelford and Allee (1913),
"The Reactions of Fishes to Gradients of
Dissolved Atmospheric Gases." For exam-
ple, they studied the ability of various spe-
cies of fish to tolerate low oxygen tensions.
One of their suggestive findings was this:
Species of fish die (in the presence of re-
duced oxygen supply) in the order of their
relation to this factor in nature. Thus, just
to make the point, Notropis, a swift-water
form, starts to die after 376 minutes' expo-
sure, while Ameiurus, typically a sluggish-
water form, does not start to die until after
1080 minutes.

A number of excellent investigations on
hydrobiology were published during this
decade. There was perhaps a growing diver-
gence between oceanography and limnol-
ogy, but the essential viewpoints of these
two fields retained much in common. The
treatise, alreadv mentioned, by Murrav and
Hjort, The Depths of the Ocean, appeared
in 1912 and helped to establish modern
oceanography on a firmer foundation. A
representative research report was that of
Petersen and Jensen (1911), who published
a comprehensive monograph on the fauna
of the ocean floor both from the quantita-
tive and nutritional aspects. This paper
discussed the techniques of bottom study
and also presented manv significant biolog-
ical data. Adams in 1913 considered it "a
verv important paper."

In addition to recognizing the importance
of Petersen and Jensen's paper, a word

should be said of Petersen himself. It is not
always recognized that this man is among
the great in the history of ecology and
hydrobiology. We should fail in our survey
if we overlooked the point. Professor E. S.
Russell, himself a distinguished hydrobiol-
ogist, in his The Overfishing Problem
(1942, pp. 68-69) pays tribute to Petersen
in these words:

"In introducing a biological and ecological
note into this discussion ... I shall follow the
lead of a remarkable man, the late C. G. Joh.
Petersen, a pioneer in fishery research and
marine ecology, whose work is unfortunately
not widely known outside fishery circles. I had
the privilege of his friendship, and the oppor-
tunity of discussing with him fishery questions
and problems of general biology— and I take
this occasion to pay a tribute to his memory.

"Petersen was for many years Director of
the Danish Biological Station, a State institu-
tion devoted to the investigation of fishery
problems, and it was his great merit that he
regarded these as being essentially problems of
ecology. He realised more vividly than anyone
else that fish must be studied, not in isolation
from their environment, or purely from a sta-
tistical point of view, but in close relation to
all the factors, including the effect of fishing,
that influence their abundance, their rate of
gro\\'th, and their reproduction."

Fresh-water investigations were also con-
tributing to the growth of ecology during
the decade. Birge and Juday were in the
midst of their long personal and scholarly
association. A representative illustration of
their then current work was the still-quoted
1911 paper, "The Dissolved Gases of the
Water and Their Biological Significance."

In 1918 Muttkowski published a sound
report covering work conducted at Lake
Mendota (Wisconsin). This paper was a
thorough treatment, with considerable tabu-
lar documentation, of the follo\\dng points:

(1) qualitative survey of the macrofauna;

(2) quantitative survey of the commoner
macrofauna; (3) ecological distribution of
the fauna: (4) breeding habits; and (5)
food relations, especially insects as food for
the fish population. In 1918 there also ap-
peared Fresh-water Biology, edited by
Ward and WHiipple. We have already sug-
gested that this source book had a firm im-
pact on aquatic ecology.

Forbes and Richardson (1919) published
a study of the Illinois River that not only
contained much of ecological importance,
but also utilized physiography as an ap-

52

THE HISTORY OF ECOLOGY

proach to ecology and presented something
of the impact of human society on a nat-
ural environment. Their interest centered
around the Illinois River as it had been af-
fected by (a) the opening of the Chicago
drainage canal into the river; (b) the con-
sequent increase in sewage; (c) the recla-
mation of river bottoms for agricultural use;
and (d) the introduction into the stream
of the European carp.

An appreciation of the amount of pub-
lished research on limnology through the
first decade can be had by examination of
the "Bibliography of Limnological Litera-
ture" compiled in the "Challenger" ofiice
and assembled by James Chumley (1910).
The reference list contains over 2500 cita-
tions.

In discussing some of the developments
of synecology during this decade, it is well
to remind the reader that many of the
papers already cited in other connections
contain much of synecological interest.
Thus, the reports of Pearse and Achtenberg,
of Petersen and Jensen, of Embody and of
Muttkowski all have direct bearing and
could be cited properly in this section.
However, we shall extend our remarks
somewhat by reviewing a few more papers
selected for the purpose.

During this decade synecological studies
were varied in character and in method of
analysis. They were dominated largely by
successional emphasis and ranged from such
papers as that of Gates (1911), describing
the distribution of summer Illinois bird life
in relation to the local plant communities,
to Wheeler's (1911) philosophical essay,
"The Ant-Colony as an Organism," in
which he pointed out some of the analogies
between such a complex, integrated popula-
tion of organisms and a complex, integrated
population of cells.

In 1912 Pierce, Cushman, and Hood pub-
lished an important paper on "The Insect
Enemies of the Cotton Boll Weevil." Al-
though this investigation was motivated by
economic considerations, it is a thoroughly
sound and stimulating analysis of biological
control, i.e., control of the boll weevil popu-
lation by predatory and parasitic competi-
tion. In an attempt to evaluate these
predatory and parasitic pressures, the au-
thors reach these major conclusions (pp. 94,
95):

1. "The control of the boll weevil by insect
enemies is sufficiently great to give it a

high rank in the struggle against the pest.
A considerable portion of the insect con-
trol would not be accomplished by any
other factor; hence it is by no means to
be neglected."

2. "The amount of control due to the var-
ious factors at work in any given place
should be increased if possible. Parasites
can be introduced into new fields."

3. "The parasites and predators which at-
tack the boll weevil are native insects, al-
ready present in a given territory before
the weevil arrives."

The synecological distinction of this paper
lies in the authors' constant emphasis on
interspecies relationships, whatever the
type. This is climaxed in an interesting dia-
gram that attempts to put in simple form
all the major relationships unearthed. Be-
cause of the novelty of this figure and
because it presages much that is to come
later in this book, it is reproduced on page
53 (Fig. 1).

The microfauna was not neglected dur-
ing the decade 1911 to 1920. Waksman
(1916) wrote cogently of it in a paper
entitled "Studies on Soil Protozoa." He dis-
cussed three aspects: (1) active protozoan
fauna in the soil; (2) numbers and types of
Protozoa in different soils at difi^erent
depths; and (3) the effect of Protozoa on
bacterial numbers and their decomposi-
tion of organic matter in the soil. His two
major conclusions were that moisture, hu-
mus content and soil structure are the most
important factors to which soil Protozoa
react, and that soil Protozoa reduce
bacterial numbers. In reference to the lat-
ter statement, Waksman makes the point
that, when conditions become favorable for
the Protozoa, the bacteria decrease. Pre-
sumably, this effect is competitive in
character, although Waksman did not ana
lyze it in any detail.

At this point attention should be called
to a considerable, early twentieth century
"Cornell School" of naturalists, including
A. A. Allen (Ornithology), A. H. Wright
(Vertebrate Zoology), and James G. Need-
ham (Entomology and Limnology), with
their students, and with the addition of
W. J. Hamilton (Mammalogy) in 1926
Cornell had become the center of entomo-
logical research and education imder the
influence of John Henry Comstock (1849-
1931), and of interest in vertebrate zoologv
under Burt G. Wilder (1841-1926). "Field
Zoology" flourished at Cornell in the varied

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

53

biotic environments aflForded by the Finger
Lakes region, with a small limnological
station and even with an occasional noc-
turnal class.

For a certain group of ecologists— a group

presented certain antecedents for this. The
chief worker was Shelford, whose writings
stress the successional development of
the animal community. Shelford's student.
W. C. Allee, also showed some interest in

THE BOLL WEEVIL COMPLEX

THE COTTON PLANT

LEAF / BOLL RICE

WORM WEEVIL WEEVIL

/ \ ^WHICH IN TURN ARE ATTACKED^

BEAN >^COWPEA
WEEVIL WEEVIL

6

HYPER -
PARASITES

Fig. L The boll weevil complex. (From Pierce, Cushman, and Hood, U. S. Department of
Agriculture, Bur. Entom. Bull., 100.)

largely stimulated by the botanist H. C.
Cowles at the University of Chicago— the
major synecological investigations of the de-
cade centered around ecological succession.
In this historical section we have alreadv

the problem both as a junior collaboratoi
and as an independent investigator. In 1911
Allee published a short paper entitled "Sea-
sonal Succession in Old Forest Ponds "
C. ('. Adams had worked with the problem,

54

THE HISTORY OF ECOLOGY

and his 2913 book, Guide to the Study of
Animal Ecology, frequently makes the
point by imphcation that this is ecology!

A word is in order about Shelford's then
current studies (1911, 1911a, 1911b, 1912,
1912a), We shall return to some of these in
a professional sense later. Historically, they
had great influence on the growth of
ecology. They were cHmaxed, integrated,
and summarized in the 1913 book. Animal
Communities in Temperate America. From
the viewpoint of succession Shelford's
greatest contribution was his interpretation
of fish succession in streams as contrasted
with that in ponds. In the former he
showed that physiographic erosion was the
important factor. In the latter, the succes-
sion was conditioned largely by biotic
factors that gradually made over the habi-
tat so that new forms could move in.

One of Shelford's more important theo-
retical discussions was his "Physiological
Animal Geography" (1911c). This paper
showed Shelford's reaction away from evo-
lution as an interpretative factor in ecology
and towards physiology and function. He
discusses briefly the point of view of the
historical or faunistic zoogeographers and
then proceeds to develop, with case exam-
ples, the alternative or physiological aspect.
Of the latter he says (p. 554) :

"There are two distinct points of view for
biological investigation. One is that of evolu-
tion; the other, that of physiology, or the ex-
planation of the organism in terms of physics
and chemistry. One may make a physiological
explanation of the behavior or structure of an
organism and in no wise explain its evolution.
On the other hand, one may make an evolu-
tionary explanation of an organism without
making any contribution to its physiology. The
study of physiological animal geography may
be conducted independently of the problems of
evolution. It does not need to be concerned
with centers of origin, or paths of dispersal, or
with other problems of faunistic animal geog-
raphy. In this paper we are concerned with
the physiological relations of animals to natural
environments."

It is only fair to state that in concluding
paragraphs Shelford does make the point
that biological science will be best served
by the wedding of these two viewpoints.
But the strong feature of his paper is its
synthesis of the ecological approach to
problems of dispersion.

In present day ecology succession no
longer occupies so prominent a place. It is

studied, but the emphasis is on the total
community, with succession essentially a
developmental phase of that total unit.
However, during the first two decades of
the twentieth century the concept was a
vital one in the historical sense; it stimu-
lated much work and provided a rational
approach for field analyses.

In 1915 C. C. Adams pubhshed his ex-
tensive monograph on "The Variations and
Ecological Distribution of the Snails of the
Genus lo." This gastropod is a river fonn
and was studied primarily in the south-
eastern and southern states. Adams states
the centi-al theme of liis study by quoting,
with patent approval, W. K. Brooks, who
wrote (Adams, p. 7) :

"Inheritance and variation are not two
things, but two imperfect views of a single
process, for the difference between tliem is
neither in fiving beings nor in any external
standard of extermination, but in the reciprocal
interaction between each living being and its
competitors and enemies and the sources of
food and the other conditions of life . . , You
will note that it is as great an error to locate
species in the external world as it is to locate
it in germ cells or in chromatin. It neither
exists in the organisms nor in the environment,
because it is in the reciprocal interaction be-
tween the two."

In this historical survey Adams' paper
makes an important point. Here was an in-
vestigation by an ecologist, utihzing ecolog-
ical techniques, that made a sincere
attempt to coordinate and inteipret the
findings as they were related to heredity
and evolution. In short, we use Adams'
paper as evidence to show that, historically,
ecology was not divorced from evolution in
the minds of many workers in the field.

Before closing this 1911 to 1920 survey,
we wish to draw attention to the point that
biometry was growing and its influence on
biologists and biology was gradually in-
creasing. The ecologist can not ignore the
importance of this fact. Much of modern
ecology is statistical and seems destined to
become more so. We have mentioned in our
review the names of Malthus, Quetelet,
Farr, Galton, Weldon, Pearson, Davenport,
Harris, and Pearl, names inextricably
woven into the history of ecology. Although
statistical methods per se did not contribute
greatly to ecology between 1911 and 1920,
they were available and were beginning to
be used. The then contemporary situation

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

55

was well stated by Raymond Pearl in a
1914 (pp. 47-48) address before the
American Statistical Association. He said:

"Statistical science has brought to biology
three fundamentally important things which it
had previously lacked. These are: first, a
method of describing a group of individuals in
terms, not of its component individuals, but in
terms of its (the group's) own attributes and
qualities; second, the concept of 'probable
error,' which makes possible an estimate of
the probable accuracy of a series of obserx^a-
tions; and third, a method of measuring the
degree of association or correlation between the
variations in a series of characters or events.
. . . By turning to statistical science for aid the
biologist has greatly augmented his powers of
analysis in the domain of his own particular
problems. While this branch of science, which
has been called into being by this coalition, is
vet too young to have shown its full capabili-
ties, yet I think its achievements have been
sufficient in qualitv and amount to justify the
belief that its position is secure and its prom-
ise bright. Biometrv seems destined to be-
come a permanent and important branch, at
once of biological investigation and of statisti-
cal inquiry."

These were prophetic and true words,
both for biology and for ecology.

1921-1930

During the decade 1921 to 1930 ecology
was expanding and maturing; expanding in
the sense that more ecological studies were
published; maturing in the sense that the
Geld was attaining greater focus. Whereas
the second decade of the twentieth century
was considerably like the first, the third dec-
ade was somewhat different, even though
much of the specific research was similar.
Ecologists were still conducting research on,
say, response physiology, or food relations
or succession, but now their work seemed to
have more of a common denominator that
took form as a "self-conscious" science.
Thus, in studies on animal responses or
succession there was greater interest in in-
terpreting these phenomena in broad eco-
logical terms. We do not imply that ecology
became a closely unified science during the
third decade. It is not that today. We sug-
gest only that it was collecting certain
varying ends, rearranging its emphases and
starting thereby on a newlv oriented course.
It is our task to examine further these
trends.

Certain books published between 1921

and 1930 reflect the temper of the times.
At the outset, two textbooks appeared de-
signed for the use of ecologists in university
classes: Animal Ecology (1926) by A. S.
Pearse, and Animal Ecology (1927) by
Charles Elton. We shall return to these
directly. There were other books basically
ecological in character. Borradaile's The
Animal and Its Environment (1923) gave
"an elementary treatment of animal ecology
including general descriptive matter from
natural history, and relatively little quan-
titative analysis of the environment"
(Chapman, 1931, p. 2). In 1922 the third
edition of Folsom's Entomology was pub-
Hshed. It is significant to note that the
author added to this edition the subtitle
"with special reference to its ecological as-
pects" and included a new chapter on
"Insect Ecology" prepared under the guid-
ance of V. E. Shelf ord. While this book
made no great impact on ecological science,
its revised publication suggests that the
ecological developments of the first and
second decades had been sufficient to cause
an entomologist to present his subject
basically from that point of view.

In 1929 Shelford published Laboratory
and Field Ecology, which was largely a
"methods" book. Although it was to serve
ecologists, it did not have anything like the
influence on ecological histor)'^ enjoved by
the author's earlier Animal Communities in
Temperate America. Elton (1930) brougrht
forth a small book entitled Animal Ecology
and Evohition. which centered around three
chief topics: "The Regulation of Numbers,"
"The Significance of Migration." and "The
Real Life of Animals." In 1927 Social Life
in the Animal World by Alverdes appeared.

From the dignified viewpoint of scholar-
ship, probably the really significant book of
the decade was R. Hesse's Tieraeos^raphie
auf oekologischer Grundlase, which ap-
peared in 1924. This treatise recognized
that there was an approach to zoogeog;raphv
other than the classical, faiinal one. Hesse's
conception of the subject is well stated in
this translated excerpt from his preface:

"Ecological animal geography is a young
science ... In this new field the fundamen-
tal questions are yet to be formulated in order
that a rich phase of biology may be opened
for further work. I hope this book may be
thought of as such an attempt; it deals largely
with problems which are taken up separately
and arranged in order, and but relatively little

56

THE HISTORY OF ECOLOGY

space is given to presenting satisfactory solu-
tions. Such treatment does show that the
problems of ecological animal geography are
capable of exact solution and indicates further
in what direction, through observation and ex-
perimentation, the solution is to be sought. I
hope that this treatment will stimulate further
ex-peditionary researches in this field. We have
had an over-supply of travel which yielded ani-
mal pelts and alcoholic material; we need
rather observations on the relations between
animals and their environment."

It is fair to state that Hesse attained these
desiderata. A tribute to his book came in
the next decade when, in 1937, W. C. Al-
ice and Karl P. Schmidt prepared a revised
edition in English and thereby made the
volume more immediately available to
American and English biologists. In their
introduction the translators said, "The ap-
pearance of Professor Richard Hesse's book
in 1924 marked the beginning of a new
phase in the development both of ecology
and of animal geography. In the latter field
it made the first serious attempt to apply
ecological methods, principles and facts to
the study of animal distribution on a world-
wide scale."

Another book on biogeography was
Willis' Age and Area, (1922). This study
did not have the weight carried by
Hesse, but it was extremely provocative and
polemic. In a historical survey these char-
acteristics, rather than its scientific validity,
may be the significant features of a work.
Another important volume of the decade
was Tier tind Pflanze in Symhiose, by P.
Buchner, which appeared in second edition
in 1930. Buchner and his students carried
out extensive studies on the importance and
mode of transmission of symbionts (p.
248).

There were other books published be-
tween 1921 and 1930 that ecologists found
useful. Some of these should be mentioned.
The Determination of Hijdros,en Ions by
Clark (1928) and Harvey's Biolos.ical
Chemistrtf and Physics of Sea Water
(1928) presented information about the
abiotic environment.* Robertson in 1923
published The Chemical Basis of Growth
and Senescence which contained a good
deal about the environment in a biochemi-

• Harvey further contributed to this topic
through publication in 1945 of a small book en-
titled Recent Advances in the Chemistry and
Biology of Sea Water.

cal sense. An important German book on
hydrobiology was Hentschel's GrundzUge
der Hydrobiologie (1923). Three limnolog-
ical books in German that appeared
during the decade should be mentioned:
Thienemann's (1926) Limnologie, Lenz's
(1928) Einfiihrung in die Biologic der
Siisswasserseen and Brehm's (1930) Ein-
fiihrung in die Limnologie. Entomologists
were active during the period. W. M.
Wheeler wrote several books, among them
Social Life among the Insects (1923),
which summarized this subject with charac-
teristic vigor and scholarship. War die and
Buckle (1923) and War die (1929) covered
certain aspects of economic entomology that
had a distinct ecological flavor.

At this point we should mention the book
by Grinnell, Dixon, and Linsdale (1930)
Vertebrate Natural History of A Section of
Northern California through the Lassen
Peak Region. This monograph is an excel-
lent example of modern natural history.
Also, its mention permits us to pay
tribute to the late Joseph Grinnell, who
was, perhaps more than any other, the
epitome of the modern natural historian. So
far as we can judge from his writings and
lectures, Grinnell was not sympathetic to
analysis of ecological problems by the
methods of instrumentation and mensura-
tion. Apparently, it was his idea that the
organism and its responses were a far bet-
ter criterion of environmental reaction than
any measurement. Once, in correspondence
with one of us, he said, "The animal is
more sensitive than any thermometer or at-
mometer."

The "Lassen Peak" study was antedated
by Animal Life in the Yosemite, by Grin-
nell and T. I. Storer (1924). This work
was equally comprehensive, although it may
not be cited so much as the former. In the
"Yosemite" volume one finds "an account
of the mammals, birds, reptiles and amphib-
ians in a cross-section of the Sierra Ne-
vada." Historically this study is significant,
not only because of its wealth of natural
history, but also because it shows how a
public preserve such as a national park can
be utilized for field research.

In the population field in a strict sense,
Ravmond Pearl published four provocative
books: The Rate of Living (1928), deal-
ing with laboratory populations; The Biol-
ogy of Population Growth (1925), deal-
ing with both laboratory and human pop-

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

ulations; The Biology of Death (1922) and
Studies in Human biology (1924), dealing
with human populations. Lotka's Elements
of Physical Biology (1925) covered certain
phases of biotic interactions from a rational,
theoretical viewpoint, and, as its meaning
is slowly assimilated, becomes an increas-
ingly distinguished contribution.

In the field of human ecology, stiaddUng
the fence between biology and sociology,
two books by Ellsworth Huntington came
out (Principles of Human Geography, 1921,
with Gushing; Civilization and Climate,
1924), along with The Population Problem,
by Carr-Saunders in 1922, and Der Gang
der Kultur iiber die Erde, by Hettner in
1923.

A rapidly advancing field during the
twenties was paleo-ecology. Although the
plant ecologists were most concerned, there
were enough general principles emerging
to warrant the attention of animal workers.
Paleo-ecology may lack the quantitative
methods of modern ecology, but it is a
necessary approach if evolutionary views
are to be applied outside taxonomic and
phylogenetic studies. A direct way to study
this subject by means of modern geological
structures was carried out by Professor
Richter and his associates in the Sencken-
berg Museum in Frankfurt. A convenient
English summary of this method was pub-
lished by Bucher in 1938. Other significant
publications were F. Clements' (1924)
Methods and Principles of Palaeo-ecology;
O. Abel's (1929) Paldobiologie und Stam-
mesgeschichte, and a summarizing paper in
the next decade (1935) by C. L. Fenton
entitled "Viewpoints and Objects of Paleo-
ecology." In 1928 a journal, "Palaeobiolo-
gica," edited by Abel, was founded and
published in Vienna.

The general ecology texts by Pearse and
Elton warrant further examination. They
show how two specialists organized ecology
during the third decade. Pearse had the
following chapter headings:

1. Introduction, ii. Physical and chemical
ecological factors, iii. Biological factors, iv.
Succession, v. Animals of the ocean, vi. Fresh-
water animals, vii. Terrestrial animals, viii. The
relations of animals to plants, ix. The relations
of animals to color, x. Intraspecific relations, xi.
The economic relations of ecology.

He thus laid a general background of phys-
ical and biotic factors and then classified
animals ecologically according to their major

Oi

habitats. The treatment was primarily
descriptive.

Elton's book appeared under the spon-
sorship of Julian S. Huxley, who said in
the Forewor-^' ^p. xiii) :

"Finally, there remain subjects which are of
such recent growth that their principles have
never yet been treated in a comprehensive way.
Such, for instance, are developmental and com-
parative physiology, animal behaviour and
ecology. From the point of view of the rapid
growth and expansion of general biology, it is
these subjects which it is at the present
moment most important to summarise in brief
text-books, since otherwise the multifarious
knowledge which we have already attained re-
garding them remains locked up in scattered
papers, the property of the specialist alone.
The present volume deals with a much mis-
understood and often underrated subject."

The emphasis that Elton placed on ecol-
ogy was different from that of Pearse, as
was the manner of treatment. This can be
seen from the following table of contents:

i. Introduction, ii. The distribution of animal
communities, iii. Ecological succession, iv. En-
vironmental factors. V. The animal community,
vi. Parasites, vii. Time and animal communities,
viii. The numbers of animals, ix. Variations in
the numbers of animals, x. Ecological methods,
xi. Ecology and evolution.

Elton was concerned more with organiz-
ing ecology around principles, and most of
his principles centered around the animal
community and the natural population. Un-
hke Pearse, he was interested, not so much
in whether an animal was found in a desert
or a lake, but rather in the environmental
factors hmiting the distribution of such a
form. Elton stressed also the quantitative
aspects, particularly in connection with the
number of animals that occupy any com-
munity and the impact that these numbers
make on their total environment. He viewed
food chains as the most important integrat-
ing factor of the community, and his treat-
ment of this subject is outstanding.

As we view the growing organization of
ecology during the period 1921 to 1930,
it looks something hke this. There was a
rough dichotomy between the physical-
chemical environment and the biotic envi-
ronment. The former was broken down into
a series of factors of greater or lesser eco-
logical significance that were studied as
"conditions of existence." This was a phrase,
apparently tracing back to Karl Semper

58

THE HISTORY OF ECOLOGY

(1881, "Animal Life as Affected by the
Natural Conditions of Existence" [italics
ours J) (see p. 22), that Shelf ord had used
ia 1918 to describe such environmental fac-
tors which, he said, "are of importance
only in so far as they affect the Hfe and
death processes of organisms." The phys-
ico-chemical conditions of existence most
studied through this decade were water,
temperature, humidity, hydrogen ion con-
centration,' oxygen and carbon dioxide
tensions, saUnity, specific gravity, molar
agents such as wind, current, and waves,
tide, substratum, and altitude. If space per-
mitted, and if it were essential for our his-
torical survey, we could discuss papers that
dealt with any or all of these factors. This
we cannot do. The major point is that ecol-
ogists had recognized the abiotic environ-
ment both as a total unit and in terms of
its components and were analyzing it from
those vantage points. The organism's re-
sponse, its growth and development, and
its toleration of these conditions of exist-
ence remained the essential subjects of
analysis.

The organization centering around the
biotic environment is more difficult to sum-
marize. In part, this means merely that
biotic relations tend to be more complex
than do the abiotic. In part, it means that
ecologists themselves had not crystalUzed

** The biologists of the twenties were amus-
ingly "pH-minded." Here was a technique,
both physiological and ecological, easily ap-
plied, far-reaching in its implications, and so
respectable! The point is well made in anec-
dotal (and true) fashion.

A well-known ecologist was setting out from
the wharf at the Marine Biological Laboratory
( Massachusetts ) to collect data about the local
distribution of certain marine organisms, partic-
ularly those factors correlated with distribution.
In true ecologist-fashion his dory was loaded
with apparatus and impedimenta of all sorts.
On the rear seat there lay a pH kit. At the
wharf to see him off was a friend, one
of America's most distinguished zoological
scholars, who asked,

"Where are you going?"

He got his answer.

"What is your problem?"

Again, an answer.

"Why do you take so much equipment?"

The ecologist tried to justify his boat load.

"Well," said the savant, pointing to the pH
kit, "that is all you'll need. Leave the rest at
home!"

Thus pH in the twenties!

this phase of their science at that time. It
is possible, however, to recognize certain
general categories into wlrich the biotic
aspects fall. These are:

1. The animal community:

( a ) Distribution

(b) Food and feeding relationships
within the community

(c) Successional and other develop-
mental aspects

2. The problem of aggregation

3. rhe population:

(a) The natural population

(b) The laboratory population

4. Parasitic-symbiotic-social relationships (in
a specific sense and distinct from the
animal community)

5. Miscellaneous:

(a) Rhythmic phenomena

(b) Dispersal phenomena

(c) Human ecology

(d) Aspects of economic zoology

We cannot take time to document this
outline in any detail, but it does seem wise
to extend our remarks by discussing briefly
the community, the aggregation, and the
population. These aspects of ecology were
developing rapidly between 1921 and 1930,
and are much studied by ecologists today.

Since Elton's treatment of communities
seems without question the best of the dec-
ade, we can do no better than examine the
state of this phase of ecology as seen
through his eyes. As mentioned earher,
Elton viewed ecology as essentially the
study of populations and communities.

Judging from Elton and the published
papers of the decade 1921 to 1930, ecol-
ogists were interested in the animal com-
munity from these aspects: its distribution
in both a geographical and a local sense;
its structure and organization; and its tem-
poral development and change. There was
not much emphasis on the community as a
"social organism," although Elton, among
others, recognized the point, nor on the
problem of biotic equiUbrium. These phases
were to come later.

Under the influence of Hesse, Shelford,
and others, ecologists were examining com-
munities on a geographical scale and were
working on the pattern of their distribu-
tion. This did not stop with mere descrip-
tion, for certain of the studies insisted that
there were basic analogies between the
communities of one area and those of an-
other. These analogies seem to have con-
vinced students that the community was a

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

59

real biological entity, irrespective of its
global location.

The "structural" studies had two major
focal points, both of which are aspects of
the same problem. On the one hand, there
were extensive studies on food and feeding
relations within the community, such as
those of Sanders and Shelford (1922) and
Summerhays and Elton (1923) on terres-
h-ial communities; of Needham, Juday,
Moore, Sibley, and Titcomb (1922) on a
fresh-water community; and of Hardy
(1924) on a marine community. On the
other hand, there was a growing interest
in "how many" animals occupied a certain
niche in a community and the effect of this
quantitative relation on the community as
a whole. This aspect was really that en-
compassed by the natural population
studies, and we shall return to it shortly.

After studying a series of papers on ani-
mal communities and working actively on
the problem himself, Elton concluded that
(p. 55):

'■ . . . Animals are organised into a complex
society, as complex and as fascinating to study
as human society. At first sight we might
despair of discovering any general principles
regulating animal communities. But careful
study of simple communities shows that there
are several principles which enable us to
analyse an animal community into its parts,
and in the light of which much of the apparent
complication disappears. These principles are
food-chains and the food-cycle; size of food;
niches; the pyramid of numbers."

It is not our task here to discuss these
problems in a technical sense. That will
come in later chapters. We are concerned
only with the historical point that the study
of natural groups or communities had ad-
vanced to such a stage in the third decade
that it was possible to conclude: (a) that
communities are integrated to a large de-
gree by the sum total of their feeding re-
lations, and (b) that these relations, al-
though they may be completely different
in detail, are the common property of all
communities, whatever the tvpe and wher-
ever located. Several other studies that ap-
peared during the period and which should
be cited are those of Weese (1924), Smith
(1928), and Shackleford (1929).

Ecologists were well aware of the signifi-
cance of temporal factors in the organiza-
tion of the community. Succession was
firmly ensconced in ecological thought as a

time factor that brought about eventual
community equiUbrium when the climax
was attained. We have now enough of a
background for this point to make unneces-
sary its further discussion. Other temporal
aspects were recognized. Some of these
were (1) day-night rhythms; (2) migra-
tions on a vertical axis that occurred at
certain intervals as, for example, plankton
migration in the sea or vertical migration
in a forest; (3) tidal rhythms; (4) climatic
rhythms of various types, including the
seasons; and (5) extramundane rhythms.
Many ecologists of the 1921 to 1930 period
were doing more than recognizing these
rhythms. They were analyzing them in re-
lation to the community constituents.

Throughout this book we shall have
much to say about the phenomenon of ani-
mal aggregations and its significance for
ecological theory. This is a phase of ecol-
ogy studied with much intellectual profit.
As such, it needs to be considered briefly
in this historical review. It is brought in
at this point in the third decade, not be-
cause the subject "originated" then, but be-
cause it was summarized and evaluated
in a paper by Allee (1927a) and thus given
impetus for further growth. Certain phases
of the general problem had been consid-
ered earlier by botanists (especially
Clements), zoologists, and philosophers, and
their contributions must not be under-
estimated. But to Allee goes the credit for
a clear statement of the problem in terms
of animal ecology and "general sociology."
In his review Allee discussed the method
of formation of aggregations: general factors
conditioning aggregations; single-species, as
contrasted with mixed-species, aggregations;
integrative phenomena within aggregations;
and the social significance of aggregations.

Despite the existence in the 1921 to 1930
period of considerable knowledge about the
physical-chemical environment, the animal
community, the phenomenon of aggrega-
tion, and, as we shall see in a moment, the
population, ecologists did not coordinate
these various phases to anv degree. When
Allee wrote his paper in 1927 he outlined
the field of animal aggregations as he
viewed it. But this did not mean that, over
night, the subject flowered and matured.

In the third decade there was fact find-
ing; there was speculation; there were
some attempts at a synthesis of ecological
principles. But there was not much syn-

60

THE HISTORY OF ECOLOGY

thesis, and, by that token, not much de-
velopment of ecology as a unified science.
It would be incorrect to say that this uni-
fication exists today, although some notable
steps were to be made during the decade
1931 to 1940.

When the zoologist started to ask him-
self the quantitative question "How many?"
in addition to the qualitative question
"What Idnd?", natural population studies
began to emerge from natural history and
community investigations. Many ecologists
felt that community analyses with their
many variables were too complex to be
feasible methodologically. Accordingly,
they sought to better the situation by count-
ing certain species of animals that hved
within the framework of the total com-
munity and were of enough ecological im-
portance to warrant such careful scrutiny.
These counts were population censuses.

It is inaccurate to suggest that such
studies appeared de novo in the third dec-
ade. There were several historical preced-
ents for them. One important precedent
lay in earlier ecological work itself, both
botanical and zoological. A basis for pop-
ulation studies had been established in the
literature before 1900 (see p. 24). In fact,
we mentioned earlier a number of papers
that could be cited appropriately. Another
precedent came from the work of biologists
with a flair for biometry and an interest
in biological groups as such. Many of these
men have already been mentioned. Still an-
other precedent stemmed from the devel-
opment of statistics as a method for han-
dling biological data, as a technique for ra-
tionalizing and formulating biological inter-
actions (e.g., Lotka, 1925; Volterra, 1926),
and as a basis for the philosophical inter-
pretation of scientific evidence. These vari-
ous fields in one way or another were
forcing themselves into the ecologist's
thinking. From them the population ap-
proach, as did many other approaches, be-
gan to crystallize.

The early work on natural populations
frequently had an economic focus and
motivation, as, to a large degree, is still
true today. The investigators were con-
cerned with certain species, frequently an
insect, that as populations in nature had
a significant relation to some problem of
human disease, diseases of other animals,
or agriculti're. Analvsis of the former prob-
lem yielded data on epidemiology, actually

an

ail excellent example of quantitative ecol-
ogy. The distinguished British sanitarian,
Major Greenwood (1932), says of this
subject:

"Epidemiology displays the general factors
which operate upon populations or aggregates,
and lead to the outbreak of a sickness afltecting
several organisms within a short time. The unit
of the epidemiologist is the population ..."

Thus many of the natural population studies
were epidemiological in character and
stressed the statistics of host-parasite inter-
action. A masterly summary of the prin-
ciples vmderlying this science was written
by Wade Hampden Frost in 1927.

Analyses of insect pest populations fre-
quently yielded many data on the abun-
dance of such forms in relation to climatic
cycles and to predation and parasitization
pressures. Some representative studies of
the decade were those of Cook (1924) on
cutwoiTn populations, Bodenheimer (1925)
on the Mediterranean fruit fly, Shelford
(1927) on the codling moth, and Swynner-
ton (1921) on tsetse fly populations as a
vector for trypanosomes.

Natural population studies also were
concerned with cycles of abundance of
mammals and birds. In the literature of
the period we find studies on lemmings,
mice, rabbits and hares, marmots, musk-
rats, and certain ungulates and birds.
While the factors controlling these cycles
were not analyzed critically in many cases,
the information in the literature suggests
that the common causes are epidemics,
variation in quality and quantity of food,
and sunspot or climatic influences. Elton
was much taken with this research, as evi-
denced by his own papers (1924, 1925)
and Chapter 9 in his text. Other represen-
tative publications are those by Hewitt
(1921) on the wolf, hare, lynx, and red
fox; Soper (1921) on hares; and Brooks
(1926) on deer.

Experimental or laboratory population
studies had their essential inception in the
decade 1921 to 1930 and grew out of two
groups of investigators. On the one hand,
ecologists with a traditional background
turned their attention, in part at least, to
such studies. On the other hand, general
biologists and biometricians interested in
the experimental approach to growth of
groups became interested in such popula-
tion studies without the impetus or motiva-

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

61

tion furnished by earlier ecological training.
These two origins were wedded later, par-
ticularly in the fourth decade. Perhaps a
brief elaboration of this subject is in order.

Approaching these studies through the
ecological door were men like W. C, Alice
and Royal N. Chapman, both feehng ap-
parently that there was much to be de-
sired in terms of environmental control
even for natural populations. The method
of such men was to bring into the labora-
tory an animal that could be cultured
there successfully and study its various
group responses under reasonably con-
trolled conditions.* Chapman transferred
his attention to the flour beetle, Tribolium
confusum, and in this organism found an
answer to his problem. His most important
paper appeared in 1928, in which he set
forth the concepts of "biotic potential" and
"environmental resistance" and substanti-
ated them with empirical evidence. We
shall return to these ideas in later sections
of the book and discuss them carefully
(p. 303).

Alice continued his work on communities
and natural populations, but brought cer-
tain phases of these problems into the lab-
oratory for solution. Particularly was this
true of his investigations on aggregations.
An examination of his writings shows that
between 1921 and 1930 he studied, as ex-
perimental populations, isopods, the brittle
starfish (Ophioderma) , the marine flat-
worm (Procerodes) and planarian worms.
Unlike Chapman, Alice's interest was not
so much in the total analysis of the pop-
ulation as in studying in the laborator)'
certain responses largely protective in
character that arose as a consequence of
aggregation or population density.

The other approach through experimental
population studies is typified by the work
of Raymond Pearl and his colleagues. Be-
tween 1921 and 1930 Pearl and his group
published an astounding amount of ma-
terial in journal, lecture, and book form
on experimental populations of Drosophila
melanogaster. It is not our province here

" A somewhat idealized definition of an ex-
perimental population would be: a group of
inbred organisms cultured under controlled, yet
manipulatory, environmental conditions for
which repeated censuses of all stages can be
readily taken. Extensions and modifications of
this definition will appear in the section on
Populations.

to evaluate these studies. It is our respon-
sibility to indicate the aspects of the sub-
ject covered by them and attempt to weigh
their impact on third decade ecology.

Pearl was interested in experimental
populations from the following five view-
points:

1. The form of population growth. This
work was largely the demonstration that
various populations (e.g., yeast, Parame-
cium, Drosophila, man) followed a sigmoid
growth curve (the "logistic").

2. The analysis of population density and
its end effects. Pearl was thoroughly con-
vinced of the biological importance of this
matter. In 1930 he said, "In general there
can be no question that this whole matter
of influence of density of population, in all
senses, upon biological phenomena, de-
serves a great deal more investigation than
it has had. The indications all are that it is
one of the most significant elements in the
biological, as distinguished from the phys-
ical, environment of organisms" (p. 145).
Population density was analyzed primarily
as it affected reproduction and mortality.

3. The problem of longevity and those
factors, both genetic and ecologic, that in-
fluence it. These studies were actuarial in
character, and the data were summarized
to good advantage in life tables.

4. The possible growth analogies be-
tween experimental and human populations.

5. An illustration of the applicability of
quantitative methods to biological research.

In sum, experimental population studies
appealed to the workers of the decade (as
well as in the 1931 to 1940 period) for
these major reasons :

1. The results can be expressed in quan-
titative terms.

2. The end responses that can be studied
include such variables of patent biolog-
ical importance as:

(a) The factors contributing to
population growth— fecundity,
fertility, fission rate, success
and rate of development.

(b) The factors contributing to
population decline— differential
morbidity and mortality.

(c) The factors concerned with se-
lection pressure.

3. There is an absence of terminology
in these studies.

62

THE HISTORY OF ECOLOGY

4. The studies are theoretically impor-
tant, especially in relation to the natural
population, the community, statistics ot
host-parasite interactions (epidemiology),
social origins and social faciUtation, and
evolution and speciation.

A final development needs mention:
Cleveland's work (1924) on the symbiotic
relationship between wood-feeding termites
and their intestinal flagellates. Here it was
demonstrated that the latter, by secreting
a cellulose-digesting enzyme, made wood
available as food for the termite colony.
In turn, the termite gut furnished a micro-
niche for the Protozoa. This study was sig-
nificant in that it placed symbiosis on an
analytical basis and furnished impetus for
excellent research in the next decade.
Cleveland himself, in collaboration with
Hall, Sanders, and Colher, brought forth
in 1934 a comprehensive monograph on
the symbiosis between the roach Crijpto-
cercus and its intestinal Protozoa.

This concludes our survey of tlie active
third decade. We have examined the trends
and developments in ecology that centered
both around the physical and the biotic en-
vironment. We have seen that this was an
era when ideas were just starting to emerge
into a broader ecological framework and
when ecological research ceased being
helter-skelter and started to acquire focus.

1931-1942

In discussing this period we shall ex-
tend the interval beyond a decade (to
1942) in order to include several significant
trends that appeared in the last several
years. In this section it is our plan to make
these points:

1. Ecology was exceedingly active, both
in terms of volume of work and in qual-
ity of ecological effort.

2. Ecology gave signs of maturation. It
began to develop, crystalUze, and coordi-
nate principles of its own.

3. There was a newborn interest in an
ecological framework of theory— a theorv
based, not on speculation, but largely on
empirical evidence.

4. By 1942 ecology, with notable excep-
tions, was in a healthy and lusty state and
was looking forward to the decades to
come.

It is not feasible to survey the progress
of this decade by the methods used in the

preceding pages of citing research papers
and suggesting their influence on the
growth of the subject. As a more mature
science, ecology in the thirties gave birth
to many sorts of activities, which index and
epitomize its growth. We shall try to in-
dicate what these activities were and then
discuss them in enough detail to deHneate
the contribution that was theirs.** This
should serve also as a sort of summary for
the entire historical treatment in the sense
that it will show the state of the science
in its most modern dress. To our minds,
these activities fall into the following larger
categories: first, books; second, journals
available to and used by ecologists both for
recording research and surveying segments
of the field; third, review articles in review
journals; fourth, symposia; and last, articles
of particular significance in the synthesis of
ecological theory.

Books

In discussing the books of the decade
we stress the point, as we have done for
all the historical treatment, that the Ust is
a sample and not a complete tabulation.
It is, however, comprehensive enough to
cover the field thoroughly. The books pub-
Ushed between 1931 and 1942 fall into
these eight categories:

(a) General texts or reference works pri-
marily ecological in character;

(b) Books emphasizing the population
primarily;

(c) Books dealing with sociality and social
organization;

(d) Books stressing the ecological aspects of
zoogeography and dispersal;

(e) Books dealing with evolutionary and
speciation aspects and containing an
ecological (as well as genetic) treat-
ment;

(/) Books on ecological aspects of behavior;

(g) Books on applied ecology;

(h) Books on theoretical and philosophical

aspects that are difficult to place in the

foregoing categories.

A fist of books according to this classifi-
cation and in the order of their pubhcation
dates is given at the end of this chapter.

At this place a word of emphasis is in
order about the Clements and Shelford Bio-
Ecology (1939) and the movement it rep-

** It is obvious, of course, that the account of
all these "activities" is reflected in final analysis
in the publication of research data.

FIRST FOUR DECADES OF THE TV^^NTIETH CENTURY

63

resents. This book assisted in drawing to-
gether the ecological researches of zoolo-
gists and botanists under a common denom-
inator. It stressed the obvious point that,
typically, there is no such thing as a plant
community devoid of animals, or con-
versely, an animal community devoid of
plants. The "bio-ecologists" work with an
ecological unit which they designate the
"biome."'

The population books deal wdth the ex-
perimental, the natural, and the human
population. We include several books on
human populations because they contribute
in a real way to the ecologist's thinking
and methodology. From certam angles the
demographers have had a more scholarly
approach to the problem than the ecolo-
gists. The books on sociality and social
organization are WTitten essentially as pop-
ulation studies from which special results
are derived. The zoogeography books focus
on distribution and dispersion in the Hesse
sense; i.e., as they are controlled by envi-
ronmental factors.

Later we shall show that during the
thirties the ecologist turned much of his
attention to ecological aspects of evolution.
His concern lay with such matters as geo-
graphic variation, isolating mechanisms,
natural selection, protective coloration,
regressive evolution, and so on.

The list of books on behavior is pur-
posely short. Despite its inextricable rela-
tion to any ecological analysis or venture,
animal behavior studies per se were matur-
ing as a separate field ("comparative psy-
chology") and thus making notable contri-
butions in their own right.

During the decade economic biologists
became interested in ecology as a solution
for their problems. Also, certain ecologists
got interested in economic biologv. Some of
this effort yielded first-rate ecological re-
search, particularly in the field of biological
control, host-parasite relations, and fisheries
investigation. The books listed document
this point, although a survey of the litera-
ture suggests that the papers published in
journals are more impressive in terms of
intellectual content than are the books.

The interest in theoretical ecologv was
acute during the thirties, but discussion of
this point is best postponed until later.

• Their usage of biome is bv no means
uniform, and is only in part that of the present
work.

Journals

With each decade the number of national
and international journals available for the
publication of ecological data and/or theory
increased. This is well illustrated by the
journals available to the ecologist during
the 1931 to 1942 period. The majority of
these journals contain many articles that
are not ecological. Of the forty-one listed
(p. 70) only four are exclusively ecolog-
ical: Ecology, Ecological Monographs, the
Journal of Animal Ecology, and the Journal
of Ecology. The Journal of Animal Ecol-
ogy was started in England in 1932 and
has been a successful medium for original
research articles. It grew out of the Journal
of Ecology, in which many first-rate articles
on animal ecology had appeared before
1932. In addition to research publication,
the Journal of Animal Ecology has helped
the ecologist to keep abreast of British pub-
lications in the several fields of ecology.*
The Foreword to the first issue is of some
historical interest. There the editor, Charles
Elton, said (p. 1) :

"The number of ecological papers dealing;
with animals is increasing, and \v\\\ imdoubtedly
increase even more rapidly in the near future.
It therefore appeared to the British Ecological
Society that steps ought to be taken now to
make adequate provision both for centralising
to some extent the widely scattered papers on
animal ecology that are now being produced,
and also, by planning well ahead, to anticipate
the future development of the subject, which
runs a real risk of becoming split unnaturally
into isolated compartments of knowledge at-
tached to specific scientific and economic
spheres, and therefore losing the advantaj^es
which come from the pooling of ideas and
knowledge in a central journal."

More or less concomitant with the found-
ing of the Journal of Animal Ecology was
the establishment at Oxford University in
1932 of the "Bureau of Animal Population"

• These "fields" as defined in the Journal of
Animal Ecologtf are: (1) "Ecological surveys
and habitat notes;" (2) "General reports and
taxonomic studies of use to ecologists;" (3)
"Animal behaviour and the action of en\nron-
mental factors;" (4) "Parasites;" (5) "Food
and food-habits;" (6) "Populations;" (7) "Mi-
e;ration, dispersal, and introductions;" (8) "Re-
ports of organizations." In this connection it is
interesting to note that Biological Abstracts also
covers the several fields of ecological literature
(see p. 43), from a less provincial point of
view.

64

THE HISTORY OF ECOLOGY

under the directorship of Charles Elton. An
initial grant from the New York Zoological
Society helped make this possible. Its ob-
jects were to conduct research on mammal
and game-bird populations, and at the same
time to act as a world clearinghouse for
literature and other information about ani-
mal populations and animal ecology gen-
erally. The Bureau has continued and ex-
panded up to the present time and has been
a thoroughly useful institution.

Ecology (founded in 1920) continued to
serve American needs both in plant and
animal fields by furnishing a place for pub-
lication of research data and by acting as
the oflBcial organ of the Ecological Society
of America. In the Foreword to Volume 1
this statement appeared:

"This journal is issued to meet the demand
for the collective publication of articles on
ecology. Its pages are open to all who have
material of ecological interest from whatever
field of biology. While the variety of fields may
cause diversity of treatment, yet the ecological
significance of the papers will make them of
general interest. Specialization is inevitable, but
makes more urgent the need for cooperation.
To approach different subjects from similar
points of view is to lay the foundations of co-
operation."

This is followed by an introductory state-
ment by Harrington Moore, the first editor,
on "The Scope of Ecology."

Ecological Monographs was founded in
193 1 to provide a pubhcation medium
for longer manuscripts covering extensive
studies on both plants and animals, partic-
ularly those written from the community
point of view.

Biological science was characterized gen-
erally during the fourth decade by the pub-
lication of many review articles, numerous
symposia, and critical syntheses of theory.
These eflForts helped scientists keep up with
current trends. We can learn much of the
growth of ecology during the period by
brief examination of these three activities.

Review Journals

The two English language biological re-
view journals of greatest circulation appear-
ing during the period 1931 to 1942 were
the Quarterly Review of Biology, edited at
Johns Hopkins University, and Biological
Reviews, edited at Cambridge, England. If
we tabulate for the former the frequency of
ecological articles relative to the total fre-
quency, the data for ten volumes look like
this:

Table 2. Frequency of Ecological Articles to the Total Number of Articles
Appearing in the Quarterly Review of Biology (1931-40)

Year

Volume

Number of
Articles

Number of
Ecological

Percentage of
Ecological

Number

Articles

Articles

per Volume

per Volume

per Volume

1931

6

20

3

15.0

1932

7

22

6

27.3

1933

8

23

5

21.7

1934

9

16

5

31.2

1935

10

17

2

11.8

1936

11

17

2

11.8

1937

12

21

6

28.6

1938

13

22

2

9.1

1939

14

16

4

25.0

1940

15

14

5

35.7

For the ter

1 volumes. .

188

40

21.3

Thus it is apparent, for this journal at
least, that a good deal of the review and
survey writing of the decade was ecologi-
cal when broadly interpreted. The com-
parable mean percentage for Biological

Reviews is lower, 11.5 per cent. It is not
clear whether this means that English stu-
dents were not so much concerned with
ecological studies as were the Americans,
whether the high percentage of the Quar-

FIRST FOXm DECADES OF THE TWENTIETH CENTURY 65

Table 3. American Ecological Symposia, 1930-42

Title of Symposium

Date of
Presen-
tation
or Pub-
lication

Sponsorship

Published

Central Ecological
Theme

' Hydrogen-ion Concen-
tration"

1930

Ecol. Soc. America

Am. Nat., 69

Determination of and
effect on animals and
plants

' Conditions of Exist-
ence of Aquatic Ani-
mals"

1933

Chicago "Century of
Progress" Exposi-
tion

Ecol. Mon., 4

Lectures on the physi-
cal and biotic en-
vironment in oceans
and lakes

'Oceanography"

1933

A.A.A.S.

Ecol. Mon., 4

Lectures on ocean bac-
teria, copepods and
marine communities

"The Species Problem"

1936

Am. Soc. Zoologists,
Genetics Soc. Amer-
ica

Am. Nat., 70

Factors in the distri-
bution of natural
populations

' Symposium on Experi-
mental Populations"

1937

Ecol. Soc. America,
Am. Soc. Zoologists

Am. Nat., 71

The nature and data
of laboratory popula-
tion analyses

' Plant and Animal Com-
munities"

1939

Cold Spring Harbor
Biological Labora-
tory

Book: Univ. of
Notre Dame
Press

Aspects of plant and
animal communities
and animal popula-
tions

'Symposium on Insect
Populations"

1939

Ecol. Soc. America,
Entom. Soc. Amer-
ica, Econom. Entom.

Ecol. Mon., 9

Aspects of insect natu-
ral populations, in-
cluding control

"Determination of Natu-
ral Population Size"

1940

Ecol. Soc. America,
Am. Statis. Assoc.

Census methods and
their evaluation -'^i

"Relation of Ecology to
Human Welfare"

1940

Ecol. Soc. America

Ecol. Mon., 10

Ecology as a technique
and point of view for
human problems

■'Symposium on Speci-
ation"

1941

Am. Soc. Zoologists,
Genetics Soc. Amer-
ica

Biological Sym-
posia, 2

Ecological factors af-
fecting speciation in
several groups

"Symposium on the Bi-
ological Basis of Social
Problems"

1941

Western Soc. Nat.,
A.A.A.S.

Biological Sym-
posia, 2

Social integration

"Symposium on Popula-
tion Problems in Pro-
tozoa"

1941

Ecol. Soc. America,
Am. Soc. Zoologists

Biological Sym-
posia, 4

Protozoan populations

'Symposium on the
Species Concept"

1941

Western Soc. Nat.,
A.A.A.S.

Biological Sym-
posia, 4

Discussion of natural
selection and insect
speciation

66

/ THE HISTORY OF ECOLOGY

Table 3. American Ecological Symposia, 1930-42 (Continued)

Title of Symposium

Date of
Presen-
tation
or Pub-
lication

Sponsorship

Published

Central Ecological
Theme

"Symposium on Tem-
perature"

1941

Am. Soc. Zoologists,
A.A.A.S.

Biological Sym-
posia, 6

Temperature in rela-
tion to evolution

'Symposium on Isolat-
ing Mechanisms"

1941

A.A.A.S.

Biological Sym-
posia, 6

Isolation in toads, gall
wasps and Droso-
phila

"The Development of
Quantitative and Ex-
perimental Work in
Ecology"

1941

Ecol. Soc. America,
Soc. Amer. Foresters

Ecology, 22

Summarizing papers;
using analytical
methods

"A Symposium on
Hydro-biology"

1941

Univ. of Wisconsin

Book: Univ. of
Wis. Press

32 papers on many
phases

"Symposium on Animal
Populations"

1941

American Soc. Mam-
malogists,
Field Museum

Census methods and

ranges of mammal
populations

"Levels of Integration
in Biological and So-
cial Systems"

1942

Univ. of Chicago
A.A.A.S.

Biological Sym-
posia, 8

Integration in popu-
lations and societies

"Dynamics of Produc-
tion in Aquatic Popu-
lations"

1946

Am. Soc. Zoologists,
Ecol. Soc. America,
Limnological Soc.
America

Ecol. Mon., 16

Population productiv-
ity

terly Review reflected the positive bias of
the editor (Pearl), or whether other factors
are involved.

Symposia

During the decade, particularly the lat-
ter part, the symposium became a more
popular and important medium for the ex-
change of ideas. It allowed speciaUsts to
meet, to discuss a topic of current interest,
and, in many cases, to pubhsh the entire
record of the proceedings. From the histori-
cal view the symposia are excellent criteria
of the temper of the times. They are data
on subject matter and personaUties. In this
light we present the following tabulation
of those American symposia held between
1930 and 1942 (including one held in
1946) that contributed more or less directly
to ecological thought (Table 3).

The following generalizations seem war-

ranted from an examination of the table:

1. There was an active interest in ecolog-
ical problems per se.

2. There was also a keen interest in co-
ordinating ecologic with other phases of
biology.

3. The two topics most frequently dis-
cussed were ecological relations of popula-
tions and ecological aspects of evolution and
speciation.

4. A variety of societies and many inves-
tigators cooperated in the entei-prises.

5. Apparently, no great difiiculties were
encountered in getting such symposia pub-
lished.

Synthesis Articles

Our final method of appraising the eco-
logical trends of the period is to examine
certain articles that were concerned with
synthesizing an aspect of ecological theory

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

67

These articles are not necessarily review
articles. They may merely collate certain
segments of information without any inter-
pretation. During the 1931 to 1942 period
the areas of ecology most frequently sub-
jected to such synthesis were {a) the
community; (i>) population problems, both
intraspecihc and interspecific; (c) society
and social integration; and {d) other
aspects.

There is not time, nor is this tlie place,
to discuss the contributions of these articles,
and others like them, to ecological theory.
That will come later when attention is fo-
cussed on specific principles. In general
terms the point can be made that ecologists
were trying to find a natural pattern into
which the data of ecology could be ap-
portioned. This was true whether the
individual, the population, the society, or
the community was studied. This led theo-
retically minded students to the question of
integration— the mechanism by which an
ecological unit maintains that unity in the
face of continual environmental impact.
Some of the analyses were mathematical,
some experimental, and some observational.
But they were all concerned with this preg-
nant question, and all seemed to suggest
that when ecology attains a greater theo-
retical orientation, it will emerge as a
science of greater stature. As pointed out in
the Introduction, this is a perspective shared
by the authors of the present book.*

CONCLUSION

This concludes our treatment of the
growth of twentieth century animal ecology.
Before closing this chapter, however, a brief
review of the forty years considered as a
whole seems appropriate. It is our wish
here to point out certain of the major his-
torical trends in ecology as well as to draw
some parallels between the growth of that
science and historical phenomena geneially.

In 1900 the basic ecological emphasis
was relatively simple. Most biologists were
aware of the fact that an organism lived in

° An interesting ecological development of
the fourth decade that deserves special mention
was the organization of field classes to study
nocturnal animal communities. Although this
was not a completely new venture, its routine
adoption did not occur until the early thirties.
A note by Orlando Park and H. F. Strohecker
( 1936 ) pointed out the potentialities of such
night study.

an exploitable environment, and now and
then this environment-organism nexus was
subjected to analysis. However, the analysis
was concerned with that problem as an
individual instance. There was not much
interest in generalization or theory We
pointed out this fact in the Introduction to
this book. The early workers, through intel-
hgent and enthusiastic labor, unearthed
many significant data, and it would be
stupid to underestimate their contributions.
As the years wore on, a need arose for the
integration of facts and concepts. This had
a salubrious effect on the development of
ecology. It sharpened the awareness of
workers to the existence of new and un-
solved problems. It brought younger
investigators into the field. It demanded the
adoption of new techniques developed by
other sciences and technologies. It increased
the outlets for discussion, pubhcation, re-
view, criticism, and intellectual intercourse
generally.

The twentieth century now can be con-
sidered briefly in a more specific way. In
the early nineteen hundreds the prime em-
phasis was on autecology. Investigators
followed either the path of natural history,
in which case they were interested, say,
in the life cycle of an organism or in its
habitat or adaptational morphology, or they
entered by the physiological route and
studied the behavior, the development, or
the toleration of an organism in relation to
its immediate environment. With the pass-
ing years, work of this type appropriated
some of the skills perfected in other
sciences, with the result that environmental
measurements became more precise and
refined. This seems to be the status of aute-
cology somewhere in the early twenties.
Thereafter, ecologists became interested in
"conditions of existence," and there arose
a more comprehensive autecology with em-
phasis on the analysis of a wide variety of
organism-environment relations. This had a
final efiFect of incorporating a large body of
autecological facts in text and reference
books, many of which have been men-
tioned.

Synecological studies lagged behind aute-
cological. There is an obvious explanation
for this. The former are inherently more
difficult and require a greater background
of fact and theory. In the early part of the
century there were some sound data on
group relations both for aquatic and ter-

68

THE fflSTORY OF ECOLOGY

restrial fonns, but the data were relatively
few, and, as we pointed out, there was little
attempt to see common denominators be-
tween the operations of one group and
those of another. Ecological succession also
furnished an important impetus for the
growth of synecology. It caused ecologists
to view groups from the long-time vantage
point of development and maturation in
fact, the early workers spoke of this ap-
proach as "genetic."

Early in the century certain botanists and
zoologists began to conceive of bio tic group-
ings as integrated wholes. These they
designated "communities." The community
concept flourished from then on and, for
a time, was identified by some as syn-
onymous with ecology. It reached a mode
perhaps in the late twenties, when overen-
thusiastic workers began manufacturing
names for ecological phenomena at a rate
that exceeded knowledge and denied wis-
dom. Fortunately, this trend is abating, and
today community studies are assuming
saner proportions and are emerging as a
significant phase of ecology. It is clear that
they owe their origin to natural history and
early synecology of the type discussed. It
is equally clear that this phase of ecology
is bringing the botanist and zoologist into
closer cooperation.

An interest in animal aggregations grew
up along with and slightly later than com-
munity studies. This interest dates far back
into ecological history as a descriptive
phase, but it did not attain more precise
treatment until the last two decades We
have shown already how this trend is cur-
rently merging into a general sociology.

Our review of ecological history also un-
covers an urge toward quantification. At the
tvirn of the century research was essentially
descriptive and quahtative, with certain
notable exceptions particularly prevalent
among the marine biologists. Later publi-
cations became increasingly numerical. This
was true both for autecology and .gyne-
cology. The former introduced simple
algebra, geometry, and graphic techniques
borrowed largely from traditional phys-
iology and the physical sciences. The latter
took over the tool of statistical methods
already well developed and applied in other
areas by the biometrician. The adoption of
these methods in synecology not only im-
proved the rigor of the evidence, but
increased as well the ecologist's awareness

of the essential nature of groups and their
properties.

We attribute in part the rise of interest
in natural and experimental populations
during the third decade to this quantifica-
tion. Ecologists apparently reahzed that
many environmental phenomena can be
stated numerically. They then found out
that upon analysis these numbers yielded
conclusions more searching than those
based upon observation alone. Such meth-
odology naturally became part and parcel
of population research (see Thomas Park,
1946).

Another trend worthy of emphasis is the
growth of apphed ecology. Early in the cen-
tury economic problems were largely those
of insect control and fisheries biology. These
problems were usually tackled in a restrict-
ed way. Later, as the economic zoologist
and the ecologist built bodies of knowledge,
we see the two turning to each other for
suggestions and advice. This now reaches a
point among the best modern workers
where data collected by one group are
directly usable by the other. This rap-
prochement is excellent.

In mentioning applied ecology, it should
be recorded here that the activities now
known as "game management" and "wild-
hfe conservation" have appropriated, in in-
creasing measure and to their advantage, a
more circumscribed ecological flavor. These
fields were foreshadowed by the splendid
book entitled The Grouse in Health and in
Disease, edited by A. S. Leshe and A. E.
Shipley (1912), and, latterly, by such
volumes as Game Management by Aldo
Leopold (1933) and H. L. Stoddard's
The Bobwhite Quail: Its Habits, Preserva-
tion and Increase (1932). Then, too, the
work of agronomists, particularly those as-
sociated with pubhc agencies both here
and abroad, has yielded knowledge valu-
able not only for the ecologist (see chap.
16), but for the general problem of con-
servation as well. In fact, we are tempted
to remark that the ecologist, given the op-
portunity, has something to say, both
scientific and constructive, about the urgent
and gloomy problem of conservation and
about the establishment of "nature re-
serves."

Although other trends could be pointed
out, enough has been said to give the read-
er the major features. In closing, we are
impressed once more by the fact that a

FIRST FOXm DECADES OF THE TWENTIETH CENTURY

69

historical development in science parallels
closely the growth of a culture or a civili-
zation. For both, there are fads, fancies,
and cycles. For all, there are good works
and poor works, and occasionally an out-
standing contribution identifies itself. We
can spot ingenuous scholars, plodders, slug-
gards, the industrious, and, frequently,
those who are more noted for what they did
not do or say than for their positive accom-
plishments. Such cross currents as these
obfuscate the story and make it hard to
decipher. But they do give it color and
even humor. It is thus that man-made
things develop, and the history of animal
ecology is no exception to the rule.

APPENDIX

A. Books published between 1931 and 1942,
arranged according to their classification.

1. GENERAL TEXTS OR REFERENCE WORKS

Chapman, R. N.: Animal Ecology with Especial
Reference to Insects, 1931.

Uvarov, B. P.: Insects and Climate (mono-
graph), 1931.

Elton, C: The Ecology of Animals, 1933.

Stork, J. W., and Renouf, L. P. W.: Plant and
Animal Ecology, 1933.

Bews, J. W.: Human Ecology, 1935.

Elton, C: Animal Ecology (2nd edition), 1935.

Hesse, R., and Doflein, F.: Tierbau und Tier-
leben in ihrem Zusammenhang betrachtet,
1935-1943 (2nd ed. by R. Hesse).

Welch, P. S.: Limnology, 1935.

Needham, J. G. (editor): Culture Methods for
Invertebrate Animals ( compendium ) ,
1937.

Bodenheimer, F. S.: Problems of Animal
Ecology, 1938.

Carpenter, j. R.: An Ecological Glossary, 1938.

Clements, F. E., and Shelf ord, V. E.: Bio-
ecology, 1939.

Just, T. (editor): Plant and Animal Com-
munities (compendium), 1939.

Morgan, A. H.: Fieldbook of Animals in
Winter, 1939.

Moulton, F. R. (editor): Problems of Lake
Biology (compendium), 1939a.

Park, O., Allee, W. C. and Shelford, V. E.: A
Laboratory Introduction to Animal Ecol-
ogy and Taxonomy, 1939.

Pearse, A. S.: Animal Ecology (2nd edition),
1939.

Calkins, G. N., and Summer, F. M. (editors):
Protozoa in Biological Research (com-
pendium), 1941.

Needham, J. G., et al. (editor): A Symposium
on Hydrobiology (compendium), 1941.

Sverdrup, H. U., Johnson, M. W., and Fleming,
R. H.: The Oceans: Their Physics,
Chemistry and General Biology, 1942.

2. THE POPULATION

Allee, W. C: Animal Aggregations. A study in
General Sociology, 1931.

H jort, J. ( editor ) : Essays on Population ( com-
pendium), 1933.

Cause, G. F. The Struggle for Existence, 1934.

Lorimer, F., and Osborn, F.: Dynamics of
Population, 1934.

Gause, G. F.: Verifications experimentales de
la theorie mathematique de la lutte pour
la vie (monograph), 1935.

Greenwood, M.: Epidemics and Crowd-Dis-
eases. An Introduction to the Study of
Epidemiology, 1935.

Dublin, L. I., and Lotka, A. J.: Length of Life;
An Introduction to the Study of the Life-
Table, 1936.

Pearl, R.: The Natural History of Populations,
1939.

Simpson, G. G., and Roe, A.: Quantitative
Zoology, 1939.

Elton, C: Voles, Mice and Lemmings. Prob-
lems in Population Dynamics, 1942.

Russell, E. S.: The Overfishing Problems, 1942.

3. SOCIALITY AND SOCIAL ORGANIZATION

Allee, W. C: Animal Life and Social Growth,
1932.

Kostitzin, V. A.: Symbiose, parasitisme et evo-
lution, 1934.

Darling, F. F.: A Herd of Red Deer. A Study
in Animal Behaviour, 1937.

Allee, W. C: The Social Life of Animals, 1938

Darling, F. F.: Bird Flocks and the Breeding
Cycle; A Contribution to the Study of
Avian Sociality, 1938.

Jennings, H. S.: The Beginnings of Social be-
havior in Multicellular Organisms, 1940.

4. ZOOGEOGRAPHY AND DISPERSAL

Rowan, W.: The Riddle of Migration, 1931.

Heape, W.: Emigration, Migration and Nomad-
ism, 1932.

Ekman, S.: Tiergeographie des Meeres, 1935.

Pearse, A. S.: The Migrations of Animals from
Sea to Land, 1936.

Hesse, R., Allee, W. C, and Schmidt, K. P.:
Ecological Animal Geography, 1937.

Moulton, F. R. (editor): The Migration and
Conservation of Salmon (compendium),
1939.

5. EVOLUTIONARY AND SPECIATION
ASPECTS

Sumner, F. B.: Genetic, Distributional and
Evolutionary Studies of the Subspecies of
Deermice (Peromyscus) (monograph),
1932.

70

THE HISTORY OF ECOLOGY

Harms, J. W.: Wandlung des Artgefiiges unter
natiirlichen und kiinstlichen Umweltsbe-
dingungen, 1934.

Prenant, M.: Adaptation, ecologie et biocoeno-
tique, 1934.

Kinsey, A. C: The Origin of Higher Categories
in Cynips, 1936.

Robson, G. C, and Richards, O. W.: The
Variation of Animals in Nature, 1936.

Shull. A. F.: Evolution, 1936.

Dobzhansky, T.: Genetics and the Origin of
Species, 1937.

DeBeer, G. R. (editor): Evolution; Essays on
Aspects of Evolutionary Biology (com-
pendium), 1938.

Banta, A. M., et al.: Studies on the Phvsiology,
Genetics and Evolution of Some Cladocera
(monograph), 1939.

Cott, H. B.: Adaptive Coloration in Animals,
1940.

Huxley, J. (editor): The New Systematics
(compendium), 1940.

Walls, G.: The Vertebrate Eye and Its Adap-
tive Radiation, 1942.

Huxley, T-: Evolution, 1942.

Mayr, E.: Systematics and the Origin of
Species, 1942.

6. BEHAVIOR ASPECTS

Russell, E. S.: The Behavior of Animals; An
Introduction to Its Study, 1934.

Fraenkel, G., and Gunn, D. L.: The Orientation
of Animals, 1940.

Warden, C. T., Jenkins, T. N., and Warner,
L. H.: Comparative Psychology. Vol. 1,
Principles and Methods; vol. 2, Plants
and Invertebrates; vol. 3, Vertebrates,
1935^0,

7. APPLIED AND ECONOMIC ASPECTS
OF ECOLOGY

Stoddard, H. L.: The Bobwhite Quail: Its
Habits, Preservation and Increase, 1932.

Leopold, Aldo.: Game Management, 1933.

Sweetman, H. L.: The Biological Control of
Insects, 1936.

Swynnerton, C. F. M. : The Tsetse Flies of East
Africa (monograph), 1936.

Riley, W. A., and Johannsen, O. A.: Medical
Entomology, 1938.

Herms. W. B.: Medical Entomology, 1939.

Metcalf. C. L.. and Flint. W. P.: Destructive
and Useful Insects: Their Habits and Con-
trol, 1939.

Clausen, C. P.: Entomophagous Insects, 1940.

Dunham, G. C: Military Preventive Medicine,
1940.

3abrielson, I. N.: Wildlife Conservation, 1941.

8. PHILOSOPHICAL AND THEORETICAL
ASPECTS

Lotka, A. J.: Throne analytique des associa-
tions biologiques, 1934.

Kostitzin, V. A.: Biologic mathematique, 1937.

Hjort, J.: The Human value of Biology, 1938.

Wheeler, W. M.: Essays in Philosophical Bi-
ology (a collection edited by G. H.
Parker), 1939.

Several comments are in order about this
list. The reader may ask: Are there treatises on
physical conditions or on communities? The
former is covered in two places: in technical
sources such as handbooks on physiology, bio-
chemist)', meteorology, and so on, and partic-
ularly in the general texts and references. Thus
CliajTman, Uvarov, Welch, Bodenheimer, and
Pearse all enter into such matters in consider-
able detail. Likewise, the community studies
are covered primarily in the general texts.
Elton (1935), Clements and Shelf ord, and Just
stressed this problem.

B. Journals containing ecological articles
published between 1931 and 1942. The list ex-
cludes provincial and governmental bulletins,
weeklies and semipopular periodicals. It is
patently biassed in favor of Enghsh-vmting
scientists. The figure following each title is the
number of the 1935 volume.

Acta Biotheoretica (vol. 1, 1937).

American Midland Naturalist, 16.

American Naturalist, 65.

Annals of Applied Biology, 22.

Archiv fiir Hydrobiologie, 32.

Archiv fiir Protistenkunde, 89.

The Auk, 52.

Biologia Generalis, 11.

Biological Bulletin, 68.

Bulletin of Entomological Research, 26.

Condor, 37.

Copeia (founded in 1913; no volume
numbers).

Die Binnengewasser, 4.

Ecological Monographs, 5.

Ecology. 16.

Entomological Society of America, Annals, 28.

Human Biology, 7.

Hvalradets Skrifter. Scientific results of marine
biological research (founded 1931; no
volume numbers).

Internationale Revue der gesamten Hydro-
biologie und Hydrographie.

Journal du Conseil. Counseil permanent inter-
national pour I'exploration de la mer, 10.

Tournal of Agricultural Research, 58.

Tournal of Animal Ecology, 4.

Journal of Ecology, 23.

Tournal of Economic Entomology, 28.

Journal of Experimental Biology, 12.

Journal of Experimental Zoology, 71.

Tournal of Mammalogv, 16.

Journal of Wildlife Management (volume 1,
1936).

Marine Biological Association, Journal, 19.

Parasitology, 27.

Physiological Zoology, 8.

Population (founded 1933; irregular volumes).

FIRST FOUR DECADES OF THE TWENTIETH CENTURY

71

Quarterly Review of Biology, 10.

Kevista de entomologia, 6.

Royal Society, Proceedings (series B), 119.

Scientia, 29.

Zeitschrift fiir Morphologic und Okologie der

Tiere, 12.
Zoogeograpliica (founded 1932; irregular

volumes ) .
Zoogeographica Argentina (founded 1942).
Zoological Society of London, Proceedings, 105.
Zoologische Jahrbiicher. Abteilung fiir Systema-

tik, okologie und Geographic der Tiere,

67.

C. Review articles of ecological interest
published between 1931 and 1942 in the
Quarterly Review of Biology,

Johnson, G. E.: Hibernation in Mammals, 1931.

Cause, G. F,: Ecology of Populations, 1932.

Gulick, A.: Biological Pecuharities of Oceanic
Islands, 1932.

Allen, W. E.: The Primary Food Supply of the
Sea, 1934.

Cravv^ord, S. C: The Habits and Charac-
teristics of Nocturnal Animals, 1934.

Higgins, E.: Fishery Biology. Its Scope, De-
velopment and Apphcations, 1934.

Severtzott, S. A.: On the Dynamics of Popula-
tions of Vertebrates, 1934.

Pearl, R., and Miner, J. R.: The Comparative
Mortality of Certain Lower Organisms,
1935.

Taylor, W. P.: Significance of the Biotic Com-
munity in Ecological Studies, 1935a.

Cause, G. F.: The Principles of Biocoenology,
1936.

Bodenheimer, F. S.: Seasonal Population
Trends of the Honey-Bee, 1937a.

McAtee, W. L.: Survival of the Ordinary,
1937.

Clarke, G. L.: The Relation between Diatoms
and Copepods as a Factor in the Pro-
ductivity of the Sea, 1939b.

Hammond, E. C: Biological Effects of Popu-
lation Density in Lower Organisms, Part
1, 1938; Part 2, 1939.

Gait, W.: The Principle of Cooperation in Be-
havior, 1940.

Lindsey, A. A.: Recent Advances in Antarctic
Bio-geography, 1940.

Park, T.: The Laboratory Population as a Test
of a Comprehensive Ecological System,
1941.

Davis, D. E.: The Phylogeny of Social Nesting
Habits in the Crotophaginae, 1942.

D. Synthesis articles representative of the
several fields of ecology published between
1931 and 1942. These papers seem to us to be
contributions to thinking as well as to fact
finding. They are arranged according to the
four categories listed on page 67, tull cita-
tion is given in the Bibfiography.

1. THE COMMUNITY

Taylor, VV. P.: Significance of the Biotic Com-
munity in Ecological Studies, 1935.

Cause, G. F.: The Principles ot Biocoenology,
1936.

Lucas, C. E.: Some Aspects of Integration in
Plankton Communities, 1938.

Carpenter, J. R.: Recent Russian Work on
Community Ecology, 1939."

Gleason, H. A.: The Inclividuafistic Concept of
the Plant Association, 1939.

Park, O.: Nocturnafism— The Development of a
Problem, 1940.

2. POPULATION PROBLEMS

Hogben, L.: Some Biological Aspects of the

Population Problem, 1931.
Chapman, R. N.: The Cause of Fluctuations of

Populations of Insects, 1933.
Hjort, ]., Jahn G., and Ottestad, P.: The

Optimum Catch, 1933.
Nicholson, A. J.: The Balance of Animal

Populations, 1933.
Ottestad, P.: A Mathematical Method for the

Study of Growth, 1933.
Allee, W. C: Recent Studies in Mass Physi-
ology, iy34a.
Smith, H. S.: The Role of Biotic Factors in

the Determination of Population Densities,

1935.
Errington, P. L.: What Is the Meaning of

Predation? 1937a.
Ford, J.: Research on Populations of Tribolium

confusum and Its Bearing on Ecological

Theory: A Summary, 1937.
MacLuHch, D. A.: Fluctuations in the Numbers

of the Varying Hare, Lepiis aniericanus,

1937.
McAtee, W. L.: Survival of the Ordinary,

1937.
Pearl, R.: On Biological Principles Afi^ecting

Populations: Human and Other, 1937.

" There has been much Russian work in ecol-
ogy pubhshed during the last ten years or so.
Unfortunately, and because of language diffi-
culties, this is essentially inaccessible to Ameri-
can ecologists. This is a pity. AU concerned
would benefit if the data and conclusions of
such books, papers, and journals could be
studied. Elton recognized the point for English
ecologists in his 1942 book (p. 69) when he
said, "Few scientists outside Russia seem to be
aware of the phenomenal growth of ecological
research under the auspices of the U.S.S.R.,
especially during the last ten years. Even con-
sidered only as a scheme of organization on
paper, these new developments take one's
breath away. A whole generation of well-
trained workers is growing up and beginning
to produce research of a high order. Car-
penter's paper forms a very useful guide to the
organization of this work."

72

THE HISTORY OF ECOLOGY

Hammond, E. C: Biological EflFects of Popula-
tion Density in Lower Organisms, 1938.

Park, T.: Analytical Population Studies in Rela-
tion to General Ecology, 1939.

Thompson, W. R.: Biological Control and
Theories of Population Interaction, 1939.

Rhodes, E. C: Population Mathematics. I, II,
and III, 1940.

Wright, S.: Breeding Structure of Populations
in Relation to Speciation, 1940.

Allee, W. C: Integration of Problems Con-
cerning Protozoan Populations, 1941.

Park, T.: The Laboratory Population as a Test
of a Comprehensive Ecological System,
1941.

3. SOCIETY AND SOCIAL INTEGRATION

Phillips, J. F. v.: Succession, Development,
the Climax, and the Complex Organism:
An Analysis of Concepts, 1934—35.

Emerson, A. E.: Social Co-ordination and the
Superorganism, 1939.

Allee, W. C: Concerning the Origin of Soci-
ality in Animals, 1940.

Child, C. M.: Social Integration as a Biological
Process, 1940.

Gait, W.: The Principle of Cooperation in Be-
havior, 1940.

Gerard, R. W.: Organism, Society and Science,
1940.

Park, O.: Concerning Community Symmetry,
1941a.

4. OTHER ASPECTS

Klaauw, C. J. van der: Zur Aufteilung der
Okologie in Autbkologie und Synokologie,
im Lichte der Ideen als Grundlage der
Systematik der zoologischen Disziplinen,
1936.

Daubenmire, R. F.: Merriam's Life Zones of
North America, 1938.

Hjort, J.: The Human Value of Biology, 1938.

Allee, W. C, and Park, T.: Concerning Eco-
logical Principles, 1939.

Note: Certain of the quotations used in this chapter
have been slightly altered without change of meaning
in the interest of brevity.

SECTION II. ANALYSIS OF THE ENVIRONMENT

4. THE GENERAL ENVIRONMENT

FITNESS OF ENVIRONMENT

We are not here concerned with an imagi-
nary ecology based upon a hypothetical
environment inhabited by fancied organ-
isms evolved in some vaguely conceived
system of life. Such a complex may exist,
for all we know, with a different chemical
and physical basis from the one we have
on the earth. It is sometimes amusing to
speculate on the possibilities of living
systems that may have developed under
conditions of low temperature that obtain,
for example, on the outer planets of our
solar system. If such life exists, its environ-
ment might conceivably be based upon and
largely determined by the properties
of ammonia. This substance boils at
—33.5° C* and has many fitnesses for
being the controlling element in an environ-
ment-organism complex which, in many
features, would not be too far removed
trom that on the earth. There is also the
more remote possibility of metabolizing, re-
producing organisms that live at tempera-
tures well above the upper limits of life
here. The organic chemistry of such
systems might perhaps be based on silicon
rather than on carbon.

Instead of dealing with imaginary situa-
tions, we are confronted by the ecology of
the earth as we know it, populated by or-
ganisms that have evolved here from the
basis furnished principally by water car-
bon dioxide, and their elements, together
with nitrogen (Henderson, 1913). These
substances tend strongly to dominate and
control both the earth's environment and
the life which inhabits it. They are aided
bv many other elements; at least thirty-six
(Fearon, 1933) and probablv forty-six
(Hutchinson, 1943, p. 342) of the ninetv-

* Unless otherwise stated, all temperatures
are given in degrees Centigrade.

six elements that are believed to constitute
the universe are major or minor constituents
of protoplasm. There is suggestive evidence
that the chemical elements essential for life
are not a random lot, but are correlated
with atomic structure (Steinberg, 1938).

On the earth, life requires the fol
lowing environmental conditions (Lafleui,
1941):

1. An available set of chemicals that will
allow variation and reproduction and will
carry on the complex processes of metabo-
lism.

2. A suitable temperature; the high tem-
perature on the average star excludes the
possibility of the organization of molecules
of sufficient complexity to serve as the basis
of life. Cold slows down chemical processes,
so that near absolute zero Life is as impos-
sible as it would be at some hundreds of
degrees higher temperature. Life in general
occurs much nearer the lowest possible
than the highest known temperatures; it is
essentially limited to relatively cold environ-
ments. Living protoplasm in latent stages
has survived temperatures as low as about
-270° C. and as high as 150° C. (see Fig
2). Practically, hfe is limited to the tern
peratures at which water is a relatively
warm solid or a cool to warm hquid, and
exists only in a narrow range of tempera-
tures far below the upper limit for inor-
ganic matter that reaches some thousands
or even millions of degrees (Huntington
1945) . Molten lava aside, life in some form
can exist at most earth temperatures.

3. The proper range of density and pres
sure; the pressure of a cool "white dwarf
star makes molecular organization impos
sible. At the other extreme in the slight
density of a diffuse nebula, it is impossible
for a molecule to collect and align needed
chemical units.

From the preceding three paragraphs it

73

74

ANALYSIS OF THE ENVIRONMENT

follows that a viscous state is necessary
which is not too near an ideal soUd or an
ideal liquid; in the intermediate colloidal
gel and sol we find suflficient solidity to per-
mit organization and enough Hquidity to
allow change. Life, as we know it, is a
matter of the colloidal state.'

ABSOLUTE
4000°

3000^

2000'

000

0^

mz^.

CENTIGRADE

3500° CARBON

1755° PLATINUM

658.7 ALUM IN Ul

100°
0°

-273*

Fig. 2. Lower end of the temperature scale,
showing melting points of carbon, platinum,
and aluminum. The cross hatched space indi-
cates the biokinetic temperature zone; dotted
spaces show temperatures tolerated by some
dry protoplasms. ( Modified from Belehradek. )

4. There must be a source or sources of
energy and of new materials; there is also
a need for controlled reaction rates. Thus in
the liberation of metabolic energy, food
stuffs are burned by oxygen at controlled
rates to supply the body needs. If these re-
actions occurred spontaneously, without
special enzymes regulating the rates, this

* We reserve judgment concerning the rela-
tion of crystalline virus to life in general.

control would be impossible, the burning
would get out of hand, and no sugar or
other food reserves could exist.** Limited
but renewable amounts of all needed mate-
rials and energy must be locally available
to permit hving processes to continue. Thus
the sun's radiation is a source of energy
that reaches the earth in limited amounts,
but which so far has been endlessly re-
newed and shows no sign of becoming
exhausted in the near future.

We have gained a much better under-
standing of energy generation in the sun in
the last few decades. The present age of the
sun is now estimated to approximate two
thousand million years. "During the next
ten-thousand-million years the sun is ex-
pected to increase about a hundred-fold in
luminosity, after which all of its hydrogen
will have been converted to hehum. It will
then rapidly dechne and disappear as a
star of the so-called 'main-sequence.' "t

5. The absorption of lethal ultraviolet
rays of the atmosphere is of great impor-
tance. Life, again as we know it, could not
occur on the earth today if these shorter
abiotic rays were not screened out. Such
rays are produced by the sun, which acts
in this respect as a black-body radiator with
a surface temperature of 6000° C. and an
internal temperature of several milhon de-
grees. Oxygen absorbs wavelengths shorter
than about 200 angstrom units (A), but
is somewhat less effective in screening out
those up to 2530 A. The absorption causes
oxygen to become ozone, which absorbs
waves shorter than 3000 A, though it does
not completely ehminate those longer than
2860 A. Today radiations shorter than 2830
A fail to reach the earth's surface.

This fifth consideration raises some in-
teresting matters that deserve brief atten-
tion immediately. The question whether the
present day type of atmospheric screening
has always existed cannot be answered with
certainty. One set of students think that
oxygen was present in the atmosphere while
the earth was cooling; others postulate a
primeval atmosphere without oxygen. Ac-
cording to the latter point of view, the
condensation of water vapor from the prim-
itive atmosphere made a shallow sea and

** Gerard, R. W., personal communication,
1942.

f Personal communication from Otto Struve,
who cites Gamow (1940).

THE GENEJiAL ENVIRONMENT

75

left a relatively rarefied atmosphere that
was probably free from oxygen and, there-
tore, from ozone.

There is a fair possibility that the early
atmosphere did lack oxygen and that the
gases then present did not act as effective
screens for ultraviolet radiation. If the sun's
full ultraviolet spectrum did reach the pri-
meval earth, some possible effects include
the following:

1. Under the influence of photochemi-
cally active radiations, the relatively inert
chemicals dissolved in the oceans might
well have formed increasingly complex or-
ganic compounds with varied colloidal
structures until, finally, Hving substance it-
self was synthesized. This photochemical
hypothesis avoids certain difficulties im-
posed by the more usual postulation of a
theiTnal activation of the beginning of Ufe.
It is pertinent that ultraviolet radiation is
reported to effect the synthesis of carbo-
hydrates from carbon dioxide and watei
without the aid of chlorophyll (Baly,
1929). Radiations of comparable wave-
length acting on modern genes accelerate
the rate of mutation. Hence, perhaps, we
could expect more rapid evolution in an
environment in which they were effectively
present in graded intensities.

2. If the initial hving material so formed
was similar to present day protoplasm, it
could have remained aUve only in or near
the shadows cast by objects Hke rocks that
are opaque to these shorter solar radiations,
or in other niches where the newly formed
life would not have been exposed for the
whole day to the action of the lethal rays.
Water could have furnished suitable pro-
tection only where it was very deep. It
follows that the presence of such abiotic
rays above the protecting umbrella of the
earth's atmosphere would probably, then as
now, kill cysts and spores that might be
drifting through interplanetary space. It
may be recalled that the theory of the ex-
tramundane origin of the ancestors of all
Ufe now found on the earth has been sup-
ported by various outstanding scientists, the
chemist Arrhenius among them. Photo-
chemical considerations are strongly op-
posed to such a possibility.

The change in the ultraviolet spectrum,
after the production of the oxygen-ozone
atmospheric screen, would account for the
apparent absence of spontaneous genera-
tion of life on the earth today when theory

apparently demands such an origin at some
time in the remote past. This whole fine
of speculation assumes that the oxygen now
in our atmosphere has been largely pro-
duced by pholosynthetic activity of plants
and, hence, that fife itself has played an
important role in estabUsliing its modern
environment. These particular speculations
are developed further by Hutchinson
(1944) and Giese (1945), who cite many
key references.

Oparin (1938) marshalls the evidence
indicating that fife evolved on the etirth
from simple inorganic materials. According
to his reconstruction, the sUghtly cooled
earth had a central molten core containing
metals acquired originally from the sun.
The core was surrounded by "a membrane
of primary igneous rocks" and enveloped in
an atmosphere made up in the main of
superheated steam. Oxygen and carbon
dioxide were not present in the original
atmosphere, but developed secondarily.
Carbon itself first appeared as carbide of
iron and other metals, all coming from the
parent sun. According to these views,
hydrocarbons arose from the action of water
on the metalhc carbides. Nitrogen also ap-
peared on the earth in the reduced state,
probably as ammonia.

Oparin summarized the essence of his
argument as follows (p. 126):

"Hydrocarbon derivatives such as alcohols,
aldehydes, organic acids, amines, amides, etc.,
undergo important transformations when tlieir
aqueous solutions are allowed to stand. In these
solutions the dissolved substances undergo re-
actions of condensation and polymerization, as
well as oxidation-reduction reactions; in other
words, every type of change occurring in the
living cell. As a result, numerous high molec-
ular compounds, similar to those present in
living cells, may appear in aqueous solutions of
hydrocarbon derivatives on long standing."

From these, given more time, comes the
origin of primary colloidal systems and
finally of organisms.

Living protoplasm is not adjusted to
meet the extreme conditions known to exist
within our solar system. Environmental ex-
tremes must not be too great, and the
transition from one extreme to another must
not be too sudden. With life based pri-
marily on water as ours is, the temperature
for active metabolism can range only a few
degrees below to a few tens of degrees C.

76

ANALYSIS OF THE ENVIRONMENT

above zeic These conditions are furnished
by the earth, which rotates on an axis while
revolving about an energy-shedding sun.

In general terms, the earth is a dense,
crusted body of sufficient size to have
strong enough gravitational attraction to
hold an extensive gaseous atmosphere, but
not strong enough to hold more than a
trace of tree hydrogen. The presence of
water and carbon dioxide in the atmosphere
seems to be a normal result of the physical
and chemical properties of water and car-
bon dioxide that have much to do with
regulating the general environment of hving
things on the earth. There is good reason to
beUeve that "water is the substance whose
movement in the organic and in the inor-
ganic world constitutes the first, the most
fundamentally important activity in the
world that we five in" (Henderson, 1922).

Water has a number of remarkable quah-
ties that make it an important factor in the
environment of hving things as well as the
major ingredient of Hving protoplasm. It is
a stable chemical compound that passes
readily through soUd, Hquid, and gaseous
states at what we call ordinary tempera-
tures. The thermal properties of water,
added to its abundance and wide distribu-
tion, make it an important temperature
regulator. Its great power as a solvent, espe-
cially of electrolytes, and its inertness,
which allows many chemicals to pass into
and out of solution readily and without
change, make it an important bearer of
chemical suppfies. The property of expan-
sion before freezing has important effects
upon fife in bodies of water that freeze
over. The high surface tension of water,
among other things, accounts for the rise
of soil water through capillary attraction,
and is important in adsorption, which, with
other properties of water, makes it of high
value in the formation of colloids. There
is also a relatively high order of trans-
parency, mobihty, and incompressibihty. In
a different field, water has a markedly high
dielectric constant and great ionizing power,
Water furnishes the basic environmental
division into aquatic and terrestrial habitats.

Another compound that, with water, is
of greatest importance in fife processes is
carbon dioxide. The environment-control-
ling properties of carbon dioxide are less
important than those of water. Carbon
dioxide enters and leaves water freely; at
ordinary temperatures its absorption coeffi-

cient approaches unity; hence carbon
dioxide can never be wholly washed from
air into water or taken from water into the
air. In water, carbon dioxide forms a weak
acid that adds to the solvent power of
water, and since the acid is dibasic, it has
marked power as a chemical buffer and so
helps maintain a near neutraUty in the
acid-base relations of the environment.

Since carbon dioxide is present as a gas
in the atmosphere and in solution in water,
and since it can readily be extracted from
both sources and also readily enters into
chemical combinations, it forms an impor-
tant nutrient for plants. Under the synthe-
siidng processes, particularly those of photo-
synthesis, carbon becomes the center of a
whole class of chemical compounds that are
so important chemically that they make up
the content of a distinct phase of chemistry,
so-called organic chemistry, which consists
of the chemistry of the carbon compounds.
Carbon has the remarkable abihty of com-
bining with itself to form the basis of
complex molecules which, when combined
particularly with hydrogen, oxygen, nitro-
gen, phosphorous, and calcium, to mention
those that, respectively, compose 1 per cent
or more of the organism (Fearon, 1933),
make a pecuHarly fit system of chemical
compounds for use by living organisms as
sources of matter and energy for the proc-
esses of metabohsm.

We are accustomed to the idea that or-
ganisms show adaptations of fitnesses to the
environment in which they five, and also to
the more general view that, everything con-
sidered, hfe in the large is well adapted to
its generahzed environment. Despite the
fact that the idea is no longer new, many
do not yet appreciate the basic ecological
principle that, given matter and energy
and the resulting probabihty that hfe when
and where it develops will be a mechanism
(a complex mechanism, to be sure), the
surface of a sohd body such as the earth-
placed as it is in relation to a central
energy-giving sun— does actually provide an
excellent general environment for the hving
organism as we know it. It is possible for
the biochemist Henderson (1913, p. 273)
to maintain without successful contradic-
tion to date that this is actually "the best
of all possible environments for hfe."

Certainly the fitness of the organisms,
which, as the idea of adaptation, Claude
Bernard urged should be the basal prin-

THE GENERAL ENVIRONMENT

77

ciple for all physiology, is only one phase of
the relationship. The environment is also
relatively a fit place for life. Reflection
indicates that both phases of this reciprocal
fitness are inherently imperative. The envi-
ronment must have been more than pas-
sively favorable; otherwise hfe would prob-
ably not have originated and persisted. This
is the primary fitness. The general adapta-
tion of organisms to their environment fol-
lows as a necessary corollary.

The developing reciprocity of environ-
ment and organism has produced funda-
mental and far-reaching results. At one
time, probably, the atmosphere of the earth
consisted chiefly of water vapor and carbon
dioxide. Cooling caused the condensation
of most of the water, and geological proc-
esses, aided in recent geological time by
the action of vegetation and the fixation of
carbon in coal and peat, have removed
nearly all the carbon dioxide. This has re-
sulted in the evolution of an atmosphere in
which inert nitrogen forms the greatest
bulk and in which oxygen is the most im-
portant active chemical element.

As a further evidence of reciprocity be-
tween living and nonliving nature, Ver-
nadsky (1929) suggests that all the free
oxygen of the earth (1.5 X 10" gm.) is
produced by life alone. Hence, not only
are organisms acted on by the environment,
but they also react upon it to produce note-
worthy changes to which, in turn, evolving
life mvist adapt itself or perish.

In discussing the general principle of the
fitness of the earth's environment as the
basis of life, certain deficiencies must not
be overlooked that make it less than ideally
fit.* Because of the relatively high opacity
of water, anabolic life is confined to a
relatively thin film near the surface, while
the intermediate reaches and the vast ocean
bottom are sparsely inhabited by sapro-
phytes and scavengers, predators and para-
sites.

The atmosphere, as a result of its low
degree of buoyancv, cannot be used as a
permanent habitat by organisms, and even
its lower reaches can be used as a passage-
way only bv accident or by highly special
ized forms. The entire ocean of air sup-
ports only a sparse and transient popula-
tion near its lower phase boundary. On

• The discussion is based on a personal com-
munication from Dr. William Etkin.

account of the same lack of buoyancy and
also because of the usually strong drying
power of the air, even earth-supported life
is limited to a biosphere which, as a per-
manent habitat for living things, never rises
more than a few tens of meters above
the earth's surface. Because of seasonal and
regional variations in distribution of heat
and water vapor, approximately half of the
terrestrial surface of the earth is an impos-
sible environment except for a sparse popu-
lation of specially adapted organisms. These
environmental deficiencies would not have
had their present values during much of
geological time (p. 8). Cold alone closes
almost all of the interior of one whole
continent, Antarctica, to endemic hfe. The
sparseness of water vapor results in large
areas being inhabited but shghtly; the Sa-
hara desert is an excellent example. Yet,
while recognizing such difficulties with the
earth as an environment for life, we are
reminded by Henderson (1917) that water
is more widely distributed over the face of
the earth than is any other known com-
pound.

To continue with the disadvantages: The
relative stability of many carbon compounds
and their insolubility in water have resulted
in a gradual piling up of carbon in coal
and peat deposits, with a resulting reduction
in the availability of this substance as a
plant nutrient. The stability of nitrogen
closes most of the great atmospheric store to
use by organisms. Such facts indicate that
despite many niceties of fit, the properties
of matter can hardly be said to be gener-
ously above the minimum required for the
origin and maintenance of living systems.

Realizing the importance of these weak-
nesses in the Hendersonian argument we
can still conclude this phase of the present
discussion with another quotation from
Henderson (1914, p. 527) :

"Just because life must manifest itself in and
tlirougfh mechanism, fust because, being in this
world, it must inhabit a more or less durable,
more or less active physico-chemical system of
more or less complexity in its phases, com-
ponents and concentrations, it is conditioned.
The inorganic, such as it is, imposes certain
conditions on the organic. Accordingly, our
conclusion is this: The special characteristics
of the inorganic are the fittest for those general
characteristics of the organic which the general
characteristics of the inorganic impose upon
the organic. This is the one side of reciprocal

78

ANALYSIS OF THE ENVIRONMENT

biological fitness. The other side may be
similarly stated: through adaptation the special
characteristics of the organic come to lit the
special characteristics of a particular environ-
ment, to fit, not any planet, but a little corner
of the earth."

VARIATIONS IN SPACE

The division into aquatic and terrestrial
organisms or habitats is primary for ecol-
ogy. The distribution of large bodies of
water is important, not alone in detennin-
ing the general outlines of the biogeography
of the world, but also in the regulation of
temperature and rainfall. Biogeographically,
the oceans provide highways for the dis-
persal of marine organisms; at the same
time they are barriers for animals of the
land, of fresh waters, and even for many
inhabitants of the shallow, inshore waters
of the sea. The present day distribution of
plants and animals depends both on the
existing configuration of bodies of land and
of water and upon the past history of these
configurations.

Here we come squarely upon an active
controversy that centers about the possible
existence of oceanic land bridges. In their
more extreme forms, the geological prin-
ciples of the relative permanence of the
present ocean basins, based especially on
the principle of isotasy, are sharply op-
posed to theories of transoceanic land con-
nections or to Wegener's idea of continental
drift. The issues involve such matters as
continental and insular isolation, the loca-
tion and duration of routes of travel, and
the methods of dispersal of organisms in
general and in particular.

The distribution of salts in water is
fundamental for large-scale distinctions in
the distribution of aquatic organisms. The
highly saline lakes or lagoons, the oceans,
and the fresh waters of the world form a
series of distinct environments. Gradual
transitions occur, and brackish water makes
a well-known transition between marine
and fresh-water environments.

The general principles and facts concern-
ing the broad temperature zones of the
world are well known. It is not so generally
appreciated that the present zonal climate
is a recurrent, relatively transitory phase
of climatic history. Throughout much of
the time that the earth has been inhabited,
the continents have stood lower in relation
to sea level than they are at present, and

relatively mild temperatures have exLended
into subpolar regions. In other words, the
strong zonal provincialism of present day
temperature belts has usually been re-
placed by a broad uniformity. One of the
unsolved problems of modern world climate
is whether we are now in another inter-
glacial period or are moving toward the
general amelioration of world climates.

The phases of temperature zonation
concerned with life zones will be considered
in more detail later (p. 114). Meantime,
it should be lecalled that many regional
or local factors act to modify the tempera-
ture in a given region from that to be ex-
pected on an idealized globe. Distance from
the ocean is one of the modifying factors.
The ocean is the great temperature regu-
lator of the world. Islands and coastal
areas, in general, undergo relatively slight
temperature fluctuations as contrasted with
the extremes found in the midcontinental
climates at the same latitude. This effect is
quite apart from a second important tem-
perature modification brought about by
ocean currents. The ameliorating action of
the Gulf Stream upon the temperature of
northern Europe contrasts with the chilling
produced by the Labrador Current at sim-
ilar latitudes along the northeastern coast
of America. Winds exert important effects
on the temperature and rainfall of a given
region. Thus, the prevailing westerly winds
accentuate the ameliorating eflFect of the
Gulf Stream on the chmate of northwestern
Europe.

Tropical and subtropical temperatures
are much more restricted along the western
coasts of the continental land masses than
they are on the eastern side. This restric-
tion is brought about either by an upwell-
ing of deeper, cold ocean water or by polar
currents, or by both acting together. Trop-
ical littoral animals are found, for example,
only from the northern coast of Peru, 5 de-
grees or less south of the equator, north-
ward to northern Mexico or southern Cal-
ifornia, a total distance of about 33 to 39
geographic degree (Ekman, 1935) (see
Fig. 3). On the eastern side of the Amer-
icas, the comparable littoral formation ex-
tends from Cape Hatteras and the Bermu-
das at 35 degrees north latitude to Rio de
Janeiro or even to the mouth of the Plata
river at 35 degrees south latitude. The situa-
tion is similar on the two coasts of the
African-Eurasian land mass and on those of

THE GENERAL ENVIRONMENT

79

Australia, although here it is less dramatic.
Another exception to the diagrammatic
expression of global temperature zones is
related to the slope of the land. Effects
of slope and exposure are more obvious on
mountains or hills than on the plains. Even
in level regions in the tundra, however, an
almost imperceptible slope toward the south
may make the difference between a rela-

The world maps of rainfall or vegetation
show a fairly definite moisture zonation
superimposed on that of temperature. From
the equatorial regions northward, with cer-
tain known exceptions, the distribution
shows the following schematized pattern:

1. A belt of heavy tropical rains with
accompanying rain forests lies near the
equator.

10° N

10° S -

10° N

0°S

Fig. 3. The temperature zones become narrower near the west coast of tropical America. ( Re-
drawn from Agassiz. )

tively abundant summer biota and a sparse
community of hardier forms that live on a
similarly slight neighboring slope to the
north.

The character of the soil also affects local
temperatures. Heavy clay soil warms up
much more slowly than does loose, sandy
loam. Alkaline soils tend to be heat ac-
cumulators, and warmth-limited organisms
which grow only on calcareous subsoil in
northern Germany and the British Isles are
not necessarily so restricted in milder
climates.

2. A region of smaller annual rainfall,
with more marked rainy and dry seasons,
supports tropical savannah or tropical
grassland; these formations lie on both sides
of the tropical rain forest.

3. Northward over much of the world,
there is an area of decreasing rainfall that
culminates in the great arid belt that con-
tains the Sonoran Desert of North America,
the Sahara, the Arabian, and Persian des-
erts. Their southern equivalents occur in
South America, Africa, and Australia.

4. Generally, the desert gives way tc a

80

ANALYSIS OF THE ENVIRONMENT

northern semidesert which, in California
and around the Mediterranean, is an area
of winter rain and summer drought.

5. To the north lies a region of moderate
rainfall that supports either deciduous
forests or grasslands in its southern phases
and a round-the-world belt of coniferous
forests at the north.

6. Farther north there is the tundra,
where the rainfall is characteristically scanty
and where even the small amount that does
fall is physiologically unavailable during the
greater part of the year.

7. Finally, as far as land is concerned,
there are the well-developed polar ice caps
in Greenland and Antarctica.

A similar set of conditions can be recog-
nized in the southern hemisphere, although,
associated with the smaller size of the con-
tinental land masses, the rainfall zonation
is not so diagrammatically developed ex-
cept for the polar ice cap in Antarctica.

The distribution of rainfall is strongly
a£Fected by mountain ranges. When these
extend east and west, as do the Himalayas,
the combined rainfall and temperature zon-
ation is accentuated. When the mountains
extend north and south, as do the Rocky
and the Andes Mountains, a secondary pat-
tern of rainfall distribution is established
which, as will be discussed in more detail
later (p. 145), runs at right angles to the
global temperature zones.

The geography of temperature and rain-
fall and of associated factors exerts a strong
influence upon the distribution of species
of plants and animals and of biotic com-
munities that is strikingly shown on the
land. Temperature also exerts a strong pri-
mary influence on the distribution of
marine organisms. The effect of rainfall on
marine life is mainly indirect and acts
through modification of salinity. Areas of
dilution occur along shores and particu-
larly near the mouths of the large tropical
and subtropical rivers where the great in-
flux of fresh water, together with the silt it
carries, inhibits the growth of coral reefs.
The opposite effect may be noted near des-
ert areas, most strikingly in the Red Sea,
which shows the effect of its location in the
great northern desert belt by the high salin-
ity of its waters, 46.5 per mille, as con-
trasted with the 35 per mille characteristic
of the surface waters of the open ocean.

The surface sahnity in the three major
oceans, and for these combined, varies from

a standard value in direct proportion to
the difference between evaporation and pre-
cipitation in the area under consideration.
Although modified by mixing with water
from 400 to 600 meters down, the differ-
ence between evaporation and precipitation
is of primary importance (Sverdrup, John-
son, and Fleming, 1942, p. 124).

Especially on land, other environmental
factors are also differentially distributed
and are important in ecological geography
and physiology. They are usually subsidiary
to the temperature-rainfall complex. Some
of the more important ones include the
length of day and the environmental con-
ditions associated with altitude and sub-
strate.

The distribution of soil types forms an
important basis of endemism in continental
areas, while the presence or absence of
traces of copper, cobalt, or selenium, tc
name no more, in the soil may have impor-
tant ecological effects (p. 221) (Godden,
1939).

VARIATIONS IN TIME

Some major variations in time have been
outhned in the preceding pages, especially
those changes that have accompanied the
evolving fitness of the physical world to
support life. The present discussion will
center about (a) changes in chmate on the
earth during geological time and (b) more
recent and present day periodicities.

Geological Climates

Physical and biological evidence both
indicate that climate during historical times
is a poor key to the more usual world cli-
mates of the past. Probably less than 1 per
cent of geological time has approximated
the essentially glacial climatic pattern that
is familiar to us. Other aspects of the late
Cenozoic and Recent epochs are abnormal.
Mountains are more numerous and stand
higher; continents are larger; there are more
volcanoes; and earthquakes come more fre-
quently than they did during the great
stretches of geologic time. We are living in
a period of geological revolution, of crustal
unrest, such as occurred on a full scale
between the Proterozoic and the Paleozoic
eras and was repeated between the Pale-
ozoic and Mesozoic eras (Brooks, 1926;
Russell, 1941).

Generally speaking, crustal stability,

THE GENERAL ENVIRONMENT

81

low average level of land masses, and wide-
spread mild temperatures have character-
ized the earth during most of geological
time. Seas were more extensive and some-
what warmer, and the Arctic Ocean was
ice-free even in winter. Precipitation was
probably less, but thanks to the higher tem-
perature of the greater proportion of water

The Pleistocene ice age is of more direct
importance for present day ecology than
are the several major glaciations of long-
past geological eras. The absence of a
Pleistocene "continental" glacier from
Siberia and much of Alaska not only af-
fected biotic distribution at the time, but
has had important influence upon the loca-

Lower
Proterozoic

Upper
Pro'crozoic

Hercynian

Alpine

\ Climate

Upper
Carboniferous

Quaternary

Fig. 4. Periods of mountain building and glaciation through the ages. ( Redrawn from Brooks. )

surface, the humidity of the air was higher.
If we can trust the generahzations based on
a correlation of red soil and salt deposits
with aridity, extensive midcontinental des-
erts were also characteristic. On this point
there is a controversy, and perhaps we may
think of these early deserts as being of a ra-
ther mild variety. The intense aridity of
modern deserts seems to be associated with
the high-standing land masses and the zonal
climate to be found in periods of geological
revolutions.

During the more usual conditions the
land areas of the earth probably had a cli-
mate much like that of present day tropical
lowlands, with forests along the coasts and
tropical grasslands in the interior. Toward
the poles, that is, above 55 to 60 degrees
north latitude, climatic zones became evi-
dent, but the shores of the perenniallv open
Arctic and Antarctic Oceans experienced
only mild winters.

The change from a normal geologic cli-
mate to a glacial one is marked for prac-
tical purposes by the formation of a polar
ice cap. An increase on the order of 1.1° C.
in the general temperature of the earth to-
day would eventually make the whole
Arctic ice mass unstable in summer, and,
if long continued, would probably clear the
Arctic seas of ice. Brooks has calculated
that an initial change of about 3° C. at
the critical temperature at latitude 50 de-
grees north would make the diflFerence be-
tween a nonglacial and glacial climate
(Fig. 5).

tion of many plants and animals today. This
last glaciation was marked by four or five
main advances of the ice with intervening
interglacial periods. In the 30,000 to
40,000 years since the last ice retreated
from low-lying regions in the middle lati-
tudes of North America and Europe, the
climate of the northern hemisphere has not
shown a steady trend toward amelioration.
The record is read, in part, from the an-

LATITUDE, DEGREES

Fig. 5. Temperature di£Ference between non-
glacial and glacial climates. (Redrawn from
Brooks. )

82

ANALYSIS OF THE ENVIRONMENT

nual layers of clay interspersed with coarser
materials deposited on the bottom of lakes.
These are called varves. The finer clays
settle slowly in the quiet water under the
ice in winter, while coarser stuflF is held
back until spring and summer. The varves
in the Scandinavian lakes have been fol-
lowed for some 13,700 years.

Tree rings have also been studied for the
light they throw on cHmatic history. As yet,
tree ring analysis covers a much shorter
period of time. Tree rings must be inter-
preted with care, since they represent, not
annual rings necessarily, but merely alter-
nating periods of rapid and slow growth. A
severe midseason drought following a good
growing period would produce a good
growth ring; another good growing season
in the same year would produce another
supposed annual ring. Also, we know that
damage caused by insects, lightning, fire,
frost, intense heat, excessive snow, sleet,
wind, and so forth, as well as drought, may
aflFect the rate of growth of trees and so
tend to modify the width of the rings of
growth (Antevs, 1938).

Past climates can also be reconstructed in
part from the succession of plant types in
peat, from ecological evidence of the shift-
ing position of the tree line in mountains or
in the far north, from the recovery of re-
sistant pollen grains in bogs, from the study
of tools, weapons, bones, and kitchen mid-
dens of men. Finally, there is the brief
period covered by more or less trustworthy
human documents.

Humphreys (1942) has a brief word to
say about one cause of long-time climatic
changes. At present the earth is nearest the
sun during the first week in January and
farthest away during the first week in July.
The difi^erence in distance, if long contin-
ued, would modify the temperature on the
earth about 4 degrees. If conditions were
reversed, as they actually were about
10,500 years ago and will be again in about
that period of time, the temperature con-
trast between summer and winter would
be definitely greater than it now is, espe-
cially in the northern hemisphere, which
contains most of the land mass of the earth.
Under present conditions of this long cycle,
winters in the northern hemisphere are
shorter and milder and summers are longer
and also milder, and the climate in general
is more equitable in our part of the globe
than would be so in any other earth posi-

tion with respect to this motion of the peri-
helion and precession of the equinoxes.

The study of climatic history since che
last glacial retreat, the Recent epoch ot
geologists, has been most pursued in
Europe. A frequently accepted summary of
the existing evidence, the so-called Blytt-
Sernander hypothesis, follows: The retreat
of the ice begun some thirty to forty thou
sand years ago and continued fairly steadily
until about 12,000 B.C.* This time of gla-
cial recession was followed by a sub-Arctic
period that lasted about 4000 years until
near 8000 B.C., when the ice had re-
treated suflBciently to allow sea water to
enter the then fresh- water Baltic lake.

Then came a warmei Boreal period char-
acterized in the Baltic area by the develop-
ment of the so-called Yoldia fauna (or com-
munity), in which the bivalve mollusk
Yoldia arctica was prominent (today this
species is restricted to salt waters that have
a temperature of 0° C. or lower). On land
the Boreal period was marked by a north-
ward movement of forests. About 5000 B.C.
the Baltic began to support animals that
live today in waters warmer than those we
now find in the Baltic Sea. This is called the
Littorina period, so named for the snail that
is prominent in the deposits of the place
and time; several species of this genus now
inhabit the shores of the north temperate
ocean. This Atlantic period lasted until
about 3000 B.C. The climate was gener-
ally warm and moist; all the mountain gla-
ciers disappeared from Europe and from
much of North America. The Atlantic peri-
od marks the climax in amelioration to date
since the last glacial retreat.

A drier sub-Boreal period followed that
came down to about 1000 B.C., but was
interrupted by floods some 300 years earlier.
It is supposed, according to the Blytt-
Sernander hypothesis, to have given way to
a milder sub-Atlantic period, which was
in typical development between about 850
and 300 B.C. The existence of the sub-
Atlantic period is questioned by some who
think that there has been a general deterio-
ration of climate from the Atlantic period
to the present, which, however, has been
interrupted by relatively small swings in
temperature and rainfall (Sears, 1935;
Trewartha, 1940).

* Deevey (1944) follows DeGeer in giving
a somewhat different time scale.

THE GENERAL ENVIRONMENT

83

Minor fluctuations of climate continued.
In the first century A.D., climatic condi-
tions were similar to those found today.
From near the end of the second to the
middle of the fourth centuries, the climate
was wet. The fifth century was dry, and
the seventh was both dry and warm, so
that passes in the Alps were in use that are
now closed by glaciers. Heavier rainfall
came in Europe near the start of the ninth
century, but Nile floods were low until
about 1000 A.D.

Warmer, drier conditions returned to Eu-
rope during the tenth and eleventh cen-
turies. Greenland was settled in 984 A.D.
and was abandoned at the beginning of
the fifteenth century. During that period its
cUmate is generally thought to have been
milder than it is today. In Europe, the thir-
teenth and fourteenth centuries were cold
and wet. Amelioration must have set in, for
the glaciers of Chamonix were small in
1580, but advanced rapidly until the
middle of the seventeenth century; then a
retreat began that lasted until 1770, when
they again advanced up until the middle
of the last century. Since that time the
glaciers have retreated approximately to
the positions held in the sixteenth century
(Brooks, 1926; Russell, 1941).

The latter part of this somewhat detailed
summary is often condensed as follows:

1. The Boreal period: warm, dry, continental
climate; birch and pine were dominant trees.

2. The Atlantic period: warmer, moist,
oceanic climate; oaks were dominant trees.

3. The sub-Boreal period: warm, dry con-
tinental climate; oaks continuing dominant.

4. The sub-Atlantic period: cool, very wet,
oceanic climate; beech and spruce were dom-
inant trees (Clements and Chaney, 1936).

The scheme may be still more simplified
to give only three stages (von Post's hy-
pothesis) of postglacial climates, namely:

1. A period of increasing warmth,

2. A period of maximum temperature, and

3. A period of fluctuating, but, on the whole,
decreasing temperature.

Climates in other parts of the world may
or may not follow the European pattern.
The climatic sequence in eastern North
America can be correlated in a general way
with that of Europe. The correlation is as
close as could well be expected, since east-
ern North America gets its climate from
the interior, while, in contrast, western

Europe is under strong marine influence. In
addition, European climates have been
much affected by the complicated history
of the Baltic Sea. The three stages of the
relatively simple von Post's hypothesis cor-
respond fairly well on the two sides of the
North Atlantic, and perhaps a still closer
correlation exists, as shown in depth pro-
files of pollen preserved in bogs; this is out-
hned in Table 4.

Table 4. Possible Climatic Correlation between

Western Europe and Eastern North America

(From Deevey, 1944, after Sears)

European
Periods

Vegetation,

Eastern North

America

Climate

Sub-Atlantic

Oak-chestnut-
spruce
(Oak-beech)

Cool, wet

Sub-Boreal

Oak-hickory

Warm, dry

Atlantic

Oak-hemlock
(Oak-beech)

Warm, moist

Boreal

Pine

Warm, drj^

Pre-Boreal

Spruce- fir

Warm, dry

Arctic
Sub- Arctic
Arctic

Missing in Nor
diagrams

th American

Shifts in the location of the tree line to
the south of the tundra and in mountains
also gives evidence of general climatic
trends. According to this criterion, there
seems to be evidence that, at present, trees
are advancing in Alaska, retreating in
southeastern Mackenzie, and apparently re-
treating in eastern Canada. The resulting
picture of current trends is by no means
clear (Raup, 1941).

Two generalizations stand out as a result
of this hasty review of past climates. The
first is the reiterated statement that the
present zonal climate, which our experience
and records indicate is normal, is highly
unusual when viewed with geological per-
spective. Through long geological eras there
has been climatic cosmopolitanism rather
than present day climatic provincialism. The
second generalization, a corollary of the
first, is to the effect that modern cUmates

84

ANALYSIS OF THE ENVIRONMENT

are unstable and have varied much even in
recent millennia and centuries.

Periodicities

Many local environmental variations re-
cur with regular rhythms, while others are
arrhythmic. The most obvious of the rhyth-
mic variations, that of day and night, is
beginning to attract the attention from
ecologists that it deserves. The day re-
presents a period of increased heat and
convection currents, as well as of in-
creased light; there is also typically a de-
crease in relative humidity. Frequently,
there are associated phenomena such as the
local changes in wind velocity and direction
that occur especially near the seashore, in
mountains, and near forest margins. Many
of these daytime changes markedly increase
the evaporating power of the air. Important
consequences of diurnal rhythms will be
discussed later.

Tides run on a shorter period. They are
periodic variations in the water level pro-
duced by the response of water particles
to the attraction of the moon and .sun.
Tidal streams result that may attain consid-
erable velocity in the shallow waters over
shoals such as those of the Newfoundland
Banks or in the neighborhood of land. The
tidal currents usually follow the direction
furnished by natural channnels, if any are
present; they become more rapid and the
tide rises higher near the head of V-shaped
arms of the sea. The len2;th of the ebb
usually equals that of the flow of the tide,
and the currents near land are in the oppo-
site direction during the two tidal phases.
In the open sea, the height of the tide is
much reduced, the rate of movement is
slower, and the general direction may be
rotary.

The oscillatory tidal movement of the
water has a normal period of 12.5 hours
(Harvey, 1928). Longer tidal rhythms also
exist. The simplest of these is the occur-
rence of a lunar cycle in tidal amplitude
in which the high spring tides occur each
fortnight when the sun and moon are ex-
erting supplementary influences. Between
the periods of spring tides, there are the
lower neap tides that come when the two
governing bodies are working more or less
in opposition to each other. The grunion.
Laureates tenuis, a small smelt of the Cal-
ifornia coast, exhibits an annual breeding
cycle that is related to this longer tidal

rhythm (p. 544; Thompson and Thomp-
son, 1919; Clark, 1925).

Many animals of the marine littoral re-
gion have lunar periodicities in their breed-
ing activities that are less obviously related
to the forces operating during a lunar cycle.
Corals, various mollusks and marine poly-
chaete worms, among others, show such
relationship. Two types of these lunar peri-
odicities have been described for annelid
worms. In one, successive breeding periods
occur during the summer season, and each
lunar cycle shows two peaks of abun-
dance. Thus, Nereis limbata at Woods
Hole, Massachusetts, ordinarily live as elon-
gate worms in burrows; they emerge
during their breeding period as short,
compact, actively swimming forms that are
only a fraction of their usual length. Each
so-called run begins near the time of the
full moon, increases to a maximum on suc-
cessive nights, falls to a low point about
the third quarter of the moon, increases to
another maximum, and finally all swimming
worms disappear shortly after the new
moon. A new run starts about the time of
the next full moon, and this double cycle
is normally repeated four times during the
summer (Lillie and Just, 1913; Townsend.
1939).

A second type of lunar periodicity occurs
when a single annual breeding swarm
makes its appearance in accordance with
some phase or phases of the lunar cycle.
The Atlantic palolo, Leodice fiicata, of Ber-
muda and the West Indies inhabits coral
reefs and spawns most abundantly during
late June and July at about the third quar-
ter of the moon, less commonly about the
first quarter. There is thus good evidence of
an internal or annual rhythm, and yet the
time of spawning is partially under di-
rect environmental control. It is delayed by
water turbulence and by lunar influence.
The cavisal factors are still obscure; neither
changing nutritive conditions, such as may
be associated with the tidal cycle, nor
changing hydrostatic pressures are impor-
tant. There seems to be a direct effect of
moonlight (Clark, 1941, 1941a).

When the average duration of illumina-
tion is increased, spawning is hastened; it
is retarded when the duration of exposure
to moonlight is decreased. If the length of
exposure to moonlight were the only factor
involved, spawning would increase to a
maximum near the time of the full moon

THE GENERAL ENVIRONMENT

85

and then decrease. As we have seen, how-
ever, the lunar periodicity of spawning in
L. fucata is bimodal, and the maxima he
about the first and last quarters, when the
duration of illumination is first increasing
and later decreasing. Something more than
a simple quantitative relationship is in-
volved. One factor that varies as does
swarming is the rate of change in the dura-
tion of moonlight. This reaches a maximum
near the time of the new and the full moon
and a minimum at the first and the third
quarters. Descriptively, then, for the At-
lantic palolo, the eflFectiveness of moonUght
seems to be correlated with some aspect of
the daily rate of change of duration. The
swarming of other annelids may be initiated
by other factors, such as a variation in the
intensity of moonlight or a change in some
direct efi^ect of the tidal cycle. At this point,
as in many other aspects of ecology, we
await further field and laboratory analyses.

The angles made by the moon and sun
with the plane running through the earth's
equator vary independently, and so does
the distance of each from the earth. The
resultant forces exerted by the two bodies
on the waters of the earth vary in a complex
fashion, one result of which is that in
addition to the daily and lunar tidal cycles,
seasonal high tides also exist that have
their due eflFect on organisms. Other tidal
compUcations may be important locally
or along long reaches of the seacoast; dis-
cussion of these does not fit into our
crowded outfine (Harvey, 1928). Some of
the complications, as well as the funda-
mentals, are treated simply and with in-
sight by Coker (1947).

Seasonal cycles in tidal amplitude and
their efiFects on littoral marine communities
are insignificant in comparison with the
seasonal changes on terrestrial communities.
As stated in Chapter 2, the study of phe-
nomena associated with seasonal appear-
ance, or phenology, has a long history. In
much of the tropics, the annual changes
are governed by rainfall and associated
factors rather than by temperature, which
exerts a controlhng influence in higher
latitudes. An intermediate climate, dom-
inated by winter rains and summer drought,
occurs typically around the Mediterranean
Sea and in much of CaUfornia. Many other
seasonal variations in climate produce dis-
tinctive efiFects upon biotic communities.

Seasonal appearance does not necessarily

follow the four conventional seasons even
in a region where temperature is a major
element in the annual cycle. In woodlands
associated with the prairie peninsula in
IlUnois, it is often possible to recognize six
seasonal subdivisions of the biotic com-
munity; these are outfined on page 53.
On the south side of the equator, in cut-
over and primeval mountain forests in the
state of Rio de Janeiro, Brazil, Davis ( 1945,
p. 294) also found the year divided into
six comparable seasons. The time hmits
in such subdivisions are only approximate
and may vary widely in diflferent years.
The exact number of seasonal subdivisions
may also differ according to the community,
the geographic and physiographic location
of the community, according to the organ-
isms used as index species and according to
individual judgment as to the time hmits
(Clements and Shelford, 1939; Wilhams,
1936).

Other Cycles

More than fifty environmental periodic-
ities had been listed by 1925; these varied
in length from a few days to nearly two
centuries. Others have been added since
that time. Cycles of solar radiation are fre-
quently discussed and are highly variable
in duration and intensity. Among others,
they include recurring periods of seven,
eight, eleven, twenty-one, twenty-five, forty-
five, and sixty-eight months' duration.
The last-mentioned runs for about 5.7 years
and is approximately half the length of the
sunspot cycle of eleven ^- years. All may be
regarded as submultiples of the cycle of
magnetic change on the sun that has a
periodicity of 276 months, or twenty-three
years. A still longer cycle, that of Briickner,
lasts from seventeen to fifty years, with a
mean length of about thirty-five years. This
may be thought of as a threefold multiple
of eleven + years or as an effect of inter-
ference between this particular sunspot
cycle and another of somewhat shorter
duration.

The literature on such cycles continues
rich in quantity and varied in quality. There
seems to be some evidence of mind-set in
discussing these problems, and judgments
differ concerning the ecological importance
of many of them. Clements and Shelford
(1939), Elton (1942) and Huntington
(1945), to mention only a few mature stu-

86

ANALYSIS OF THE ENVIRONMENT

dents, are usually more or less favorable.
On the other side, Russell (1941, p. 92)
wrote: "Though firm advocates of climatic
cycles will sharply disagree, such facts as
we possess today neither definitely demon-
strate nor disprove the existence of any
real cycle. Such climatic variability as has
been observed may be explained as result-
ing from random fluctuations."

rhe sunspot cycle of shghtly more than
eleven years has attracted much attention
from ecologists and others. The underlying
causation ot this cycle is still unknown. The
cycle itself consists of the periodic varia-
tion in numbers of sunspots and is charac-
terized, in part, by the tendency to remain
at one length of period during a number
of repetitions and then to shift to some
other value that is again repeated for a
time. Since 1750 the periods have varied
from approximately eight to sixteen years.
Even average values vary between 11.13
and 11.6 years, and a period of 10.2 years is
seriously advanced for the sunspot series
between 1615 and 1788 A.D. (Douglass,
1936). The variation reflects the continuing
inexactness of the basic data regarding
weather and chmate (as well as population
density), combined at times with the ac-
ceptance of indications as a substitute for
rigorous proof.

Solar radiation appears to be less when
there are few (or many) sunspots; a maxi-
mum of radiation is reached when the sun-
spot number is about 100. It appears that
the temperature at the earth's surface tends
to be highest when the actual solar radia-
tion is least during this particular cycle of
radiation. The reasons for this paradox are
not yet wholly clear. Shifts on the order
of 1 or 2 per cent in intensity of radiation
are matters of record. If other conditions
remained constant, as they would not do,
an increase of 1 per cent in solar radiation
would produce a rise of about 0.75° C. in
the mean temperature at the earth's sur-
face, since this temperature varies essen-
tially as the fourth root of the intensity of
the radiant energy received from the sun.
The reasoning that other conditions would
not remain stationary while the inten-
sity of solar heat varies is based, in part,
on the knowledge that resulting variations
in tempera. Mre bring about important
changes in atmospheric pressure, and the
final effect is to decrease temperature in
areas cold for their latitude, while those

wann for their latitude have increased
warmth (Brooks, 1926).

It has been estimated that temperature on
the earth might vary about 0.6° C during a
sunspot cycle. Small as this amount is, it
represents an appreciable fraction of the
lowering of temperature that would bring
about an ice age. A more recent test of the
correspondence between sunspot cycles was
made by using temperature records from six
scattered tropical stations, covering a period
of fifty-eight years. Tropical stations were
chosen, since many writers have stated that
the closest correlation between sunspots and
weather is to be found in the tropics. When
the available records were combined in
cycles equal to the sunspot cycle of eleven
years, a mean temperature ampHtude of
0.22° C. was found. The correlation be-
tween sunspot number and the annual tem-
perature was found to be —0.37, a correla-
tion which, although low, probably indi-
cates statistical vaHdity (Elton, 1924;
Adams and Nicholson, 1933).

Brooks (1926, p. 409) summed up the
situation about sunspot cycles as follows:
"The most perfect example of a solar rela-
tionship hitherto found in purely meteor-
ological data is shown by the level of equa-
torial Lake Victoria. Generally speaking the
eleven-year cycle is characteristic of equa-
torial regions while the thirty-five year
Briickner cycle is characteristic of higher
latitudes. The amplitude and regularity of
the eleven-year cycle decreases toward the
poles, those of the Briickner cycle increase
from the equator toward the North Pole at
least."

The most discussed biotic cycles include
(1) the lemming and mouse cycle of three
to four years; (2) the varying hare and
lynx cycle of somewhat less than ten years;
(3) a cycle corresponding to the sunspot
cycle of somewhat more than eleven years
which we have been discussing; and (4)
another cycle corresponding to the Briick-
ner cycle of about thirty-five years.

As critical studies accumulate, it becomes
difficult to discover biological phenomena
exactly coinciding with the last two cycles,
even as it is difficult to find a sound envi-
ronmental periodicity that corresponds with
the first two cycles just listed. Goldie's
(1936) suggestion of maxima as related
to the mean cycle of annual air drift over
the northern part of the British Isles that
recurs at an interval of somewhat less than

radiation: a general introduction

87

four years remains for the present a
suggestion only. Clements and Shelford
(1939), although they are, in general, fa-
vorable to the idea of a correlation between
the eleven-year sunspot cycle and biological
events, are able to cite few well authenti-
cated cases, and they emphasize, rather,
cycles that are near or under ten years in
length. Elton (1942), following MacLuiich
(1937) and his own unpublished data, has
definitelv abandoned the suggestion that
the rabbit cycle of the Canadian forests cor-
responds to the eleven-year sunspot cycle.
Even the oft-cited cycles in tree rings of
the giant sequoias of California were re-
ported by Huntington (1932) to supply
"another type of evidence of this same cycle
of about ten years." Douglass (1936) re-
cords cycles in tree rings of 5.7, 8.5, 10, 14,
17, 19 or 20, and 23 years and "certain

cycles close to 12 years in length." It is
perhaps worth noting that the much-dis-
cussed eleven-year cycle is not listed in this
latest summary. This point seems to trouble
Douglass (1936, p. 132), who remarks that
"the disturbing feature in all comparisons
between solar and terrestrial cycles has been
the presence of other cycles on the earth
of yery different lengths and only rarely one
of 11 years." Because of his hypothesis of
a cycle complex, he concludes, however,
that "We feel justified in assuming the
hypothesis that there is a physical relation-
ship between our climatic conditions and
the sun." Elton (1942) records his behef
that we will eventually be led "back to very
curious meteorological and perhaps astro-
nomical processes as well as to new rela-
tions between climate, physiology, and
disease."

5. RADIATION: A GENERAL INTRODUCTION

The eflFective environment is holocoenotic;
it is a whole composed of many parts as a
rope is made of many strands. For the next
several chapters holocoenotic aspects will
be mainly disregarded, and the approach
will be frankly analytical; near the end of
the discussion, however, an attempt will be
made to bring the strands together again
into a unit. For the moment we will focus
on one factor or on one set of factors at a

time.

RADIATION

Radiation that reaches the earth from the
sun as heat and light has obvious impor-
tance for living things. All functional ecol-
ogy is closely related, directly or indirectly,
to the capture of radiant energy that origi-
nates in the sun. Radiations are transmitted
in straight lines and are usually thought of
as consisting of waves or pulsations which
although of different lengths, travel at a
velocity of about 3 X 10'" cm. /sec. Some
phases of the physics of radiation are most
readily explained on the assumption that
the radiating units are corpuscles rather
than waves. This phase of the matter can
be left to the physicists, since ecological as-
pects can be stated with approximate
accuracy in terms of the wave theory.

The lengths of the waves, or pulsations,
differ tremendously. They extend from the

long waves of radio, thousands of meters in
length, to the x-rays, gamma rays, and
cosmic rays only a small fraction of an
angstrom unit long (an angstrom unit (A)
equals 1 X 10"* cm.). Those of known
ecological significance are (a) the infra-red
rays that are important for the heat they
carry and that range from about 0.1 mm.
(100 M') or somewhat longer to 7700 A
(1 H = 10,000 A) and are not visible to the
human eye. Then (b) comes the narrow
octave that we know as light; this extends
from 7700 to 3900-4000 A and transmits
heat as well as light. The exact limits of
visible light yary from person to person and
from one species of animal to another. Be-
yond these are (c) the ultraviolet rays,
which, like those of the infra-red region,
are invisible to man. Solar radiation re-
ceived at the earth's surface extends from
about 135.000 to about 2860 A and lies
mainly ^^^thin the wave lengths of 30.000
and 3000 A. There is a sharp maximum at
4700 A. The earth radiates as well as re-
ceives radiations. Coming from a cool body,
these lie mainly between 40.000 and
500,000 A (4-50 n), with a maximum at
95.000 A.

Water vapor absorbs solar radiation dif-
ferentially, with the absorption mainly
taking place in wave lengths of 8000 A or
longer, a region that lies well beyond the

88

ANALYSIS OF THE ENVIRONMENT

point of maximum intensity of incident
radiation. The absorption on a clear humid
day rarely amounts to more than 15 per
cent of the incident energy. Thus 85 per
cent of radiations from the sun that are not
stopped by other causes pass the water
barrier in such an atmosphere. In contrast,
water vapor absorbs almost all the terres-
trial radiation. If the atmosphere holds only
the equivalent corresponding to 1 cm. pre-
cipitation, it absorbs 72 per cent of the

TOTAL RADIANT ENERGY

The mean value of the amount of radia-
tion received from the sun at the upper
level of the earth's atmosphere is 1.94 gm.
calories per square centimeter per minute.
This is called the solar constant. If this
amount of heat could be absorbed and re-
tained, it would warm a layer of cool water
1 cm. deep at the rate of 1.94° C. per
minute. The atmosphere screens out inci-

en °

I-

2! oo
o mi;-

— fON

-■y RAYS— »

■X-RAYS-

ULTRA

VIOLET

♦ ksUISHJ *

INFRA

L

RED

VISIBLE

•HERTZIAN

Fig. 6. The electromagnetic spectrum. (Redrawn with slight changes from Heyroth's revision

of Ellis andWeUs.)

earth's radiation. This phenomenon is called
the "greenhouse effect" and acts so that
solar radiation is transmitted and the earth's
radiation is retained. The effect is still
strong in a relatively dry atmosphere.

Scattering and reflection brought about
by dust particles in the atmosphere produce
an "inverse greenhouse effect." The sun's
radiation is screened out by such particles,
and the earth's radiation is not affected. The
"greenhouse effect" results in a warmed
earth, and the "antigreenhouse effect" pro-
duces a lowering of the surface temperature
(Laurens, 1933). The portion of the sun's
ultraviolet radiation that passes through
the earth's atmosphere approximately coin-
cides with the so-called near ultraviolet.
The middle and extreme ultraviolet rays
have many biological effects and great
theoretical value, but so far as we now
know they are not important in outdoor
ecology. The parts of the whole radiation
spectrum that are ecologically significant
will be considered in the following chapters
in the order of their decreasing wave-
lengths.

dent energy the more, the greater the
distance of air mass that is traversed, the
greater the amount of water vapor in the
air, and the more dust (Brooks, 1926), The
amount of energy that reaches the earth's
surface is also affected by the distance of
the earth from the sun and by variations
in the energy radiated by the sun. Other
conditions being equal, the solar radiation
received in early January is about 7 per
cent greater than that of early July, since
the earth is nearer the sun in January (see
p. 82).

The amount of water vapor in the atmos-
phere decreases, in general, with latitude
and distance from the ocean, and increases
with temperature. Radiation intensity is de-
creased on the order of 2 per cent by an
increase of 1 mm. in water vapor pressure.

The intensity of solar radiation differs
greatly at different points on the earth and,
at the same point, at different hours of the
day or night. At Washington, D, C, 127
meters above sea level, the amount of
energy received at noon is on the order
of 60 per cent of the mean solar constant.

radiation: a general introduction

89

The value falls to about 10 per cent of this
constant when the sun stands just above
the horizon (Kimball and Hand, 1936).
Then the rays pass through 14.5 times the
air mass that they have to traverse at noon.
These radiations were measured at right
angles to the rays of the sun. For many
ecological purposes, the total amount of
radiation, both direct and indirect, is more
important. This is better approximated by
using the vertical component of the total
solar radiation that falls on a given point.

for a given interval of time can be calcu-
lated from the formula:

Q. = Qo[a +(1.00 -a)S]

in which S is the percentage of possible
hours of sunshine; Qo is the radiation re-
ceived from a clear sky, and Q, is the
amount received from a more or less over-
cast sky; a is a so-called constant the
value of which varies with the character
of the clouds, with dust in the atmosphere.

Jan.

21

Feb

20

Mar.

22

Apr
21

May
21

June
20

July
20

Aug
19

5ept
18

Oct.
18

Nov.

17

Dec

17

y

~~,

N.

^

/

/

/

N

\

N

■■

/

^

—

—-.

^

\

N,

..

/

'

/

ty

--

-

^

s

N

\

\

\

8

/

/

/

V

/

/

P

^

"S

■v,

\

s

k,

\

\

\

s,

\

\

/

/

C

/

/

^

-L

—

\

\

\

N

\

S,

\

s

'/

/I

/

/

y^

^

^

— .

■^

>

\

\

\

s

\

N

N

y

/

1

/

/

/

/

/

z'

/

/

y

\

1

N

\

N

\

\
\

^

\

\

\

^

■•

3
2

^

K

^

/

k

/

/

/

^

\

\

S

\

\

^

^

!-,

=

^"

kJ

■"y

/

/

y

/

\

\

^^

^

^

^

^

^

y

v

s

>

-^

==

:=

=

-^

1

^

~-

L-

"

r

•1000

900
800
700
600
500
400

300
200
100

Fig. 7. Smoothed annual variation in the total radiation received on a horizontal surface.
A, Oatside the atmosphere at the latitude of Washington.

Cloudless sky: B, Twin Falls, Idaho; C, Washington.

Average cloud conditions: D, Twin Falls; E, Washington; F, Chicago. ( Redrawn with modi-
fications from Kimball and Hand.)

The radiation received may consist of (a)
direct sunhght, {h) diffuse sky radiation,
skylight, and (c) radiation from trees or
other objects of the environment.

The distribution of solar insolation is
such that only two-fifths as much heat is
received per unit surface at the poles as at
the equator, and the polar ice and snow
cap may reduce the effective insolation still
more (WiUett, 1931).

All solar radiation is much affected by
the amount of cloudiness in the earth's at-
mosphere. In general, the proportion of
direct sunlight varies inversely with the
amount of skyUght. The effect of cloudiness
on the vertical component of incident hght

and also perhaps with surface conditions.
A commonly accepted value for a in the
eastern United States is 0.22, and the basic
equation becomes:

Q, = Qo(0.22 + 0.78S)

The maximum amount of sunhght re-
ceived at the latitude of Washington is 1.5
gm. cal./min./cm."; this is equivalent to an
intensity of about 10,000 foot candles. On
Mt. Whitney, at an altitude of 4420 meters,
the amount may reach 1.67 gm. cal./min./
cm.*, or approximately 11,000 foot candles.
The distribution of total radiation from the
sun at the earth's surface is as follows: The

90

ANALYSIS OF THE ENVIRONMENT

radiation in the remote infra-red supplies
an insignificant amount of energy. On some
days the infra-red energy between about
20,000 to 30,000 A and 7700 A, the begin-
ning of visibiUty, may be greater than that
carried by visible Hght. In general, 50 to 58
per cent of radiant energy lies in the visible
range, and 1 to 5 per cent Ues in the ultra-
violet region, with less than 0.1 per cent

tive solar energy is such that the entire field
of the ultraviolet gives only a small fraction
of the caloric energy to reach the earth,
while the nonvisible, infra-red rays carry
about one-half of the heat received. Data
from the latitude of Cleveland, Ohio, are
summarized in Figure 8. The maximum in-
tensity of the sun's energy as it reaches the
earth Hes at 4700 to 5000 A. with the sun

50-

10-

2

1.0

7

AIR MASS

1.50

2.37

•

9 1-

< '

-

a:

< 0.5^

1-

o

•" 01-

;

u.

o

2

u

o

-

Q- 0.01 -I

UJ

Q.

0.005-

0.001-

0.000

1 IIIILIZ3Z:

I niiiEziz:

I ninrzs:

Fig. 8. Spectral distribution in percentage of total solar radiation at the latitude of Cleve-
land, Ohio. I, Below 3100 A; II, below 3250 A; III, below 3500 A; IV, below 4000 A; V,
4000 to 7000 A. ( Drawn from data reported by Forsythe and Chiistison. )

in wave lengths shorter than 3130 A
(Bracket, 1936; Kimball, 1924; Ellis and
Wells, 1941).

Figure 7 gives the annual variations and
total radiation received on a horizontal sur-
face with a clear sky or with average cloud
conditions for three widely separated
stations in the United States. The record
for Washington, D. C, which is also given,
shows weekly variations as great as 50 per
cent of the normal values and from 30 to
40 per cent of the mean solar constant.

Spectral distribution of ecologically eflFec-

near the zenith. The region of the greatest
intensity is displaced toward the longer
wavelengths with decreasing altitude of the
sun and is located at about 7000 A at 80°
incidence when the rays are passing through
nearly six air masses; the shortwave Umit
is similarly shifted toward the red under
these conditions (Forsythe and Christison,
1930; Bundesen, Lemon, et al, 1927).

Approximations are sometimes more re-
vealing than exact statements. Roughly one-
third of the radiation reaching the earth's
atmosphere is thrown oflF into outer space

IIEAT

91

again without making any change either on
the earth or in its atmosphere. Roughly
another third is absorbed by the atmos-
phere, and the final third is absorbed by the
earth itself. These are average figures for
the earth as a unit when all seasons are
considered.

On a clear day, when the sun stands
overhead at the zenith, approximately 92
per cent of the radiation at sea level comes
from the sun directly; the other 8 per cent
comes from the sky. The relative differ-
ences decrease until they are equal, though
both are much less, when the sun is some 8
degrees above the horizon. The intensity of
direct radiation from the sun increases with
an increase of height above sea level; con-
versely, the intensity of sky radiation
decreases with altitude. When the sun is
overhead in an overcast sky, if the cloud
layer is uniform, the brightness is surpris-
ingly uniform; brightness decreases about
10 per cent 45 degrees from the zenith and
about half of that at a point almost at the
horizon (Humphreys, 1942).

ECOLOGICAL RADIATION UNIT

Under many conditions, the amount of
radiation received in a given biotic com-
munity, or a fraction thereof, can be
summarized by the ecological radiation unit
that may be stated in terms of energy or of
light intensity (O. Park, 1931). This unit

represents a summation of (1) the inten-
sity (a) under the open sky, (b) under
different degrees of shade, and (c) in sun-
flecks under a canopy of vegetation; (2) the
area in the community which receives ra-
diation of each of the recognized inten-
sities. In a representative case, the ecologi
cal radiation unit of the forest floor can be
calculated as follows:

Let A =: unit area

P = portion of unobstructed radiation
Shi = shaded portion of density 1
Sh2 = shaded portion of density 2
S = portion covered by sunflecks
Q = intensity of unobstructed radiation
Qi = mean intensity in sunflecks
qi = mean intensity in Shi
qz = mean intensity in Sh2

When P + S -^ Shi -t- Sh= = A, the fol-
lowing simple formulation can be stated:

PQ + SQ, + Shiq, + Shsqj

AQi

= Ecological radiation unit.

The ecological radiation unit may sum-
marize all radiation, or it may be broken
into different fractions, as, for example, the
originally proposed ecological light unit (O.
Park, 1931; Strohecker, 1938). The latter
has distinctly different values in the several
stages of the dune-forest succession.

6. HEAT

EFFECTS OF HEAT ON THE PHYSICAL
ENVIRONMENT

Heat is a form of energy, of which two
important ecological factors may be recog-
nized. There is (1) the intensity factor,
temperature, and (2) a capacity factor,
heat capacity. Temperature is measured in
degrees on some temperature scale; in this
book the centigrade scale will be used un-
less otherwise stated. The capacity for heat
is defined as the quantity of heat taken to
raise the temperature of the given substance
through 1° C. The standard unit, the
calorie, is the quantity of heat required to
raise 1 gm. of water from 15° to 16° C;
this is a gram-calorie and represents a rela-
tive!" <unall amount of energy. When large

quantities are involved, the kilogram-calorie
is often used as the basic unit, especially in
human nutrition; this is 1000 times larger
than the gram-calorie.

Ecologists, and biologists in general, fre-
quently use the words "heat" and "tempera-
ture" as though they were synonyms; often
they are. A familiar phenomenon will illus-
trate one basic difference. Much heat
energy must be spent to melt ice, yet, until
it is melted, the temperature remains con-
stant. It requires 3500 gram-calories per
square centimeter of surface to change the
ice on Lake Mendota at Madison, Wiscon-
sin, to water; an amount equal to the heat
from some 195,000 tons of anthracite coal
is necessary to melt the ice on the whole
lake, yet the temperature of the water is

92

ANALYSIS OF THE ENVIRONMENT

unchanged by the process. This high latent
heat of fusion of water is one of the prop-
erties that make it a remarkably fit sub-
stance for life and, therefore, a major envi-
ronmental factor (see p. 76).

The amount of heat present affects both
living organisms and their environment.
While much of the space available for this
discussion will be devoted to the more
direct effects of temperature on plants and
animals, it is useful to remember that the
heat relations of the environment are also
important ecologically. For example, sur-
face layers of suddenly warmed rocks flake
off from the cooler inner layers; the flak-
ing is often produced by the heat expanding
trapped water into steam. Winter cold also
disintegrates solid structures, primarily as a
result of the force exerted by expanding ice.
These matters are commonly treated in
physiography. Other aspects of the effect of
heat on the physical environment can be
effectively summarized and possibihties can
be suggested by considering the tempera-
ture relations of lakes. A lake affords a
rather neat environmental unit, many
phases of which, temperature included,
have been studied intensively. Good sum-
maries of the literature should be consulted
for interesting general features and for
details.*

THE HEAT BUDGET

Outside the tropics, the water of a lake
accumulates heat during one portion of the
year and gives it off at another. Although
the processes involved are complex, they
can be summarized in terms of the annual
energy budget of the lake. This may be
considered as being composed of the energy
received from the sun and sky each year
and is substantially balanced by the outgo
of energy from the lake water. In simplest
terms, the annual heat budget of the lake
is based upon the amount of energy in
gram-calories required to raise the tempera-
ture of the water, including the energy
used in melting the ice, from the winter
minimum to summer maximum.

Different lakes vary greatly in heat bud-
gets. The general principles involved can be

• Birge and Jtiday, 1911, 1912; Birge, 1915,
1916; Needham, Juday, Moore, Sibley, and
Titcomb, 1922; Welch, 1935; Hesse, Allee, and
Schmidt, 1937, and Juday, 1940.

illustrated from the data concerning the
well-studied Lake Mendota. This lake has
an area of about 40 square kilometers, a
maximum depth of 25 meters, and a mean
depth approximately half of that. The
minimum balance occurs late in December,
when the lake freezes over, near enough to
the end of the year so that, for practical
purposes, the fiscal year for Lake Mendota
corresponds with the calendar year The
mean energy receipts from sun and sky
radiation from April, 1911, to March, 1939,
inclusive, are shown in Figure 9. The aver-
age total of annual receipts is 118,872
gram-calories per square centimeter of sur-
face, expended as indicated in Table 5.

Table 5. Estimates of Energy Expenditure of
Lake Mendota (see Juday, 1940)

Gram-calories

per Square

Centimeter

of Surface

For raising temperature under ice 1800

For melting ice in spring 3500

For raising water to summer tem-
perature 22,400

For raising temperature of bottom,

net 1500

For evaporation 29,500

For energy used by organisms,

maximum 1000

For surface loss' 28,500

For loss by conduction, convection

and radiation 30,000

Total 118,500

* Types of surface losses include reflection,
upward scattering by particles in suspension,
and absorption by snow and ice.

The bottom of the lake has a heat budget
of its own. At four different stations where
the depth of the water ranged from 8.0 to
23.5 meters, the budget ranged from about
3000 gram-calories at the shallower station
to about 1100 for the deeper. The mean
for the lake is near 2000 gram-calories per
square centimeter of surface, of which
some 500 are used in heating the water
under the ice in winter.

In general, lakes in eastern North
America that do not present imusual
features and that lie between 40 and 60 de-
grees north latitude have similar heat bud-
gets. When lakes are about 10 kilometers

HEAT

93

long, 2 kilometers wide, and have a mean
depth of 30 meters and a maximum of 50
meters or more, the annual heat budget of
the water, rather than of the lake as a
whole, is between 30,000 and 40,000 gram-
calories per square centimeter of surface.
Different types of soils also have charac-
teristic heat budgets, and supposedly rivers
do, too, although knowledge concerning the
heat budgets of rivers is remarkably scanty.

THERMAL STRATIFICATION

One important relationship between fresh
water and temperature is outstanding. As

Deeper lakes, if covered with winter ice
for some weeks and not exposed to unu
sually strong or direct wind action, show
an annual cycle that is closely associated
with the four seasons of the year. The se-
quence is: (1) Under the ice in winter, the
lake is stratified inversely with the colder,
lighter water at the surface floating on the
denser water, which has a uniform tempera-
ture of about 4° C. (2) There is an over-
turning and a circulation of water through-
out the entire lake in the spring that results
from the surface water becoming warmed
to 4° C. when it has the same density as

Jan.

Feb. Mar. Apr. May June July Aug- Sept. Oct.

Nov.

Dec.

Fig. 9. Mean annual energy receipt at Lake Mendota. Ordinates are in thousands. (Data

from Juday.)

the temperature approaches 4° C. from
either direction, the density, but not the
viscosity, of the water increases. With fur-
ther cooling or warming, the density falls.
This point of maximum density of fresh
water at 4° C. has an importance in the
temperature relations of a lake that is some-
what comparable, when broad implications
are considered, with the fact that the freez-
ing point of such water lies where it does
on the absolute temperature scale.

the deeper water. It is then easy for the
spring winds to produce the spring over-
turn, (3) With rising temperature, the sur-
face water is soon warmed above the point
of maximum density and floats upon the
colder, denser water below (Fig. 10). The
spring warming takes place mainly in the
water near the upper surface, since from
two-thirds to nine-tenths of the incident
radiation is cut off, either by surface reflec-
tion or by absorption by the first meter of

94

ANALYSIS OF THE ENVIRONMENT

COIl-

the summfir temperature
rise, since water is a poor

water. As

tinues to

conductor of heat, a direct stratification is

established in which the warm upper layer

of water, the epihmnion, now considerably

expanded, passes by a narrow transition

stratum, the thermocline, to the cold lower

the drop in temperature is at least 1° C. per
meter of depth. Above and below, the rate
of decrease is less. Within the thermocline
it may be much greater. The depth of water
down to the thermocline and the depth of
this zone of rapid change may vary in the
same lake at a given time, both in thick-

MIDSUMMER TEMPERATURE
LAKE GEORGE

Wind-stirred water
Shifting temperatures
Plenty of light and air
Abundant plankton

TRANSITION AREA

65% of fall in temperature is here

Still water

No light

No wind

Scanty growth of plant plankton

No weather

Maximum range of temperature

for the year, about 5° C.

Fig. 10. Summer stratification in Lake George, New York. (Redrawn from Needham, Juday,

Moore, et al. )

water, the hypolimnion. The temperature
of the epilimnion lags somewhat behind the
seasonal march of the temperature of the
air. Its waters are kept in a fairly homo-
geneous condition by wind action. Their
oxygen content is high. The water of the
hypolimnion is seldom disturbed by sum-
mer winds, and it, too, becomes fairly
homogeneous, but with lower oxygen con-
tent, frequently very low indeed.

As becomes a transition zone, the ther-
mocline is not sharply marked off from the
region above and below, though the transi-
tion is usually more abrupt from the epihm-
nion than from the hypolimnion (Fig. 11).
Birge's arbitrary rule for the location of the
thermocline fimits it to the region in which

ness and in depth with seasonal changes.
A thermocline to physical oceanographers
means the layer of water in which the tem-
perature shows maximum change with
depth. They carry this practice over to
studies in lakes. Thus, Church (1942, p.
14) speaks of a thermocline in Lake Michi-
gan in which the mean temperature change
was 2.5° C. in 20 meters. This double usage
is unfortunate.

To return to the outline of the seasonal
cycle in lakes: The fourth stage comes with
autumnal cooling, followed by a complete
circulation of the water until shortly before
the lake becomes covered Vidth ice. The ver-
nal and autumnal overturns not only
equalize the temperature relations through-

HEAT

95

out the lake, but also serve to distribute
oxygen and other dissolved materials uni-
formly from top to bottom in decided con-
trast to the conditions found during winter
and especially during summer stratification.
The usual geographic classification of
lakes of the world is based on these relation-
ships. It was originally suggested by Forel

II.

Order 3. Temperature of bottom water
similar to that of surface
water; circulation tends to be
continuous except when sur-
face is frozen

Tempera'.e lakes: surface temperatures

vary abc-ve and below 4° C.

Order l. Temperature of bottom water
at 4° C. throughout the year;

10 12 14 16 18

Temperature in Degrees C.

22

Fig. 11. Summer temperature-depth curves for the major Finger Lakes in New York, shown
to the depth of 50 meters. ( Redrawn from Birge and Juday. )

(1901), and, after being modified by
Whipple (1927) and Welch (1935), is as
follows:

I. Polar lakes: surface temperatures never

above 4° C.

Order 1. Bottom water 4° C. through-
out the year; one circulation
period possible in summer,
generally none

Order 2. Temperature of bottom water
varies, but not far from 4° C;
one circulation period in
summer

two circulation periods pos-
sible (one in spring and one
in autumn), but generally
none

Order 2. Temperature of bottom water
varies, but not too far from
4" C; two circulation periods
(one in spring and one in
autumn )

Order 3. Temperature of bottom water
similar to that of surface
water; circulation more or less
continuous except when sur-
face is frozen

96

ANALYSIS OF THE ENVIRONMENT

III. Tropical lakes: surface temperature al-
ways above 4° C.

Order 1. Temperature of bottom water
near 4° C. throughout the
year; one circulation period
possible in winter, but gen-
erally none
Order 2. Temperature of bottom water
varies, but not far from 4° C;
one circulation period in
winter
Order 3. Temperature of bottom water
similar to that of surface
water; circulation at all
seasons

From this classification, which sums up
in a general way a great deal of limnologi-
cal research, it is clear that temperate lakes,
especially of the second order, are regularly
stratified with respect to temperature. Third
order lakes generally are too shallow to
allow such stratification. In tropical lakes,
where the temperature never falls to 4° C,
if other conditions are favorable, there is
always a direct thermal stratification. Polar
lakes, where the temperature is always be-
low 4° C, show an inverse stratification.

These essentials of thermal stratification
must be kept in mind when considering the
stratification of lake communities (p. 443),
and in many phases of the physical environ-
ment as well, notably as regards dissolved
chemicals (p. 202) and dissolved atmos-
pheric gases (p. 193). Despite earlier sug-
gestions by others (p. 41), we owe much
of the background to Birge, whose ecologi-
cal studies have extended over some fifty
years. He introduced the term "thermo-
cline" in 1897 and the terms "epilimnion"
and "hypolimnion" in 1910; the amounts of
heat acquired and lost by a lake over a
year's time were organized and presented
by him (1916) as the heat budget. The
study of heat budgets should be of great
use in the comparison of thermal stratifica-
tion in difiFerent lakes, but has been little
used (Rawson, 1939).

Seasonal variation in the progress of
thermal stratification has been considered
for numerous lakes sufiicientlv to establish
its generality for second-order temperate
lakes over the world. A paired example
must sufiice: Lake Waskesiu, Saskatchewan,
and Lake Mendota, Wisconsin, among
others, were compared seasonally. In the
order listed, vernal overturn began in mid-
dle May, as opposed to middle April; sum-
mer stagnation with thermocline formation

began the first week of June in the northern
lake, as against the end of May for the
more southern one; winter stagnation wdth
ice cover and reversed gradient became
established in middle November in Lake
Waskesiu and in middle December at Lake
Mendota (Rawson, 1939). These general
thermal cycles vary as much as one or two
months for the same lake in difiFerent years.
The lake cycle also varies with bottom
characteristics, altitude, and latitude, but
the process itself is universal for suitable
lakes and plays a major role in community
development both directly and indirectly.

The rate of change of water temperature
may prove important in the organization of
the lake community. This has been studied
in Linsley Pond and Lake Quassapaug,
Connecticut, and derived for Lake Men-
dota, Wisconsin, from Birge's table of mean
temperatures, by Hutchinson (1941). Such
data indicate that the hypolimnion can be
divided into an upper clinolimnion, in
which the rate of heating falls exponentially
with increasing depth, and a lower bathy-
limnion, in which the rate of heating ap-
proaches a constant value independent of
depth.

Not only is there a vertical gradient in
thermal stratification, but there is a horizon-
tal sjradient as well, at least in large lakes
such as Lake Michigan (Church, 1942).
Many factors are involved in establishing
such gradients; for example, radiation,
evaporation, conduction, mixing, chilling by
snow, hail or sleet, and condensation. Sea
water, unlike fresh water, continues to be-
come denser with cold until it freezes at
about —1.9° C, when the salt content is
35 "'',„. As the surface water evaporates,
it becomes more salty and therefore more
dense. Even though warmer than underly-
ing water, it sinks until its density matches
that of the colder, deeper water. At such a
depth there is a zone of rapid temperature
chance that may approach the abruptness
of Birge's thermocline; this is usually found
at depths between 50 and 150 meters. Since
ocean water does not show a change in den-
sity at 4° C, but continues to increase in
density tmtil the freezing point is reached,
the abyssal waters of the oceans are cooler
than those of lakes and ranee from about
— 1' C, in regions of cold currents to
slightly above zero elsewhere.

Thermal stratification also occurs on land
with depth in the soil, and it is particularly

HEAT

97

important in deserts and other hot, dry
lands, where the animals by burrowing can
escape midday heat and the great fluctua-
tions that characterize desert conditions (p.
219). Temperature stratification is found
also in the air, especially in regions covered
by relatively dense vegetation; such strati-
fication is well developed in forests. There
the daytime temperature gradient is largely
a result of dijfferential insolation and tends
to disappear on heavily overcast days, es-
pecially if there is little air movement.

Vertical gradients of other environmental
factors also exist under the forest canopy as
well as in lake or soil. For the forest, the
more notable ones include differential dis-
tribution of intensity of sunhght, of wind
velocity and of evaporating power of the
air. These matters are considered in some
detail in dealing with biotic factors of the
environment (p. 228) and especially in
stratification in communities, as discussed in
Chapter 28.

EFFECTS OF HEAT ON ORGANISMS

The relation of animals to temperature
supplies another basic ecological division,
that between the animals whose body tem-
perature approximates the temperature of
their environment, the poikilothermous
animals, as contrasted with the so-called
warm-blooded or homoiothermous birds and
mammals. The body temperature of the
homoiotherms may be independent of that
of the environment within rather wide
hmits. Of the million or so known species
of animals, all but about 20,000 are
poikilotheiTnal. Homoiothermal mechanisms
are not required or fully acquired before
hatching or birth. The ability to maintain
a given temperature normally improves to
a steady state with early development. In
seasonally variable chmates, many species
of birds and mammals hatch or are born
near a time of optimal temperature, but
there are so many exceptions that it is
questionable whether this involves a definite
adaptation or is merely a tendency. The
degree of approximation between the body
temperature of cold-blooded animals and
their immediate environment may be
close; the earthworm Lumbriciis agricola,
for example, when immersed in water, be-
comes adjusted to a change of 10 degrees
within two minutes with an accuracy of
0.05 degrees (Rogers and Lewis, 1914,
1916).

Small aquatic animals, especially if tlieir
muscular activity is low, have a body tem-
perature that closely approximates that of
the surrounding water. Active fishes, how-
ever, may show temperatures some 10 de-
grees above their environment, and with
passivity the temperature of the surround-
ing water is approached slowly. Under
many conditions the body temperature of
fishes is about that of the surrounding water
(Clausen, 1934).

Terrestrial poikilotherms, especially in-
sects and other similarly active forms, may
have their body temperatures raised above
that of the surrounding air as a result either
of their own activity or of insolation. With-
in a period of ten minutes, the air tempera-
ture remaining constant at 28° C, the
internal temperature rose from 27.9° in
the shade to 42.7° C. when a third instar
grasshopper nymph {Locusta migratoria)
was directly exposed to sunrays that had
an intensity of 1.07 gram-calories (Strel'-
nikov, 1936). As might be expected, the
body temperature of black-brown locusts
exposed to the sun is higher than ttiat ot
green ones. The amount of air movement
in the micro-habitat is an important agent
in lowering the temperature of insects and
other animals. This becomes more eflFective
when combined with the evaporation of
water from the exposed body surface. Thus,
land amphibians usually maintain a body
temperature below that of the surrounding
air as a result of constant water loss.

Aggregations of insects may have tem-
peratures within the aggregations decid-
edly higher than the surrounding air even
when there is Httle integration between the
members of the aggregation. Social insects,
notably the honeybees in their winter clus-
ters, are much more independent of en-
vironmental temperature. As a result of the
heat produced by muscles in vibrating the
wings and as a further result of the insula-
tion furnished by the covering shell of bees,
such a cluster may maintain a tempera-
ture decidedly higher than that outside the
cluster. At high temperatures honeybees are
able to lower their temperature slightly,
probably by increased evaporation. Such
social insects show partial control over their
immediate microclimate and have become
facultative homoiotherms as a result of so-
cial activities (Pirsch, 1923; Phillips and
Demuth, 1914).

The distinction between cold-blooded

98

ANALYSIS OF THE ENVIRONMENT

and warm-blooded animals tends to break
down, also, when approached from the
homoiothermal side of the division line.
Monotremes afford an example of forms es-
sentially transitional between poikilothermy
and homoiothermy. The body temperature
of Echidna varies from 26.5° to 34° C.
without correlation with air temperature
(Semon, 1894). Nesthng birds may start as
poikilothermous animals and, later in ontog-
eny, develop the ability to regulate their
temperature. Hibernating mammals become
essentially poikilothermic during hiberna-

tures near maximum toleration; the safet)^
factor is much greater at the lower than at
the upper end of the tolerated temperature
range. This relationship is illustrated in a
generalized fashion in Figure 12; it holds
for the majority of aquatic as well as for
terrestrial forms.

As with several other environmental fac-
tors, the temperature extremes that an ani-
mal can tolerate depend on a complex series
of relationships. Some of these are: (1) the
species or other taxonomic subdivision; (2)
the external temperature at which the spe-

INCREASING TEMPERATURE

HEAT COMA

Fig. 12. The effect of temperature on activity of animals. (Redrawn after Verwom.)

tion. The temperature of the extremities of
homoiothermal animals fluctuates somewhat
with the environment (see p. 120), and the
body temperatures of small birds and mam-
mals are independent of the environmental
temperature within a narrow range only
(Kendeigh and Baldwin, 1928; Rasmussen,
1916; Chevillard, 1935; Gerstell, 1939).
Thus, adult passerine birds have a nor-
mal body temperature between 38.9° and
44.6° C. The environmental temperature
can carry body temperatures of these birds
down to about 23.9° and up almost to
46.7° C. for short periods without being
necessarily fatal for the birds. The lower
margin of safety is rather large, some 15
degrees; this is not the extreme Limit of tol-
erance, for the tiny house wren {Troglo-
dytes aedai parkmani) has survived after
its body temperature was lowered to
16.7° C. for a short time. The upper mar-
gin of safety between the highest normal
and the lethal body temperature is much
less; it is only 2.1 degrees for adult passer-
ines. This condition seems to be general
(Kendeigh, 1934). Most animals, cold-
blooded as well as warm-blooded, operate
most efficiently in many ways at tempera-

cies normally li\'es and that in which the
given individuals have lived recently; (3)
the length of exposure; (4) the internal
body temperature; (5) the rate of change
of internal temperature as extremes are
approached; (6) for low temperatures, the
presence or absence of internal ice; (7) the
general condition of the individuals as re-
gards items hke water content and thermal
insulation (cf. Luyet and Gehenio, 1938).
Roughly stated, most poikilothermous or-
ganisms are active at temperatures between
6"^ and 35° C. Numerous exceptions are
known, and even the more generous limits
from about —37° to -f64° C," such as
have been found on the surface of the Lake
Michigan dunes, do not reach the extremes
of endurance of active animals. Entire Ufe
histories are passed both above and below
the usual temperature limits.

* Higher surface temperatures are known.
Geiger (1927) cites 71.5° at Tucson, Arizona,
and 69° C. at Agra, India. Johnson and Davies
( 1927 ) estimate that it is unlikely that the sur-
face temperature of the soil will ever exceed
200° F. (93.3° C), and the highest soil
temperature that may be expected is about
180° F. (82.2° C).

COLD HARDINESS

We shall first consider relations of or-
ganisms to lower temperatures. A cave sil-
phid beetle (Astagobius angustatus) is
known to carry on its lite cycle in ice grot-
toes where the temperature range is be-
tween — 1.7° and +1.0° C, and the marine
bivalve mollusk, Yoldia arctica, is confined
to ocean water with a temperature of
0.0° C. or lower (see p. 82).

Organisms fiom the north temperate re-
gion can be divided into three main groups
on the basis of their resistance to low tem-
peratures. These are:

1. Those that can survive exposure to
temperatures that approach absolute zero
(-273° C).

2. Those that are killed at or near their
freezing point, usually relatively near the
freezing point of water.

3. Those that die when chilled to some
point above freezing.

The first assemblage includes plants ana
animals that, at some stage in their hfe his-
tory, can withstand desiccation and, when
dried, become tolerant of extremely low
temperatures. This group includes, among
others, plant spores and seeds, protozoan
cysts, rotifers, tardigrades, and nematodes.
The last three, if refrigerated slowly 'Aath-
out preliminary desiccation, can sinrvive a
temperature of -253° C. (Rahm, 1922);
certain bacteria, yeasts, and other fungi can
live similarly in extremely low temperatures.

The majority of plants and poikilother-
mous animals of our latitudes belong to the
second group and are killed at tempera-
tures somewhat below, but still relatively
near, zero Centigrade. Two general sub-
divisions of these cold-tolerant forms are
known: First, those that can hve until their
body temperatures fall some 10 to 30 de-
grees below zero; often they can survive the
formation of much ice withvii their bodies.
They can recover after being frozen hard
and brittle with the cold, and apparently
die from cold only when the last of their
cellular liquids solidify. Cold-hardy insect
larvae and many woody plants react in this
way. This more or less artificial ecological
class passes, by continuous gradation, into
the second category, which includes a
larger assemblage of forms that are killed at
or near the freezing point of water.

The third group, killed at some point
above freezing, is illustrated by some of the

HEAT 99

higher plants, by most mammals (except
certain hibernating ones) (see p. 105), and
by certain poikilothermal animals. Some
cladocerans— A/o/na macrocopa, for exam-
ple—become chilled and cease swimming
movements after continued exposure to
10° C; they settle to the bottom and die,
since the gill chambers become clogged
with mud and debris (Brown, 1929).
Long-continued exposure to low tempera-
tures well above freezing may cause
death even when the animals can with-
stand shorter exposure to cold of the
same intensity (Leeson, 1941). From
the httle that is known about cold
death of poikilothermous tropical animals,
it appears that they are probably killed by
only relatively low temperatures; fish near
subtropical Bermuda were killed in num-
bers by wdnter temperatures when the air
did not go below 7° C. (Verrill, 1901),
and breeders of tropical fishes know that
death occurs from cold at temperatures at
which more northern fishes thrive. In freez-
ing weather along the Florida coast there
is differential killing of the tropical element
of the fish fauna.

It is probably not an overstatement to
summarize our knowledge of cold death by
saying (cf. Luyet and Gehenio, 1938, p.
88) that, with the exception of a few or-
ganisms that are killed at temperatvures
above zero, plants and animals of the tem-
perate latitudes either die when chilled
relatively near to their respective freezing
points or are not killed by any low tem-
peratiu-e to which they may be subjected
in nature.

Winter's cold may be escaped by migra-
tion or hibernation, or it may be resisted
by the development of protective coverings
of fat, fur, or feathers, by the seasonal eUm-
ination of activities that consume much
energy, such as those concerned with repro-
duction, or by the development of in-
dividual or racial cold hardiness. Frequently
there are effective combinations of these
methods for successful overwintering.

The problems differ for warm-blooded
and for cold-blooded animals. Unhke birds,
even highly motile terrestrial insects seldom
execute geographic migrations. Butterflies
do so more than most insects, and even
with them, periodic seasonal migrations on
a geographic scale are rare. The massed
autumnal migrations of the monarch butter-
fly (Danaus plexippus) to the south, and

100

ANALYSIS OF THE ENVIRONMENT

their less conspicuous northward migrations
in the spring, are noteworthy, since only in
this species are the same individual butter-
flies known to make the return journey
(Williams, 1930, p. 323; 1938; Beall,
1941). Usually, as winter approaches, in-
sects of the tree tops or other exposed
places migrate, a short distance to less ex-
posed niches where they escape full ex-
posure to the cold.

The phenomenon of supercooling appar-
ently plays an important role in the cold-
hardiness of insects. The body temperature
of the adult or juvenile insect, or of eggs,
falls, with that of the environment to a criti-
cal point at which the temperature re-
bounds to a brief equilibrium between the
heat of fusion and the radiation of heat into
the immediate environment, and remains
there until the heat released by freezing

,B = DEATH
Sc^A — Permanent heat stupor

Temporary heat stupor
= Beginning of heat stupor
•Supra-optimal zone
OPTIMUM
■< Suboptimal zone

(frozen
fluids)

T2=Criticab state T3 ^^^^^
Fig. 13. Temperature relations of an insect. (Redrawn from Uvarov, after Bachmetjew.)

The overwintering of insects has been
much studied both by ecologists and by
physiologists. Here, as elsewhere, it is hard
to separate the work and interests of these
two groups. Usually, insects from exposed
positions, such as those of tree tops, migrate
a short distance to protected micro-habitats
and so escape from the full impact of win-
ter conditions. A number of adaptive
processes take place as temperature falls:

(1) The activity of the insects decreases;

(2) production of metabolic water lessens;

(3) the percentage of salts and colloids in
body liquids increases; (4) other colloidal
relations with water may change. From the
integration of insect behavior with biophysi-
cal and biochemical processes, most insects
pass the winter without being frozen, even
though they are living in a continental cli-
mate in the middle latitudes.

The temperature relations of insects in
such climates are summarized in Figure 13.

becomes dissipated; then the temperature
drifts downward again to stable equilibrium
with the environment. The freezing tem-
perature in insect bodies is not closely
related either to the limit of liquid under-
cooUng or to what is sometimes called the
rebound temperature. In insects, supercool-
ing before ice formation starts within the
body and carries down from a few degrees
below zero to —40° or —50° C. (Salt and
Mail, 1943).

It is difficult even to approximate the
true freezing temperature of an insect. The
rebound point is often taken as the freez-
ing point, a concept now known to be
erroneous, since the two may differ by as
much as 25 degrees. Water in bulk under-
cools several degrees; it undercools still
more in an emulsion, and when subdi-
vided, as a fog, it has been undercooled to
— 155° C, in the laboratory. Natural fogs
occur with temperatures of —25° C. to

HEAT

101

-50" C. (Salt and Mail, 1943). Some de-
terminations made in the light of these con-
siderations by puncturing the insects with a
thermocouple are given in Table 6.

For the great group of insects of the tem-
perate regions that do not survive under-
cooling and subsequent freezing, the larg-
est class of insects in temperate North
America, the undercooling temperatures ap-
pear to be more important than the freezing
temperatures. The other tvi'O classes of in-

artificial dehydration, and others are not so
affected. Lepticoris may lose a fifth of its
weight by artificial drying without showing
any change in the critical supercooling
temperature. The converse is also true: an
increase in water content by natural means
may, or may not, alter the ability to under-
go supercooUng.

Animals exposed annually to seasonal de-
creases in temperature typically show sea-
sonal variation in the location of their

Table 6. Mean Undercooling and Corrected Freezing Temperatures Based

on Ten Insects of Each Species (Data from Ditman, Voght,

and Smith, 194S)

Hibernating

Undercooling
Mean

Freezing

Carpocapsa pomonella larvae

8.9

10.4

6.4

4.2

1.8

Pyrausta nubilalis larvae

3.1

Diatraea cramhidoides larvae

1.3

Heliothis armicjera pupae

0.9

Active

Anasa tristis female adults
Anasa iristis male adults. .
Pyrmtsta nubilalis larvae . .

sects with relation to cold hardiness are:
( 1 ) those that are extremely cold hardy and
survive undercooling and freezing and are
killed only by long exposure to low tem-
peratiires or by one or more sudden changes
in temperature; and (2) the noncold-hardy
nonhibemating insects that succumb to low
temperatiires even without freezing.

The development of ice crystals within
the body is much more harmful than is a
lower temperature when ice formation is
avoided as a result of supercooling. An in-
crease in the degree to which supercooling
occurs in connection with cold hardiness is
usually associated with dehydration, but
the whole set of relations is far from simple.
For example, full-grown larvae that are still
feeding, prepupae, and pupae of the moth
Ephestia all have nearly the same percent-
age of water, yet they show critical su-
percooling points of —5.8°, —8.0°, and
—21.3° C, respectively. Some animals may
have their critical temperature lowered by

critical supercooling temperature. Beetle
larvae that bore in oak wood, for example,
showed a supercooling point of —22° and a
reported freezing point of —12,8° C. in
February. In July, similar larvae super-
cooled only to —2° and froze at —0.8° C.
Insects from stored grains that are not or-
dinarily exposed to temperature extremes do
not exhibit this seasonal periodicity, al-
though they show much variation in cold
hardiness.

The physiological explanations of these
complex phenomena are obscure. Although
supercooling presents certain resemblances
to the physical supercooling of distilled
water, the phenomena associated with cold
hardiness are much more complicated. Wig-
glesworth (1939, p. 366) sums up the
physiological situation thus:

" , , . Ice crystals forming in solutions rich
in hydrophyllic colloids are liable to become
covered with a sheath of dehydrated colloid
and thus fail to 'seed' the entire solution; the

102

ANALYSIS OF THE ENVIRONMENT

quantity of such colloids may be a factor in
super-cooling. It has been suggested that the
water which remains unfrozen at —20° is
'bound' to the tissue proteins; but there is at
present no way of distinguishing 'bound' water
from water which is supercooled from some
other cause."

Not all insects undergo a decrease m
percentage of body water in winter. The
mound-building ant, Formica exsectoides, of
which many individuals retire to points
near the water line of the soil before hiber-
nating, has practically the same percentage
of body water at all times during the year.
In fact, hibernation is often associated with
a retreat into a more protected situation.
The more protected micro-habitats do not
reach the freezing points of supercooled

for long periods are least likely to be im-
mobilized by chilling to zero degree Centi-
grade. Even acclimatization to higher
temperatures of stages in the life history
that are normally exposed to low tempera-
tures for long periods may not raise the
temperature at which chill coma sets in.
Later stages of the same insects, stages that
ordinarily Hve at higher temperatures, are
more easily aflEected by cold and have the
temperature at which they pass into coma
raised by exposure to warmth (Mellanby,
1940). Many poikilothermal animals of the
higher latitudes are able to withstand being
frozen. Normally, both freezing and thaw-
ing take place slowly, and this seems to be
important so far as thawing is concerned.
There is laboratory evidence that quick

SPECIES

Alloeapnia mystic a
Allocapnia recta
Allocapnio vivipara
Taeniopteryx nivalis
Allocapnia gronulata
Strophopteryx fasciota
Perl/ net I a dry mo
Isoperia bilineota
Neophasganophora capitato
A toper la ephyre
Perlesta placida
Acroneuria arida
Acroneuria internata
Neoperia clymene
Acroneuria abnormis

NOV

AUG.

Fig. 14. Seasonal succession in stone flies of Illinois. The width of the spindle suggests abun-
dance. (Redrawn from Frison, )

body liquids, and the most protected niches
of soil and forest do not normally reach the
freezing point of water even in climates
such as those of northern Illinois (Bach-
metjew, 1907; Payne, 1926; Holmquist,
1926, 1931; Dreyer, 1938).

The development of cold hardiness in in-
sects has a species as well as an individual
basis. This is illustrated by the seasonal suc-
cession of stone flies. About one-third of the
species of the order Plecoptera in Illinois
emerge as adults, mate, feed, and carry on
all essential activities during the coldest
months of the year (Frison, 1929, 1935).
Certain of these seasonal relations are
shown in Figure 14. The racial character
of cold hardiness is further illustrated by the
fact tliat those stages of arctic insects that
are mrmally exposed to low temperatures

freezing at low temperatures is less harm-
ful than is the slower process; but such
freezing is presumably rare in nature
(Uvarov, 1931; Parker, 1930; Zeuthen,
1939).

Animals frequently develop structures
that aid in over\vintering. The gemmules of
sponges give an illustration. Fresh- water
sponges usually form large numbers of re-
sistant gemmules in the autimin and then
disintegrate. The gemmules can vdthstand
freezing and drying and begin growth
anew under favorable conditions. In nature,
they normally start to develop in the spring
when temperatures rise. Freezing and dry-
ing are not always necessary; gemmules of
Spongilla have hatched out after two week's
exposure to a temperature of 22° C. in the
autimm. The accelerating eflPect of exposure

HEAT

103

to cold, perhaps combined with maturation
phenomena, is shown by the fact that simi-
lar gemmules hatched after three days in
similarly favorable temperatures in the
spring. After extended hibernation, develop-
ment of this sponge takes place at tempera-
tures as low as 2^^ to 5° C.

The accelerating effect of exposure to low
temperatures, freezing included, is also
shown, among others, by grasshopper eggs
in the middle latitudes. These develop more
rapidly if placed at low, even freezing,
temperature and later transferred to a
liigher one, and the process appears to be
related, superficially at least, to the so-
called vernalization of plants. Seeds of win-
ter cereals can be "vernahzed" by adding
water until the seeds barely sprout and then
chilling them to 3° or 5° C. or even freez-
ing for an average length of thirty-five to
forty-five days or over winter. When
planted in the spring, such seeds develop as
though they belonged to spring varieties
(Miller, 19cj8). From naturafistic evidence,
Darling (1937, 1938) has suggested that
cold, as such, may act on the gonads of
birds and mammals as a stimulating agent
alternative to the well-authenticated stimu-
lation produced by light (p. 121), and
that low temperatures may be responsible
for the marked development of the gonads
of hibernating mammals before they emerge
from hibernation. C. R. Moore and his co-
workers (1934) had already tested this
point with the ground squirrel, Citellus. The
reproductive system of this rodent under-
goes a marked regression after the annual
spring breeding season. Experiments with
diet, darkness, constant warm or cool tem-
peratures, brief transfer from hibernation to
warm surroundings, and similar manipula-
tions have yielded essentially negative
results so far as the induction of develop-
ment of the male reproductive system is
concerned during seasons when such
development is unusual in the ground squir-
rel. In the female, however, with constant
cold and darkness, periods of oestrum have
been induced and maintained for many
months at times in the year when the ani-
mals are ordinarily sexually inactive.

In medial latitudes, cold-hardy animals
that emerge in early spring frequently have
northern afiBnities or a northern origin,
while forms that appear in late summer
tend to have a southern origin or southern
afiBnities.

HEAT HARDINESS

Hot springs furnish the warmest environ-
ments known to be inhabited by active
organisms. The blue-green algae Fhormid-
ium bijahense and Oscillaria filiformis ap-
parently hold the record for multicellular
plants. They five in the thermal waters of
Yellowstone National Park at a temperature
of 85.2° C. Living bacteria have been found
in even hotter water at 88" C. (Copeland,
1936).

Brues (1939) discounts, pending more
evidence, certain reports that larvae of
brine flies (Ephydridae) Live at tempera-
tures of 55° and even at 65° C. and that
rhizopod protozoans have been taken from
water at 58° C. He regards 50° to 52° C.
as the highest temperature compatible with
the fife of plants other than those just men-
tioned, and of active animals. Encysted
animals and plant seeds resist much higher
temperatures. An examination of the tem-
peratures at which animals have been taken
in thermal waters indicates that the major-
ity of these heat-tolerant animals live in
water below 40° C. As the temperature de-
parts more and more from the usual opti-
mum, the number of species that can
tolerate such a temperature becomes re-
duced. This is a phase of a much more
general principle that can be stated as fol-
lows: Wherever and whenever conditions
approach a pessimimi (see p. 213), the
biota becomes impoverished, the more so,
the closer the approach to the Hmits of tol-
eration. In East Indian thermal springs,
Brues records fifty-seven species from 36
to 40°; twelve, from 41 to 45°; four, from
46 to 50°; and four from above 50° C.

Deaths from heat may occur at much
lower temperatures. Some one-celled, snow-
dwelling algae cannot resist temperatures
higher than 4° C. (Luyet and Gehenio,
1938). The alga Phacocystis poucheti dies
at 11.6° C, and, despite fittle exact work
on the subject, it is known that many sorts
of animals are killed by heat before the
temperature reaches 20° C. In general,
fishes and marine invertebrates are less re-
sistant to heat than are terrestrial insects
or mammals, and animals from streams are
less resistant than are related animals from
small ponds. Such differential resistance is
probably a result of natural selection. One
reason for the lack of more data concerning
the point at which heat deaths occur is

104

ANALYSIS OF THE ENVIRONMENT

that in the lower ranges of lethal tempera-
tures, the effect of heat is a function of the
duration of exposure as well as the absolute
temperature. No satisfactory generaUzed
formulation of these time-temperature rela-
tions has been worked out.

There is a large, though somewhat con-
fused, hterature dealing with heat death;
much of it has been summarized by Heil-
brunn (1943, 1946). Death from heat does
not necessarily occur during or immediately
after exposure and, if deferred, is not evenly
distributed through the passing horns.
Rather, as the hfe history develops, death
may be restricted mainly to times of
greatest physiological activity when there
is need for the closest interaction between
crucial processes (Larsen, 1943).

Attention has been paid to heat hardiness
as distinct from heat death. Adaptive proc-
esses that make for heat hardiness include,
among others, evaporation of water from
skin or lungs (p. 183), evaporation of
water from the nests of social bees or
wasps (p. 215), and aestivation in some
more or less completely quiescent stage (p.
185). Many animals emigrate, burrow, or
become nocturnal, and so escape more ex-
treme heat. Others apparently acchmate
themselves by synthesizing Hpoids with a
higher melting point. As with cold hardi-
ness, for many organisms, heat hardiness is
favored by a decrease in the amount of the
water content of the organism (Heilbrunn,
1943).

Ecology, like genetics and evolution, can
gain much sound knowledge from the study
of domestic animals. Not only do the breeds
of cattle wdth a smaller body size tend to
have greater heat hardiness (Davidson,
1927), as called for by Bergmann's rule (p.
119), but a tentative scale of heat hardiness
has been worked out by Rhoad (1941) as
follows :

100- lO(Tb-lOl) zzHt

The formula is based on 101.0° F. as the
normal body temperature for cattle: 100
represents perfect efficiency in maintaining
body temperature at 101° F.; 10 is a factor
to convert degrees of deviation in body
temperature from the temperature scale to
a convenient unit basis; Tb is the observed
body temperature under conditions of a
"severe" test such as would be furnished by
exposure under field conditions to a tem-
perature of 95° F.; r. h. of 72 per cent with

a wind velocity of only 4.5 miles per hour;
Ht is the heat tolerance index. A group ol
cattle with a mean body temperature of
104.3° F. would have an indicated heat
tolerance of

100 - 10( 104.3 - 101 ) = 100 - 33 = 67

Using tliis scale, Rhoad reports that cattle
tested under comparable conditions on a
Texas farm showed the heat tolerance as
follows: Braliman cows, 93; Jersey cows,
86; Hereford steers, 73; and Aberdeen
Angus cows, 56. These indexes of heat tol-
erance are approximate and tentative. The
method is promising and can be applied
widely.

HIBERNATION, AESTIVATION, AND DOR-
MANCY OF VERTEBRATES

Considered in a broad sense, hiberna-
tion and aestivation are related phenomena
and are united under the concept of dor-
mancy. A period of dormancy under con-
ditions of heat and drought is much more
familiar in invertebrates than in mam-
mals (p. 185). The passage of a dry season
in summer (and by extension, of any dry
season) is commonly referred to as aestiva-
tion, which bears directly on the water
relations of animals (pp. 183-189). Over
wintering in a dormant state is commonly
referred to as hibernation.

Poikilotherms

The overwintering of cold-blooded ver-
tebrates in temperate and northern latitudes
is much like that of insects and other inver-
tebrates. Most fishes, amphibians, and
reptiles are killed by complete freezing, but
not by freezing of the extremities. Perma-
nently frozen subsoil accordingly Hmits their
northward spread. The Alaska blackfish
(Dallia pectoralis) is said to survive being
frozen solid. In some fresh-water fishes ol
the temperate zone there appears to be a
tendency to suspend feeding and to form
loose or even compact aggregations, some
times in the water, sometimes in the bottom
mud (Norman, 1931, p. 243; Anonymous,
1943, p. 129). Many fresh-water fishes are,
of course, active throughout the winter.
Marine fishes are not known to exhibit any
suspension of activity in the cold season.

An aestivation period becomes a fixed
part of the life cycle of many tropical fresh-
water fishes in regions with a sharply de-

HEAT

105

fined alternation of wet and dry seasons.
This is especially exemplified by the African
and South American lungfishes, which re-
main in the mud of the drying ponds or
marshes and form mucous-lined individual
cells in which they stay until the rising
water level again frees them. This hfe-
history cycle is essentially hke that of fresh-
water animals in general in temporary
ponds, illustrated among North American
fishes by the mud minnow (Umbra limi).

Overwintering of both frogs and sala-
manders frequently involves aggregation in
large numbers in a moist terrestrial situa-
tion (like a large rotten log), in the
bottom mud of marshes and ponds, or in
springs. In springs partially torpid frogs
may be seen swimming in exaggerated slow
motion in midwinter. Toads and the more
terrestrial types of frogs may pass the win-
ter in solitary burrows on land (Noble,
1931). Aestivation of frogs during a dry
season is reported. The Central Australian
Chiroleptes platycephalus is said to fill its
urinary bladder, lymph spaces, and the
body cavity with water and to pass the dry
season in this condition in a mud cell much
like that of a lung fish (Spencer 1896, p.
164). Holzapfel (1937), by experiment,
demonstrated a relation between the hiber-
nation of the common leopard frog in North
America and the seasonal cycle, independ-
ent of actual temperatures, since the normal
dormant condition was readily assumed
during the winter months, while frogs sub-
jected to low temperatures in the summer
months did not become dormant, and died
in a relatively short time. The latitudinal
gradient of hibernation in frogs has not
been studied. The relation between hiber-
nation and the sexual cycle varies geo-
graphically; thus the leopard frog may
breed in the fall in the southwestern United
States.

The more terrestrial reptiles accomplish
overwintering mainly by retreat into rock
crevices or burrows, often in considerable
aggregations; such aggregations may in-
clude several species. More aquatic turtles
spend the winter buried in mud beneath
ponds and stream borders. Hibernation of
solitary individuals has been observed in
^n American lizard, the anole.

Duration of the period of dormancy is
proportional to the length of the winter or
dry season. It is evident also that there is
no period of dormancy in the life cycle of

fishes, frogs, and reptiles in tropical regions
without an extended dry season, and that
the latitudinal gradient with respect to win-
ter dormancy deserves further examination.

Hibernation of Mammals

Hibernation of warm-blooded animals,
i.e., of mammals, difi^ers conspicuously from
overwintering of poikilotherms. Observa-
tion, study, and experiment combined have
disclosed further problems and have led to
a great diversity of opinion and hypothesis
as to the immediate causes of hibernation,
i.e., the factors that induce its deathlike
torpor in individuals. Such studies fre-
quently disregard evolutionary and ecologi-
cal causes and relations. The fact that the
temperature of the mammal drops below
the normal level in aestivation as well as in
hibernation points to the functional relation
of lowered metabolic rate for surviving an
unfavorable season. (Rasmussen, 1916;
Johnson, 1931; Benedict and Lee, 1938;
"Hamilton, 1939).

Factors that have been thought to induce
the torpor of hibernation are low tempera-
ture, especially gradually decreasing tem-
peratures; inadequacy of heat-regulating
mechanisms; lack of food; dryness of food;
concentration of carbon dioxide in hiber-
nacula; accumulation of fat; and glandular
disturbance. The operation of special hiber-
nating glands has been postulated, but the
supposed glands (in the marmot) prove to
be merely masses of fat. Freedom from ex-
ternal stimuli appears to contribute to
maintenance of deep torpor, since under
experimental conditions activities of the ex-
perimenters have been observed to arouse
animals from dormancy. The most remark-
able feature of hibernation physiologv in
mammals is the low internal temperature
reached, which approximates that of the
environment and falls as low as 1° C:
body temperatures of dormant mammals
average about 1 degree above that of the
environment. Death appears to ensue if
temperature falls to freezing; such a fall
may arouse the animal and thus save its
life, a reaction with obvious survival value,
especiallv for mammals that hibernate in
relativelv exposed situations. Where the
soil is permanently frozen beneath the level
reached by summer thaws, hibernation of
burrowing mammals can not take place,
and hence is absent in polar regions.

106

ANALYSIS OF THE ENVIRONMENT

Among the characteristics of torpor in
mammals are the reduced rate of breathing,
with complete suspension of breathing for
several minutes at a time; lowered body
temperature; and persistence of the heart
beat, as in cold-blooded vertebrates, when
the animal is decapitated.

Relations between hibernation and sleep
have been investigated somewhat. Sleep of
mammals in which heat regulation is imper-
fect, and in which hibernation can develop,
diflFers conspicuously from sleep of the nor-
mally winter-active mammal. Relation of
the period of hibernation to the sexual cycle
is far from clear, but there may be active
development of the gonads during hiber-
nation. This is especially conspicuous in
Columbian ground squirrels, in which the
long period of aestivation and hibernation
concentrates active life into about five
months. The remarkable interruption of
early stages of embryonic development in
various mammals exhibits no correlation
with hibernation, since it occurs in non-
hibernating mammals as well as in hibernat-
ing forms.

The zoological dispersion of hibernation
among mammals is not especially illuminat-
ing, since closely allied forms (e.g., the true
squirrels and the ground squirrels) may
diflFer radically in this respect. Hibernation
is reported for the orders Monotremata,
Marsupialia, Insectivora, Chiroptera, Ro-
dentia, and Camivora. Hibernation of carni-
vores appears to diflFer in important respects
from that of other orders and might be re-
ferred to as pseudohibernation. The tenrec
of Madagascar, a remote relative of the
hibernating hedgehog, is often cited as an
aestivating mammal. Various ground squir-
rels of western North America have a well-
defined period of aestivation combined with
hibernation, since they disappear into their
burrows early in August and do not appear
until the following March. Ground squirrels
in Turkestan have the same habit.

Among gradients connecting the hiberna-
tion habit with more normal life histories,
there may be mentioned storage of food by
nonhibemating rodents, the storage of con-
siderable supplies by some that hibernate,
and of small amounts, perhaps as bedding
rather than food, by those in which the
hibernating habit is fully developed. Aggre-
gation associated with hibernation is wide-
spread among tropical bats as well as those
of temperate regions. Aggregation of hiber-

nating mammals is otherwise rare. It is
reported for family parties of skunks.

Great diflFerences exist in the extent to
which the hibernating animal withdraws
from the effects of the environment. Some
bats merely enter crevices beneath loose
bark, while others congregate into vast win-
ter colonies in caves, where the temperature
is constant. Burrows of more familiar hiber-
nating rodents extend well below the frost
Une into relatively constant temperature,
while pseudohibernation of carnivores may
occur in hollow logs, in snow-covered de-
pressions, or even on level ground beneath
trees.

We do not find an ecological study of
the north-south gradient in the hibernation
of mammals with a considerable latitudinal
range, but it is obvious from scattered in-
formation that such behavior clines must
exist, though less directly correlated with
environmental climatic factors than are
similar clinal gradients in overwintering be-
havior of poildlothermic animals.

In general, it appears that several en-
vironmental factors combine with interna]
factors (which in turn have been modified
in evolutionary adaptation to hibernation)
to induce dormancy in mammals. Hiberna-
tion and aestivation are to be compared
with seasonal migration or with food storage
as evolutionary adjustments for the passage
of an unfavorable season. According to this
view, the partial poikilothermy of homoio-
therms with the hibernating habit is
secondary and, at least in higher orders, is
a degeneration.

ENVIRONMENTAL TEMPERATURE AND THE
RATE OF BIOLOGICAL PROCESSES

Metabolic rates increase almost to the
upper temperature limits at which the or-
ganism is normally active.* The possibility
that such ecological accelerations are funda-
mentally similar to the effect of heat upon
the speed of chemical and physical reac-
tions has attracted much attention. Chemi-
cal reactions in a laboratory test tube, and
rate of living of adult organisms, speed of
embryonic, larval, or pupal development,
rate of living of adult organisms, speed of
locomotion, and other behavioristic reac-
tions, are all accelerated with higher
temperatures. These results are more ob-
vious in poikilothermal organisms than in

• For exceptions, see Barnes, (1937).

HEAT

107

homoiotherms, and they affect such impor-
tant ecological phenomena as the time span
of the life history or the length of any given
stage.

A number of mathematical formulae have
been used in attempts to express accurately
the relation between temperature and the
velocity of biological processes. These are
of two main types; the first type is based
on the chemical "law of mass action,"
which, in its simplest form, states that the
rate at which any chemical reaction pro-
ceeds is directly proportional to the concen-
tration of substances actually taking part in
the reaction. According to this law, tem-
perature influences can be expressed by an
exponential curve. This means that the ve-

whatever the location on the effective tem-
perature scale.*

Reaumur suggested in 1735 (see p. 18)
that the sum of average daily temperatures
during the growing season bears relation to
the time at which fruits ripen. This idea
can be expressed by the equation

in which y represents time, t, temperature,
and k is a. constant. The velocity (u) of the
process in question can be calculated, since

v = -; therefore, v = tk. As stated pre-

y ^

viously, the ecological zero (see p. 110) of
a given process usually fails to coincide with

° ° o ^ ,

-o

o

o

o

o

35 4 36

(l/-^,0

ABC

Fig. 15. Velocity of ameboid movement in relation to temperature. A, Log of velocity plot-
ted against inverse of absolute temperature. B, Three straight lines fitted to the same points.
C, Log of velocity plotted against log of temperature. (Redrawn from Belehradek, after
Pantin. )

locity constant is an exponential function of
effective temperature, which, in turn, means
that the effect of an increase of 1 degree
in temperature differs, depending upon the
location of the increase on the temperature
scale. Van't Hoff's rule (1884) and the
formula of Arrhenius (1915) are exponen-
tial expressions of this type, while the more
empirical catenary formula of Janisch
(1932) and the equally empirical one of
Belehradek (1935) are also exponential. In
the second type, the relationship is based
on the observation that in many biological
processes, the product of temperature and
time to a given end point is a constant.
Sanderson and Peairs (1914) state this
generalization, and the formula of Krogh
(1914) gives the usual form of expression.
According to this idea, the effect of an in-
crease in temperature of 1 degree is similar.

the freezing point of water. It is a variable
quantity and depends on the process and
the organism. The last equation must,
therefore, be modified by the parameter c,
which, practically speaking, shows the loca-
tion of the intercept of the straight-line por-
tion of the velocity curve on the tempera-
ture axis (Belehradek, 1935; Powsner,

" Roughly approximated, the overworked
Van't Hoff rule, often known as the Qio rule,
states that the rate of reaction is often doubled,
or more, for each 10° C. increase in tem-
perature in the median range. The use of these
temperature coefficients is not a subject for the
unwary. Accounts by Shelford ( 1929 ) and
Chapman ( 1931 ) contain helpful discussions
of general temperature relations. Readers with
stronger physiological inclinations can consult
such references as Kanitz (1915), Belehradek
(1935), Barnes (1937), and Heilbrunn (1943).

108

ANALYSIS OF THE ENVIRONMENT

1935; Krogh, 1914a) (see Fig. 16), so that
the expression becomes:

V = tk + c

If graphed, the equation yt = r gives

a rectangular hyperbola, the reciprocal of
which is a straight hne that theoretically
intercepts the temperature axis at zero; the
value of c in the modified formula shows
the shift of the intercept.

TEMPERATURE

Fig. 16. A generalized time-temperature hy-
perbola and its straight-line reciprocal. (Re-
drawn from Belehradek. )

This statement of the case means that the
velocity of the process under consideration
is a linear function of temperature, a rela-
tionship that has been found to hold for
many physiological events that have ecolog-
ical significance. This is by no means a
perfect working tool. It is most useful when
few measurements are available at tempera-
tures some distance apart. This makes the
Sanderson-Peairs-Krogh formula serviceable
for field workers who need to make an
approximation of the length of various
developmental stages at different times of
the year.

When the effects of temperatures closely
spaced along the whole range of effective
temperatures are examined, the resulting
velocity curve remains Unear only at inter-
mediate points. Near the ecological zero for
the reaction (s), the effect of a degree rise
in temperature is greater, and near the up-
per temperature limit it is less than at the
intermediate points. A more or less sigmoi-
dal curve results (see Fig. 17), the form
of which is not given by the equation.

In summary, we are forced to the con-
clusion that, despite a great mass of work
by ecologists and physiologists, there is as
yet no generaUzed expression for the known
relations throughout the range of effective
temperatures that is soUdly based on
theory. A new approach may be necessary
in order to express mathematically the sub-
tility of the effect of temperature on the
rate of Hving processes. Even an approxi-
mate solution of the problem may depend
on accumulation of more complete knowl-
edge of temperature relations of enzyme
systems. The difficulties involved are in-
creased by the readiness with which organ-
isms acclimate to temperature, and accUma-
tization has ecological impUcations with
both ontogenetic and evolutionary values.
For example: There are five species of Rana
in the eastern United States and Canada.
The species whose range extends farthest
north, R. sylvatica, tolerates lower tempera-
tures, breeds earher in the season, and has
the most rapid rate of development at a
given temperature. Similar relations hold
with regard to the rate of development of
Ambystoma (Moore, 1939) and various
other aquatic animals, both fresh-water and
marine. It follows that the rate of metabo-
Usm in animals of the colder waters is not
necessarily retarded as would be inferred
from temperature formulae.

The different chemo-biological tempera-
ture formulae have received attention from
ecologists and others out of proportion to
their proved usefulness; even so, they are
suggestive. The known importance of tem-
perature upon such processes as the rate of
egg laying, of development, and of death
of insects, among others, indicates tha*
velocities dependent on temperature affect
population ecology as well as the ecology
of individuals. The probable significance of
favorable temperatures in relation to out-
breaks of insect plagues and the useful-
ness, here as elsewhere in ecology, of an
approach to mathematical formulations are
reasons for continued preoccupation with
these problems.

VARYING TEMPERATURES

The most densely inhabited portions of
the earth's biosphere have daily or seasonal
fluctuations in temperature or both. Many
well-occupied habitats, on the other hand,
have nearly or quite constant temperatures
the year around. Even under arid condi-

HEAT

109

tions, soil temperature is nearly constant at
a depth of 2 meters, although the soil sur-
face may vary more than 56 degrees in one
day (p. 219). This state can be reached
much nearer the surface under less extreme
conditions, and in steadily shaded portions
of tropical rain forests, where even sun-
flecks are absent, temperatiire on the forest
floor may show little variation from one
year's end to another. Deeper parts of caves
characteristically have constant tempera-
tures.

the Bay of Biscay is one example of such
a situation (Sverdrup, Johnson, and Flem-
ing, 1942).

At the surface of the sea, diurnal tem-
perature changes average not more than
0.3° C; hence, short-Uved populations in
surface waters often live out their life cycle
without great temperature fluctuation. In
deeper waters, according to records taken
at fifteen stations in the Atlantic and Atlan-
tic-Antarctic oceans, the mean temperature
variation between depths of 4000 and 2000

14°

IQo 20° 22° 24° 26' 28° 3C° 32'
TEMPERATURE C°

Fig. 17
reciprocal
from Kro£

. Duration of incubation of chrysalids of Tenebrio molitor in hours (T); V is its
and shows velocity of development; it approximates straight line (v). (Redrawn
;h.)

The bottom waters of lakes of the first
and second order (p. 95), whether arctic,
temperate, or tropical, do not vary much
from 4° C. at any season. Similarly, animals
that live in the ocean, the upper part of
the lighted zone alone excepted, meet little
if any temperature variation. Even well
within the Hghted region, at a depth of 100
meters in many temperate locations, annual
variations are on the order of 1° or 2° C;

meters averaged only 1.3° C, with extreme
ranges from 0.3° to 2.3° C. At these depths
a great many generations must live under
temperature conditions that approach or
equal those we call constant in experimental
laboratories. We do not know what would
be the effect of subjecting animals from
such steady environments to the controlled
constant or variable conditions that charac-
terize studies in experimental ecology. This

no

ANALYSIS OF THE ENVIRONMENT

leaves a large gap in fundamental knowl-
edge concerning temperature relations of
animals.

It is not fair to compare the biological
eflFects produced by constant temperature
with those obtained under the variable con-
ditions found in nature, since, for many
animals, laboratory life at its best is highly
artificial. In the laboratory, organisms ex-
posed to variable temperatures frequently,
perhaps usually, show accelerated develop-
ment as compared with those held at a
constant temperature of the same mean
value, if other conditions remain equal. This
generalization might not hold true for ani-
mals that live in the steady temperatures
such as obtain in even the upper layers of
the soil of the tropical rain forests or at
some depth in lakes or oceans. Exceptions
have been recorded for terrestrial animals
from the middle latitudes (Headlee, 1914;
Ludwig, 1928).

The amount of acceleration varies with
diflFerent stages in the life history, with
diflFerent species, and with the combinations
of temperatures that are used. When the
range is held between the minimum effec-
tive temperature and the maximum for the
given process, blowfly larvae and pupae
showed acceleration (Peairs, 1927). The
codling moth (Carpocapsa pomonella) was
accelerated between 7 and 8 per cent for
egg, larval, and pupal stages. Grasshopper
eggs held at 22° C. for sixteen hours and 5,
10, or 15 degrees higher for eight hours
daily showed an average acceleration for
Melanopltis mexicamis of 38.6 per cent and
for Camnula pellucida of 30.5 per cent, as
compared with the rate of development at
comparable constant temperatures. Grass-
hopper nymphs reared in such alternating
conditions were accelerated some 12 per
cent over expectation based on results from
constant temperatures (Parker, 1930).

Certain complexities involved are illus-
trated by relations reported for the flour
beetle, Tribolium confusum. The eggs
develop more rapidly in constant tempera-
tures than in comparable variable ones
without reference to the position of mean
temperatures, except when the upper tem-
perature lies below the optimum and the
lower temperature is at the developmental
zero. In the pupae, if the mean of the com-
bination lies above or at the optimum with
symmetrical alternating temperature, devel-
opment is delayed; if the mean lies below

the optimum, development is accelerated.
With Tribolium, the most rapid acceleration
comes with an alternating amplitude of 5
degrees. The thermal optimum in the action
of alternating temperatures may be different
from the optimum for constant tempera-
tures.

Alternating temperatures may affect sur-
vival as well as the rate of development
Tribolium shows increased longevity with
alternating temperatures, especially in the
lower part of its favorable thermal range.
The range of constant temperatures, with a
survival greater than 50 per cent, is nar-
rower than the range of mean alternating
temperatures that produce the same result.

Relations between effects produced by
variable as contrasted with constant tem-
perature of the same mean value depend,
among other things, upon the species, the
process measured, the location of the tem-
peratures used in relation to the effective
temperature range, and the length and
amplitude of the thermal cycle. It is unfor-
tunate that the majority of the experimen-
tal work has been done with terrestrial
insects and plants that are sensitive to
changes in humidity and under conditions
that at times leave doubt concerning the
exactness of the control of the latter factor
(Mikulski, 1936, 1936a).

From an entirely different approach,
there is much evidence that man thrives
best and works most efficiently when ex-
posed to dailv or weekly changes of wea-
ther rather than in the same locality in
periods of relative constancy. Further
changeable temperate climates are more
stimulating to man than are relatively con-
stant tropical regions (Huntington, 1924;
Taylor, 1927).

ECOLOGICAL TEMPERATURE ZERO

The lowest temperature at which a given
physiological process, or development
through a given stage in the life history can
be carried through to completion is the ef-
fective temperature threshold for the
function under consideration. In the lower
temperature range, the highest externally
imposed temperature at which such a
functional unit cannot he successfully com-
pleted is its ecological temperature zero.
This is a new name for an old idea. The
concept of an ecological zero of develop-
ment is a general one; the ecological tem-

HEAT

111

perature zero represents only one of its
many components (see below). In studies
related to life histories and behavior, the
first temperature just given is often referred
to as the "threshold of development" and
the second approaches the "developmental
zero." Actually, some development fre-
quently takes place at temperatures that
will not allow the successful completion of
a given stage or process; hence the ecologi-
cal zero represents a somewhat higher
temperature than the developmental zero
(Parker, 1930; Belehrddek, 1935; Powsner,
1935). The biological zero of Belehradek
denotes "the temperature at which a given
protoplasmic action is arrested by cold
without formation of ice." Its precise re-
lation to the ecological zero has not been
determined; presumably it approximates the
developmental zero. Belehradek (pp. 139-
145) bsts the biological zero for diverse
processes in different organisms. These
"zeros" and "thresholds" may be affected by
time of exposure as well as by temperature
and such physiological factors as age, pre-
vious conditions, and temperature adapta
tion.

The location of the ecological tempera-
ture zero for a given process or stage in
development can be determined only by
experimentation. Its position can be approx-
imated rather closely at times by plotting
the point at which the straight-line recipro-
cal of the temperature hyperbola crosses
the temperature axis (see Fig. 17). This is
the so-called alpha point for the curve; it
has theoretical rather than ecological signifi-
cance. Usually the true ecological zero lies
at some lower temperature than is indicated
by this intercept, a fact that has aheady
been considered in connection with the
correction of the simple equation for the
hyperbolic temperature curve (see p. 107).

The effects of exposing organisms to
temperatures between the ecological zero
and the complete stoppage indicated by
the developmental zero and then returning
them to higher temperatures are not uni-
form. In some instances, acceleration up to
80 per cent occurs (Parker, 1930; for
grasshopper eggs). In other cases there is
retardation (cf. Ludwig, 1928; Powsner,
1935). This discrepancy may be interpreted
as follows: Some processes always continue
at temperatures at which the organism is
not killed by cold. The effect of low tem-
perature is probably differential, and some

processes are slowed down more than
others. If not pronounced, this may weL!
have an accelerating influence when the
organisms are placed in medial tempera-
tures. The "disorganization" that results
from chilUng increases with time; if the
temperatures again become favorable, a lag
period follows before metabolic processes
become sufficiently well correlated to pro-
ceed normally. If the "disorganization" has
gone too far (and this depends on the
length of exposure as well as on low tem-
perature), the process cannot be completed
or completion will follow only at a retarded
rate.

SUMMATION OF HEAT

The useful ecological practice of sum-
mation of temperature represents in reaUty
an attempt to find an index for a summa-
tion of the heat energy required to complete
a given stage in the life history of an ani-
mal or plant. As such it has a theoretical
basis in the "law of constant heat summa-
tion" of thermochemistry. This generahza-
tion states that the quantity of heat
involved in a chemical process is the same
whether it takes place in one or in several
steps (Getman and Daniels, 1931). Actu-
ally, summation of the capacity aspect of
heat (see p. 91) has not been practiced
by ecologists in connection with life his-
tories; they have used temperatures instead.

Modern ecological summation of tem-
perature developed from the extended
experience of the phenologists that the ac-
cumulation of a given daily excess of tem-
perature above some convenient base will
approximately coincide with the completion
of a given stage in development. This
amount is usually found by summing so-
called day-degrees. In present usage, a day-
degree represents 1 degree of mean temper-
ature above the ecological zero lasting for
one day. The needed accumulation to a
selected end point is called the thermal
constant for that set of processes. In more
refined usage, especially for shorter fife his-
tories, hour-degrees are used for summa-
tion, and attention may be turned from
such environmental units to reciprocal
values called developmental units. A de-
velopmental unit is defined as the amount
of organismic development produced in a
given amount of time, frequently one hour,
by an increase of 1 degree of medial tem-
perature. Developmental units are obtained

112

by dividing the whole developmental period
by the number of days (or hours) taken for
its completion; they represent the reciprocal
of the fraction of the whole development
that takes place in a day (or an hour).
Practically, the medial temperatures are
those at which the increase in rate of devel-
opment is directly proportional to the rise in
temperature, i.e., to the region expressed
by a straight line in the temperature veloc-
ity curves shown in Figure 17. Develop-
mental units can be used to demonstrate
that the rate of development under variable

ANALYSIS OF THE ENVIRONMENT

environmental or in organismic units, tem-
peratirre summation, in the present sense,
finds its mathematical formulation in th&
equations of Sanderson and Peairs and of
Krogh, and their modifications.

Supporting evidence for the validity of
the concept of temperature summation is to
be found in the work of many biologist?
who have studied a wide variety of organ-
isms, both under natural conditions and in
the laboratory; special attention has been
paid to the length of life histories of insects.
The straight-line portions of the preceding

.14

.12

.10

.08

.06

.04

.02

r — 7^/T"

/ / Y

_A —^ ;f

10

15

20

25 30

TEMPERATURE c!

35

Fig. 18. Comparison of the rates of development of each stage of the Japanese beetle: P,
Pupa; £, egg; 1, 2, and 3 represent respective larval instars. ( Redrawn from Ludwig. )

field conditions approaches expectations
based on the theory of temperature summa-
tion and on laboratory experience with
controlled temperatures. When summed for
a given process, the total number of such
units for completion of the process in
question is a more refined expression than
that given by the thermal constant obtained
from the summation of day-degrees (Shel-
ford, 1929, p. 368). Whether measured in

temperature-velocity curves all imply that
temperature summation is relatively accu-
rate within the indicated medial tempera-
tures. Similar data from one more set of
experiments are summarized in Figure 18,
for the developing Japanese beetle, Popillia
japonica. This is a good final test case for
several reasons. Data for eggs and pupae
fit the theory rather well. Those for larval
stages show irregularities produced, in part

HEAT

113

by variations in food and presence of a
resting stage in the first instar. The third
instar runs contrary to expectation; this in-
dicates correctly that laws governing heat
summation for biological processes are im-
perfectly known.

Experience suggests, however, that
summed temperatures are frequently related
to the development of many animals and
plants in a fairly exact manner. The close-
ness of fit is readily disturbed and is par-

among poikilothermal animals is correlated
with Rubner's hypothesis that within a
given genetic combination, longevity is
inversely proportional to the intensity of
living. In other words, this hypothesis
states that a definite sum of hving action
determines the physiological end of life.
Although originally advanced as a result
of studies with mammals, this hypothesis
is now generally restricted to cold-blooded
forms in medial ecological conditions. For

Table 7. Evidence That, for Many Processes, the Total Amount of Work Done Is Relatively
Constant at Medial Temperatures, Regardless of the Velocity Imposed by Temperature

Temperatures

Organism

Process

Temperature
Range

Observer

Low
a

b

c

High
d

Rubner (1908)

Proteus

Days until gills
disappear.
Coefficient, N2
utilization

30
0.75

22
0.73

20

0.75

8
0.75

8°-21°C.

Krogh (1914a)

Tenebrio
molitor

Total CO2 pro-
duced in liters
per kilo.

59.6

59.1

58.0

59.3

20.9°-32.3°C

MacArthur and
Baillie (1929)

Daphnia
magna

Days duration

of life.
Days of life

times mean

heart rate per

sec.

108.2
182.5

41.7
177.5

25.6
175.0

8°-28° C

Parker (1930)

Melanoplus
atlantis

Days duration

nymphal

stage.
Mg. dry wt.,

consumed

food

94
4079

54
4311

27
4098

25
3988

22°-37° C.

Filinger (1931)

Phlyctaenia
ferrugalis

Days to 50%

prepupae.
Food consum-
ed, mg.

58
868

22

870

12.5

815

11
735

15°-30° C.

dcularly faulty when resting stages occur
during one of the developmental stages.
The relation between summed calories as
contrasted with summed temperatures
awaits appropriate experimentation.

RUBNER'S HYPOTHESIS

The existence of a "developmental total"
or "thermal constant" for m.any processes

example, at medial temperatures, such
forms live longer at low temperatures than
at higher ones, and various types of
measurements indicate that total energy
transfomnation is approximately the same
regardless of the length of natural life.
Some of the supporting evidence for this
generalization is given in Table 7.

Rubner's hypothesis finds general support

114

ANALYSIS OF THE ENVIRONMENT

in many other data that suggest that ani-
mals with a higher rate of metabolism tend
to live for a shorter time than do others of
the same species with lower metabolic rates.
Frequently these data are not carefully
quantitative. It is, for example, rather gen-
eral experience that the rate of metabo-
lism of males is higher than that of females
and that the latter live longer. Fairly exact
information has been collected on this sub
ject for the cladoceran, Daphnia magna
The rate of heart beat gives a rough indi-
cation of the rate of metabolism of this or-
ganism (Table 8).

15° than at 26° or 30° C. and much
greater in light than in darkness (Northrup
1926; MacArthur and Baillie, 1926).

THE LIFE ZONE CONCEPT

Merriam's so-called temperature laws on
which his life-zone system for North Amer-
ica is based, were stated in 1894. They
grew out of the idea of temperature sum-
mation and find their modem basis, in part,
in the hyperbolic temperature equations.
The forerunners of the scheme included the
system of faunal areas worked out by
American naturalists and Merriam's own

Table 8. The Mean Age in Days and the Average Number of Heart Beats per Second, Times

the Average Age for Males and Females of D. magna

(Data from MacArthur and Baillie, 1926, 1929)

Sex

Length of Life in Days

Heart Beats per Second X Days of Life

1926

1929

1926

1929*

Male

37.8
43.8

38.6
44.7

161.5
162.1

178 4

Female

174.5

* Data for 1929 taken at 18° C.

In these tests the females lived approxi-
mately six days longer than the males, yet
the product obtained by multiplying the
average Hfe-time rate of heart beat by the
duration of life in days is of the same order
of magnitude for the two sexes in each
test. The greatest discrepancy given here is
that for 1929; the males lived 13 per cent
shorter time than did the accompanying
females and showed a life-time heart beat
of 2 per cent more; unfortunately, we can-
not give a statistical evaluation of these
results. Regardless of differences produced
by temperature or sex, the average total
number of heart beats for this race of
Daphnia was estimated to be on the order
of 15.4 X 10\

Not all the tests of Rubner's hypothesis
have given positive results. In one set of
experiments, aseptic cultures of Drosophila
were reared under controlled conditions,
and their total carbon dioxide was measured
as an indication of total metabolism. The
amount produced was not constant, as re-
quired by the hypothesis; it was greater at

sound field studies on vertical zonation of
animals and plants in the Rocky Moun-
tains (Merriam, 1890, 1892, 1894, 1899,
1899a; Allen, 1892; Daubenmire, 1938).

When isotherms were drawn through
localities said to have equal sums of effec-
tive temperatures, certain of these coincided
suggestively with the northern boundaries
of distributions of certain animals and
plants. This is summarized in Merriam's
first temperature "law," which states: "Ani-
mals and plants are restricted in northward
distribution by the total quantity of heat
during the season of growth and reproduc-
tion." A similar study of the mean tempera-
ture for the six hottest weeks of summer led
to the second "law," which is: "Animals
and plants are restricted in southward dis-
tribution by the mean temperature of a
brief period covering the hottest part of
the year."

The division lines ran roughly east and
west over the plains and prairies, but made
a sharp dip southward in the mountains.
Along the western mountain and coastal

HEAT

115

country, the diflFerent zones made a com-
plex system. The zones and their supposed
temperature limits are bsted in Table 9.

The transition zone and the two zones
south of it were later divided into a dryer
western and a moister eastern region at
the 20 inch line of equal annual rainfall
(isohyet) which, in these latitudes, happens
to approximate the 100th meridian. Mer-
riam planned to use 6 degrees as the base
for temperature summation. Through a mis-
understanding, the temperatures were
summed with zero Centigrade as a base,
although only for those days on which the
mean temperature was above 6° C. The
values for summed temperature are there-
fore much too high, and the agreement be-
tween Merriam's isotherms and the bound-
aries of his life zones becomes a proof of
the incorrectness of his use of temperature
in setting up the scheme. Corrected tables
have never been published.

at what purports to be the root of
Merriam's life zone concept have actually
affected its usefulness very little. This para-
doxical situation results from the fact that,
from the very beginning, the life zones
have been mainly delimited by the ob-
served distribution of life-zone indicators
rather than by temperatures. The life-zone
concept owes its considerable vitality to the
soundness of the work of the early natural-
ists on which it was really based. The
weakness of the false fagade of tempera-
ture relations has been a factor in prevent-
ing ecologists generally from taking the life
zones seriously. Also, their studies of the
distribution of biotic communities have
shown that, south of the Canadian forests,
temperature is only one of a number of
environmental factors that regulate bio-
tic distribution (Sanderson, 1908; Shreve,
1914; Allee, 1923; Kendeigh, 1932; and
Shelford, 1932).

Table 9. The Temperature Values in Degrees Centigrade Assigned to
Merriam's Life Zones

Zone

Arctic*

Hudsonian

Canadian

Transition

Upper austral or upper Sonoran
TjOwer austral or lower Sonoran .
Tropical t

Northern Limit

in Accumulated

Day-Degrees

5500

6400

10,000

14,500

Southern Limit;

Mean Temp, of

6° C. Hottest

Summer Weeks

in Degrees,

Latitude

10
14
18
22

2(i

* Northward, ground never thaws,
f Southward, no freezing.

This application of temperature "laws"
to latitudinal distribution zones overlooks
entirely the phenomenon of winter killing
and cold hardiness, an oversight that biolo-
gists soon emphasized. Further, in hot
weather, daily maxima are more important
limiting factors than are daily means, and
still further, high temperatures may be an
effective limiting agent for the distribution
of organisms at other times besides the six
hottest weeks of summer.

These inadequacies and others that strike

Students of the distribution of birds and
mammals in the Rocky Mountains and
westward in North America continue to
find this scheme of hfe zones useful in a
descriptive sense. The distribution of verte-
brates in the Yosemite region of California
is given in Figure 19 in terms of the local
life zones. Even in these vertical distribu-
tions, the temperature implications of the
life-zone system present an oversimplified
picture. In northern California, for example,
the following environmental factors are

116

ANALYSIS OF THE ENVIRONMENT

ZONATIONOF THE MAMMALS OF THE YOSEMITE REGION

c

c
-'if)

c
o

c
o

c
o

■o
o

c:
o
o

c
o
c
o
w

X

<u

c

<t:
<

San Joaquin Pocket Mouse

.^►

Son Joaquin Kit Fox

Pacific Pallid Bat

..^-

....

Sacramento Cottontail

^

Long-toiled Harvest Mouse

^■-■,

California Spotted Skunk

-^;-

Striped Skunk

-

California Badger

^^_

►

■-■/

Mariposa Meadow Mouse

*■:.'■:'.-:■_

California Pocket Mouse

r-/\

Californio Ring-toiled Cot

-r:-:-

California Gray Squirrel

Norttiwestern Mountain Lion

Mountain Coyote

-•-::-:..

.•....:

.' ■■

Steptiens Soft-tiaired Ground Squirrel

^■y-:

Desert Jock Rabbit

■^

Wostiington Cottontail

-

Great Basin Pocket Mouse

Yosemite Stirew

n

-

Mono Chipmunk

Sonora White-footed Mouse

Yosemite Meadow Mouse

_^ . ■ . ■

;.-: ■■

Sierra White-tailed Jock Rabbit

■ ■

^■.■.■■.

Pacific Mink

-"-

■■;:-■■ ^■

Allen Chipmunk

Sierra Chickaree

■ ■ ■

::■■.■,■■

Sierra Mountain Beaver

!>■-..■:

Sierra Nevada Wolverine

—

■^

■'■'■■■■■'■•

Sierra Least Weasel

<^

-

Gray Bushy-toiled Wood Rat

^__

^^T

Yosemite Cony

■.;■.■■.-•'

^-^

-—

Mountain Lemming Mouse

-^

—

Fig. 19. Distribution of vertebrates in the Yosemite region of California in terms of Merriam's
life zones. ( Rearranged from Grinnell and Storer. )

HEAT

ir

known to aflFect the distribution of animals
(Grinnell and Storer, 1924; Grinnell, Dixon
and Linsdale, 1930) : vegetation, food, rain,
humidity, soil moisture, pH of soil, tem-
perature, altitude, atmospheric density,
available breeding niches, available refuge
niches, light, cloudiness and competition.

The distribution of animals in many parts
of the world is more or less closely tied up
with temperature. For instance, temperature
races of the fruit fly, Drosophila funebris,
exist in Europe. Northwestern populations
are more resistant to cold; southwestern
ones, to heat. All eastern populations, whe-
ther from the northeast or the southeast.

ture of their respective habitats. Doubtless
their distribution is also affected by other
environmental factors.

THE BIOCLIMATIC RULE

In spring and early summer in temperate
latitudes, periodic phenomena, such as be-
ginning of blossoming for a given species,
ripening of fruit, or appearance of active in-
sects, usually come three or four days later
for each higher degree of latitude and for
each 100 to 130 meters of latitude from any
given base. In late summer and autumn,
similar relations can be recognized, but in
the reverse direction. In certain regions tem-

Fig. 20. Approximate distribution of three "temperature races" of Drosophila funebris. (Re-
drawn from Timofeeff-Ressovsky, in Huxley.)

show high tolerance to both heat and cold.
These differential resistances are correlated
with the respective temperatures of the re-
gions under consideration. Figure 20 shows
that the January isotherm of —5 degrees
runs from northern Norway to southeastern
Russia. The July isotherm of 20 degrees
runs from Lisbon on the Atlantic eastward
and then northward up to about 63 degrees
latitude in Russia. The spread between
these isotherms in the east encloses an area
with a seasonal diflFerence of 25 degrees
and reveals the continental climate of this
inland region. Thus, even a coarse analysis
of these temperature races of D. funebris
shows a high correlation with the tempera-

perature relations vary also along a given
set of meridians. Thus Hopkins, who def-
initely formulated the bioclimatic relations
for the United States as a "law" of nature,
added that there was a seasonal retardation
of four days from west to east for each 5
degrees of longitude. This rule was origi-
nally worked out on the basis of observa
tions on North American and European
phenology.

The speed of migration of birds gives a
convenient test for the application of this
rule to one prominent periodic phenomenon
in animal life. From New Orleans to south-
ern Minnesota, the average speed of migra-
tion for all species of birds is close to 23

118

ANALYSIS OF THE ENVIRONMENT

miles a day. The speed for individual mi-
grants or for a given twenty-four hours may
be much greater or much less, but the aver-
age holds, and this average brings the mean
rate of migration within the rule as stated.
Northward, the rate of migration is faster,
probably because some of the slower spe-
cies have stopped to nest, and so the aver-
age rate is increased, and because, once
started, the season develops faster in north-
em latitudes.*

In China, the bioclimatic rule was fol-
lowed, in whole or in part, by eleven
species of Lepidoptera (41 per cent of
those studied). Three of these species have
one annual generation only and overwinter
as pupae. Of the sixteen species (59 per
cent) that did not conform, six overwin-
tered as larvae. The rule seems, in general,
to be a useful summarizing statement of a
situation that holds for some, though by
no means for all, seasonal events. It must
be remembered that seasonal changes are
aflFected by differences in length of day and
frequently by rainfall and other conditions
as well as by changes in temperature.

STRUCTURAL MODIFICATION INDUCED
BY TEMPERATURE

It is easy to produce changes in metabo-
lism in response to changes in temperature.
Such functional modifications, important as
they are at times, are usually reversible and
transitory. Those modifications of function
that result in phenotypic changes in struc-
ture attract more attention because they are
both rarer and more obvious. Changes in
temperature are known to produce struc-
tural modifications, and numerous instances
can be cited with the well-studied Droso-
phila melanogaster alone, in which, among
other structures, temperature affects the
number of facets in the eyes, the size of
vestigial eyes, and the presence or absence
of supernumerary legs (Goldschmidt,
1938).

Cyclomorphosis

A most striking instance of a relation be-
tween body form and seasonal change in
temperature is the phenomenon of cyclo-
morphosis in some small aquatic organisms,

" Selected references include Cooke (1917),
Hopkins (1918, 1920), Clarke, Margerie, and
Marshall (1924), Chapman (1934), and Mell
(1935).

including Cladocera, some simplified as-
pects of which are illustrated in Figure 21.
The facts as collected from observations in
nature are: In Danish waters, at least, a
change of form in whole populations of wild
Daphnia follows a rise in temperature to
between 12° and 16° C. (or to above 19°
in Connecticut; Brooks, 1946). The head
projections or helmets become fully devel-
oped in a few weeks and thereafter remain
at their summer size; hence, there is little

SUMMER

Fig. 21. Cyclomorphosis in Cladocera, show-
ing identical winter forms, but contrasting
summer forms. ( Redrawn from Coker. )

correlation between the degree of warmth
of the water and the size of the helmet. A
gradual reversion to the round-headed win-
ter form may occur in the autumn; in sum-
mer, perhaps after the formation of ephip-
pial eggs, the daphnia may disappear to
reappear in autumn as ephippial round
heads. Correlation of helmets with abun-
dance of food, if indeed it exists, is only
partial, and all size relations, both with
temperature and food, fail when popula-
tions from different waters are compared.
There is a rough, partial correlation be-
tween the size of the body of water and
degree of helmet development, with larger
helmets in larger bodies of water and their

HEAT

119

almost complete absence in laboratory
dishes. In different locations and despite in-
dividual variations, the general form of the
helmet is characteristic for the several pop-
ulations.

Two main theories have been advanced
to explain the phenomenon of cyclomor-
phosis in cladocerans (Coker, 1939). The
first, the buoyancy theory, is based on the

related species from warmer waters. The
low temperatures retard the rate of growth
and delay the appearance of sexual activity;
this delay tends to produce larger forms. In
marine copepods, for example, there is an
inverse correlation between body size and
temperature. The relation to temperature
may be more indirect, since the viscosity of
warm water is so much lower than that of

Fig. 22. The decreasing size of ears of Lepus from south to north. A, Arizona, jack rabbit ( L.
alleni); B, jack rabbit from Oregon (L. californicus); C, varying hare from northern Minnesota
(L. americanus); D, Arctic hare from the Barren Grounds (L. arcticus). (Redrawn from
Hamilton. )

fact that the floating power of warm water
is much less than that of cold water, and
there is the suggestion that protuberances,
whether spines or helmets, will aid the flo-
tation process in summer. The other theory
holds that the protuberances are directive
and stabilizing surfaces that function as do
rudders or keels.

Jordan's Rule

Jordan's rule that fishes in low tempera-
tures tend to have more vertebrae than do
those in warmer waters holds true in gen-
eral; however, this is not the only factor
that affects the number of vertebrae of
closely related fish. One of the exceptions
is illustrated by the observation that the
average number of vertebrae of young coal-
fish, Gadus viens, is lower for small fish
than it is for large ones of the same year
class. A possible, though unproved, explana-
tion for the relations found in coalfish may
be that small eggs produce smaller fish lar-
vae than do larger eggs and that such ef-
fects persist in later life. In such an in-
stance, temperature is involved only indi-
rectly (Dannevig, 1933).

Cold-water forms of many sorts are fre-
quently larger than are individuals or

cold water that the larger forms would be
handicapped in their efforts to maintain po-
sition in warmer seas (Hesse, Allee, and
Schmidt, 1937; Coker, 1934).

Bergmann's Rule and Allen's Rule

Homoiothermal animals from colder cli-
mates tend to be larger in size and hence
to have less surface in proportion to body
weight than do their relatives from warmer
regions. This phenomenon occurs widely
even though not universally among birds
and mammals and is usually interpreted in
relation to heat conservation in the north
and to heat radiation in the south. This is
Bergmann's rule. Allen's rule is correlated
with it and is concerned with the marked
tendency toward the lessening of extremi-
ties in colder climates (see Fig. 22.) Allen
based his conclusions on measurements of
animals killed in nature. His observations
have many confirmations both from field
and laboratory studies, especially when
rather large differences in temperatvire are
considered. For example, mice reared at
31 to 33.5° C. have longer tails than those
of the same strain reared at 15.5 to 20°
(Allen's rule), and the latter have larger
and stockier bodies and hence are decidedly

120

ANALYSIS OF THE ENVIRONMENT

heavier (Bergmann's rule) (Allen, 1905;
Ogle, 1934). Similarly, the young of the
common domestic fowl kept at 6° C. during
their third and fourth months of life were
shorter in body length, gained more weight,
and had shorter tarsi and tails than did their
former flock mates, which were kept
throughout at 21 to 24.5°. The birds from
the lower temperature also had larger
hearts, as has been reported for birds in
nature (Hesse, 1921; Hesse, Allee, and
Sclimidt, 1937; Allee and Lutherman,
1940).

An interesting sidelight on the relation
between internal and external temperatures
with respect to extremities throws important
light on phenomena such as those that
doubtless underlie Allen's rule. Red bone
marrow, ordinarily absent from the distal
regions of the tail in many animals, will
form if the intact tail tip is inserted into and
retained in the warm body cavity by a sim-
ple surgical operation (Huggins and Block-
som, Jr., 1936). Conversely, spermatozoa
of certain animals with pendant testes, such
as sheep, will not develop if the tempera-
ture is raised to that normally found within
the body cavity (Moore and Quick, 1924).

Poikilothermous terrestrial animals tend
to have their species and individuals with
largest size in warmer, rather than in colder,
climates. In this, a main trend in their sur-
face-mass geographic relations difi^ers from
the general rule for homoiotherms. Terres-
trial lizards, snakes, and many insects have
their larger species, or individuals within a
species, in the warmer parts of their range.

Exceptions occur to both the homoiother-
mal and poikilothermal phases of this rule.
Among mammals, there are many, of which
racoons (Prociion) aflFord an example, in
which the body size becomes smaller to-
ward the north. The reduction in body size
corresponds with an invasion of a less suit-
able climate. Hibernating mammals and
migrating birds escape the full rigors of
winter cold and may show no relation be-
tween body size and environmental tem-
peratures. Small birds have difficulty in
maintaining an even, high body tempera-
ture in a variable climate and may be lim-

ited to the tropics except for summer
migrations; the hummingbirds give an ex-
ample. An exception to the usual rule that
in terrestrial, cold-blooded forms the body
size is largest toward the tropics is fur-
nished by bumblebees, which are fuzzier
and larger in the northern part of their
range. These are evidently adjustments that
conserve the body heat generated by the
action of large wing muscles. Here we have
another example of the frequent experience
of ecologists. When different principles
come into conflict, only a direct inquiry can
determine which will be followed in any
given instance. It is worth repeating that
while we can discern and outline many
broad general ecological principles with
confidence, their application in a given
situation is frequently a matter for empiri-
cal research.

CONCLU