Bulletin


Bulletin
OF THK

Illinois State Laboratory
ov

Natural History

Urbana, Illinois, U. S. A.

STEPHEN A. FORBES, Ph.D., L.L.D.,

Director

Vol.. XI. July, 1915 Article I.

AN OUTLINE OF THE RELATIONS OF ANIMALS
TO THEIR INLAND ENVIRONMENTS

BY

Charles C. Adams, Ph.D.



ERRATA AND ADDENDA.

Page 50, second column, line 13 from bottom, for Danais arcMppus read Anosia
plexippns ; line S from bottom, for melliflca read mellifera.

Page 51, line 11 from bottom, for Danais read Anosia.

Page 159, at right of diagram, for Bracon agrilli read Bracon agrili.

Page 289, second column, last line but one, for Scalops real Scalopus.

Page 294, line 3, for c.atesheana read catesiiana.

Pages 327 and 330, line 12, for oreus read oreas.

Page 347, line 4, for Cecidomyidae road Cecidomyiidae.

Page 356, line 7, for Anthomyidse read Anthomyiidae.

Page 368, line 18, dele second word.

Page 373, after line 10 insert as follows: 53a, suipruinosa Casey, 1884, p. 38.

Page 375, after suXimucula Le Conte, 48, insert subpruinosa Casey, 53a.

Page 377, after line 7, insert as follows:

—

1884. Casey, Thomas L.

Contributions to the Descriptive and Systematic Coleopterology of
North America. Part I.

Page 379, line 11 from bottom, for sen.iu lata read sensu lato.

Page 382, line 12, for VII read VIII.

Page 408, line 2, for the next article in read Article VIII of.

Page 410, line 6 from bottom, for = 4 read '11.

Page 412, line 7, for 31 read 30.

Page 421, line 17 from bottom, insert it before grows.



CONTENTS

PAGE

The (lynaiuic rel;itions of aniinals 1—17

1. Introductory note 1

2. The relations of animals to their environment 1

3. Optima and limiting factors 8

4. Determination of dynamic status 9

5. Animal responses 5

6. The interrelations of animals 12

7. Ecological units for study 14

8. The animal association 15

9. Associational succession 16

The dynamic relations of the environment 17—31

1. Introductory 17

2. The dynamic and genetic standpoint 18

3. Dynamic and genetic classification of environments 21

References to literature 31-32

I



ArticIvE I.

—

An Outline of the Relations of Animals to their In-

land Eni'ironinents. By Charles C. Adams, Ph.D.

The Dynamic Relations of Animals

i. introductory note

As creatures of habit, the attitude of mind with which we approacli

a scientific problem has much influence upon what we see in it or get

from it. Although the essence of life is activity—the response of the

changing organism to its changing environment—yet this dynamic
concepticm of animal relations, and all that it implies, has not become
as prevalent a mental habit among biologists as one might expect.

While some naturalists view the animal from a more or less dynamic
standpoint, they do not include a similar conception of the relation of

an animal toits environment. Still others view the environment more
or less dynamically but do not extend this conception to the animal,

and thus both of these conceptions lack completeness and are not thor-

oughgoing and consistent. The study f)f activities, or in other words
the stuily of processes, has made great progress in the allied sciences,

much to their advantage, and undoubtedly the prevalence of similar

conceptions will lead to similar advances in biology.

In the present brief paper I have attempted to discuss only certain

phases of the problem with the idea of emjihasizing tlie general jirin-

ciples involved, and in the hope that it may aid in making these con-

ceptions of more practical value in investigation, and also facilitate an
understanding of the discussion contained in a report on the inver-

tebrates of the Charlestfm (Illinois) region, to appear in a subsef|uent

paper of this volume of the Laboratory Bulletin.

2. THE relations of ANIMALS TO THEIR ENVIRONMENT

The study of animal ecology may I)e taken up from many sides and
in many ways. One of the most interesting and fundamental of these

is tliat which considers the dependence of the animal upon its environ-

ment, and at the same time orients it in the gamut of energies and
substances. Many phases of this discussion, though elementary and
for this reason easily overlooked, are yet of fundamental importance.

Every boy who has kept pets in confinement, .and wlio has had the re-



sponsibility of caring for tliem, and every one who has cared for

domestic animals, knows what constant attention must be given to keep

them suppHed with food, water, shelter, and other "necessities of life."

And who can overlook the fact that it re(|uires attention to maintain

his own physical health? In the laboratory this dependence upon the

envirtmment is readily tested experimentally by any method of isola-

tion which will prevent an animal from securing any "vital necessity" :

as air—when sealed in a vessel; or food—when locked up without it;

or a favorable temperature. No animal can survive such isolation

from its normal environment. Every student of animals in nature

must also realize that similar supplies and conditions determine and
control the existence and welfare of all wild animals. The animal is

not self-sustaining, but requires a constant intake of energy and sub-

stance from its environment. Chemical methods will readily show the

source from which the materials composing the animal body have been

derived. The ash came from the soil or rock, and shows the animal's

dependence upon the solid earth; the liquids came from the water of

the earth and constitute from fifty to ninety-five per cent, of the bulk

of the animal's body, showing that a relatively large quantity of this

substance is essential to all living animals ; the abundant gaseous ele-

ment was derived from the atmosphere, to which it will again return.

The substance composing the animal body is thus derived mainly from
the water and the air rather than from the relatively inert and stable

earth. It will be profitable for us to imagine these proportions so

changed that the solids instead of the relatively mobile Hcjuids and
gases form the principal mass of the body, keeping in mind meanwhile
the slow rate of chemical change in solids compared with the change

in substances in a finely divided condition, such as liquids and gases.

If the solids predominated, the rate of the chemical change, upon
which the active life of animals depends, would be greatly retarded,

and animals, including man, would be stolid beyond comprehension.

Furthermore, we must not overlook the fact that animals are not main-

tained solely by substance, because substances are also carriers of en-

ergy, substance and energy never being separated. The living animal

is not a producer: it can make neither substance nor energy, nor is it

a kind of energy; it is solely a traiisforiiicr, a chemical engine which

changes the form of substance and chemical energy and produces new
combinations from the old. The living plant transforms energy and
inorganic substance, from the air, water, and earth, into complex
chemical compounds, and thus concentrates powerful chemical energy

in such a form that the animal, by a further change, is able to set it

free and to utilize it. Sugar, starch, and gluten are familiar examples



of this "tablet" or "cartridge" form of chemical energy wliich animals

explode or set free and then use in maintenance. During this trans-

formation, in which chemical energy is set free, waste products—inert

chemical substances—arc formed which if not eliminated from the ani-

mal system will prevent its operation, just as ashes if not removed will

check a furnace. Respiration aids in the removal of carbonic acid

gas— a waste product—from the body, but we often forget that the

chemical energy derived from the oxygen is an important feature in

respiration. By another process the h'(|uid and the solid waste is re-

moved. Thus gases, liquids, and solids are taken into the body and
later returned to the environment in a different chemical condition,

thus completing a cycle of transformation. That the animal body is

so largely made up of solutions and gaseous substances is an important

factor in its relatively unstable chemical condition, a condition of un-

stable cqiiilihn'imi, which determines the active and dynamic character

of the animal. Since, then, chemical activity is one of the essential

characteristics of a living organism, its influence forms one f)f the main
problems of the zoologist when studying the changes in animal activi-

ties ; their orderly sequence and the laws which govern them.

On account of the fact that the animal is a chemical engine, it is

able to use chemical energy to the fullest extent. If we assume a hier-

archy in the forms of energy, chemical energy seems to belong to the

upper class ; for though some forms of energy are not readily trans-

formed into chemical energy, chemical energy can be transformed into

all others. As a result the animal, being a cliemical engine, has, as it

were, an "inside track" to tiie main sources of energy, and thus by
transformation is able to utilize chemical energy to form light, as in

the fireflv, or electricitv, as in tlie electric eel; and other forms of en-

ergy useful to the animal are similarly derived. This study of the

activities of living animals, as contrasted with the study of dead ones,

is a phase of the general science of energetics, a science which fur-

nishes the basis for the correlation of many diverse branches of knowl-

edge.

The activities and transformations within tlie ;inimal bodv show
lis very clearly how an animal is dependent upon environmental condi-

tions. The animal transforms air, w.ater, and rock, and all animal

habitats and environments must contain these clcmenls. In nature

these are combined in a multitude of ways. The interrelations of these

fundamental environmental imits have been strikingly expressed by
Powell ('95: 22-23) .-is follows:

"The envelopes of air, water. ;md rock are so distinct tinat they can

be clearly distinguished ; and yet, wiien they arc carefully studied, it is



6

discovered that every one encroaches upon the territory of the others,

not only by interaction, but also by interpenetration. It has already

been shown that the water penetrates deep into the rock. Every spring

that falls from the hillside gives proof that the r(icks above its level

hold water, which they yield slowly as a perennial supply ; and the in-

numerable hills of the continents and islands have their innumerable

springs. Every well proves that there is water below ; every artesian

fountain shows the existence of underground waters ; and every boring

in the crust of the earth, and every excavation in underground min-
ing, discovers the presence of water.

"Wherever water flows, air flows with it, and all natural waters

are permeated with air.

"Tile aqueous envelope is everywhere permeated with rock, which
it holds in solution or suspension, and there is no natural water abso-

lutely pure. The sea is full of salt. Salt lakes are more than full of

salt, and so they must throw it upon the bottom ; and the waters hold

lime and many other substances. Not a drop of pure water can be

found in the sea ; not a drop can be found in a lake ; not a drop of pure

water can be found in any river, creek, brook, or spring; and not a

drop of pure water can be found underground : it is all mixed to some
degree with rock.

"All natural waters are aerated. No drop of water vmmixed with

rock and air can be found, except by the process of artificial purifica-

tion.

"But surely there is pure air? Nay, not so. There is no natural

air unmixed with rock and water. All the air that circulates above the

land and sea, within the ken of man, and all the air which circulates

underground, is mixed with rock and water.

"Pure air is invisible: it will not reflect light; it is transparent, but

will not convey light. Light is conveyed through the atmosphere by
ether, and is reflected and refracted by rock and water; and it seems
to be largely afifected in this manner bv rock. If the ambient air of

the earth were pure, there would be no color in the sky, no rainbow
in the heavens, no gray, no purple, no crimson, no gold, in the clouds.

All these are due largely to the dust in the air. The purple cloud is

painted with dust, and the sapphire sky is adamant on wings.

"Land plants live on underground waters : were there no subter-

ranean circulation of water, there would be no land plants. Fishes

live on under-water air : were there no circulation of subaqueous air,

there would be no fishes in the sea. The clouds are formed bv par-

ticles of dust in the air. which gather the vapor : were there no dust in

the air, there would be no clouds ; were there no clouds, there would
be no rain."



Up to this point we have considered mainly the processes of main-
tenance of the animal body, but there are other processes as well which
must be called to mind, such as growth, development, multiplication,

and behavior. Physiologically considered, none of these activities are

essentially different from the fundamental phases of metabolism and
all are dependent upon it ; they are special forms of the transformation

of substances and energy within the animal. As the individual animal

grows and develops in its life cycle, its metabolism, form, and behavior

change in an orderly manner, and this transformation is in the main
a continuous process like the other transformations of matter and en-

ergy. The changes which take place during entogeny are often greater

than the differences which e.xist between very distantly related adults,

and these differences result in very different roles which the animal

often plays in the economy of nature.

Comparable to the responses of the animal to its environment, and
indeed essentially of the same kind, are the responses of any part of

an animal to all its other parts, the entire organism, in this case, being

considered as a unit. The environment of an internal parasite is

formed by the body of its host, and in a similar sense the different

parts of the body are parts of the environment of the other

parts. The different parts of the animal body are what they are

on account of three conditions. The first is determined by its

relative position and responses as a member of a series of

successive generations. In this way the hereditary potential-

ities are determined. Ecologically considered heredity may be

regarded both as the response of individuals (unicellular) and
germs to the conditions of life, and as the mutual responses of

different germs to one another. The crossing and intermingling of

germinal elements is as truly a response as are other forms of activity.

Secondly, there is considerable evidence which indicates that at sonic

stage in the development of an animal any part is potentially capable

of developing into any other part. The character of development,

then, is conditioned by the character of the cell-environment—its rela-

tive position, and all that implies with regard to environment. A frag-

ment of a regenerating animal develops differently according to its po-

sition, and this is a response to its relative position in the cell commu-
nity. Thirdly, the development of an animal is conditioned by its ex-

ternal environment. The external conditions influence animals by
changing their internal activities. The internal changes modify the

cell community and change development. In this manner every part

of the animal is influenced by the conditions of its existence.

The processes of metabolism are continuous as long as life lasts.

Thus, as an animal respires there is a gaseous exchange, from the



earliest stages of its existence until its maturity and death. Eggs re-

spire as surely as larva? and adults, and the chemical, physical, and

physiological changes within them vary with their growth and develop-

ment. Some of these changes are primarily dependent on the orderly

course of development during the life cycle, and are therefore irrever-

sible processes, because no higher animal which is mature may reverse

its development and become young again. At different stages of de-

velopment different enzymes and harmones appear v.hich modify the

physiological conditions of growth, development, and behavior. Envi-

ronmental changes, persistent and uniform, or periodic in character,

tend to modify and alter these internal processes, and are an additional

source of change, which is particularly shown in behavior.

It is interesting to observe in this connection that certain factors

are important as they Jiastcn or retard other processes. Thus enzymes

iiastcn chemical changes which without them would take place at a

very slow rate, and they set free much energy in a relatively short

time. Temperature is another hastener of chemical reaction. Not
only is it a condition which sets limitations on the chemical reaction in

animals, but it also influences their optimum, and with increasing tem-

perature chemical changes take place within the animal irrespective of

the control of the animal, except in the warm-blooded animals, where
a mechanism exists which regulates, within certain limits, temperature

conditions.

3. OPTIMA AND LIMITING FACTORS

We have seen that the animal is dependent upon its environment
for both substance and energy. If, therefore, the environment does

not contain, in available form, both substance and energy, animals will

not be aljle to live in it permanently, although with energy stored in

their bodies they may be able to make more or less prolonged and suc-

cessful invasions into such an environment. The optimum is the most
favorable condition for anv function. We may consider optima cor-

responding to units of different rank: a single cell or tissue in action,

an organ or system of organs, the animal as a whole, a taxonomic
unit—and so on, to an animal community or association. There are,

then, many kinds of optima, and the study of the conditions which pro-

duce them is a complex subject. The optima for different fimctions

may differ much; for example, that for growth is often different from
that for reproduction, and the optima may also change greatly with the

development of tlie animal. Optima, therefore, are not fixed condi-

tions, even though they do represent a condition of physiological rela-

tive eqiiilihriuni. The amount or intensity of substance and energy

which produces an optimum is limited above by the maximum and be-

low by the minimum. Thus departures from the optimum, toward an



9

increase or a decrease, are departures from the most favorable condi-

tions toward less favorable conditions, and hence toward limiting con-

ditions. This form of expression is mainly that of the laboratory; it

is desirable therefore, in addition, to express it in terms of the normal
habitat. In nature we look upon the optimum as that complex of

habitat factors which is the most favorable, and departure in any di-

rection from this optimum intensity is in the direction of a less favor-

able degree of intensity or into unfavorable conditions. From this

standpoint any unfavorable condition is a liniHting factor and may re-

tard, hasten, or prevent vital and ecological activities. Optima arc

thus almost ideal conditions, and are probably realized in nature only

to a limited degree; in other words only approximalely. Here also,

as in the laboratory, they represent a condition of relative equilibrium.

The laws of the transformation and development of optima are of

great ecological importance, as I pointed out several years ago ('04).

In field study probably the most valuable criterion to be used in the

recognition of ecological optima is the normal relative abundance and
influence of animals in their breeding environment.

In the preceding discussion no special emphasis has been placed

upon the time element, or the rate at which changes may take place.

Natural environments are complexes, in the composition of which sev-

eral factors are involved. This being true, it is desirable to recall the

fact that the rate of change is determined by the pace of the slowest

factor, or, as Blackman ('05:289) has expressed it: "When a proc-

ess is conditioned as to its rapidity by a number of separate factors,

the rate of the process is limited by the pace of the 'slowest' factor."

This is a general law and applies to all changes, internal as well as

environmental.

In closing this section, I wish to call attention to another conclu-

sion of the English plant physiologists Blackman and Smith. They
state ('11) that from experimental study of the assimilation of water

plants, the conception of the optima is untenable, and that the phe-

nomena are better explained as the result of "interacting limiting

factors than by the conception of optima" (p. 412). This principle is

formulated as follows (p. 397) : "When several factors are possibly

controlling a function, a small increase or decrease of the factor that

is limiting, and of that factor only, will bring about an alternation of

the magnitude of the functional activity." It will be of much impor-

tance to test the application of this idea to animal responses.

4. DETERMINATION OF DYNAMIC STATUS

In any study of the energetics of organisms it is desirable to have
clearly in mind one of the fundamental conceptions of this science

—



10

the dynamic status. The law of conservation of energy teaches us

that energy can not be destroyed ; that it is transformed only, and thus

undergoes a cycle of changes. The animal or an animal community,

as a unit and as an agent or transformer, is constantly transforming

energy, setting it free. In this sense it originates, but not at a uniform

rate. At one time much energy may be transformed, and at another

very little. When a great amount of energy is being set free, when
the animal or community is exerting much influence, we may look upon
it as producing pressure or strain. A condition of stress is not a per-

manent one, because the pressure tends to cause such changes as will

equalize or relieye this condition. This is considered as the process

of adjustment to strain, and is called Bancroft's law ('ii). An ani-

mal in an unfavorable condition is stimulated, its normal activities are

interfered with, and a physiological condition of stress is produced

which lasts until by repeated responses or "trials" the animal escapes

stimulation or succumbs and a relative equilibrium is established. An
area may become overpopulated and consequently there may be estab-

lished a condition of stress, which results in an adjustment by a reduc-

tion (through many causes) in the excess of population and a restora-

tion of the normal, or a condition of relative equilibrium. From these

examples it may be seen that the dynamic status means the condition

of a unit or system with regard to its degree of relative equilibrium.

The cycle of change may be considered to begin at any point. I have

taken as the initial stage of the cycle the condition of stress or pres-

sure, and have indicated how this condition tends to change in re-

sponse to pressure, bringing about the process of adjustment to strain,

and leading to the condition of adjustment to strain, or that of relative

equilibrium. The activity of the agent produces the condition of

stress, the process of adjustment to the strain follows, and this leads

to the product—the establishment of the condition of adjustment or of

relative equilibrium.

These conceptions are very suggestive when applied to various

phases of organic activity, and aid greatly in utilizing the dynamic con-

ceptions which are in constant use in many of the physical sciences.

But we can not assume that these ideas will take definite form unless

the student makes some special effort to master the principles involved.

5. ANIMAIj RESPONSES

The gcner.'d character of the changes within the animal, which re-

sult in the transformations of energy and substance, or the process of

metabolism in its broadest sense, is the basis of all animal responses.

It is well known that growth, development, and behavior are condi-



11

tioned by certain metabolic processes, the rate of which are further

conditioned by the presence of certain substances, as enzymes (from
liver, etc.), and internal secretions (from thyroid, testes, adrenals,

etc.). The influence of certain physiological conditions or processes

is thus well known to affect the behavior of animals. The changes

of instinct through the removal of the testes or ovaries, may be cited

as examples of this influence. An animal whose metabolic processes

have reached a certain stage is said to be satiated; later it is in the con-

dition of incipient hunger ; and still later, in the physiological condi-

tion of intense hunger. These internal changes cause the animal to

react very differently to any food which is in its immediate vicinity.

These changes in physiological conditions are strictly comparable to

the change which an animal passes through in its ontogeny ; to the life

cycle of an insect, for example, in which the physiological conditions

and behavior of a caterpillar are very different from those of the pupa
and of the adult or moth. One of the higher animals, a dog, for in-

stance, will undergo internal changes which will completely alter its

responses at the sight of an old rival or enemy. Such considerations

as those just cited show clearly that extensive internal physiological

changes take place in animals, and that while some of them are very
gradual others are exceedingly rapid. These internal conditions or

changes have been well characterized by Jennings ('06:289) ^s fol-

lows : "The 'physiological state' is evidently to be looked upon as a

dynamic condition, not as a static one. It is a certain way in which
bodily processes are taking place, and tends directly to the production

of some change. In this respect the 'law of dynamogenesis,' pro-

pounded for ideas of movement in man, applies to it directly (Bald-

win, '97: 167) ; ideas must indeed be considered so far as their objec-

tive accompaniments are concerned, as certain physiological states

in higher organisms. The changes toward which the physiological

state tends arc of two kinds. First, the physiological slate (like the

itlea) tends to produce movement. This movement often results in

such a change of conditions as destroys the physiological state under
consideration. But in case it does not, then the second tendency of the

physiological state shows itself. It tends to resolve itself into another

and different state."

I may thus summarize the relation of metabolic processes to

physiological conditions and processes of behavior by the following

table.



12

Table ].

—

The Dynamic Relations of Animal Activities

The Animal as an Agent
(Activiiil tif (III Afient)

The animal as an agent transforms

energy and substance by its motalj-

olic processes. These are acciim-

panie<l by physiological conditions

or states; they constitute a condi-

tion of unstable equilbrium. The
transformations take place as

—

(1) (lontinuous and irrevorsilile

processes, as development, differen-

tiation, etc.; or are^-

(2) Periodic or rhythmic proc

esses, as digestion sexual activity,

etc.

Processes of Activity

This unstable internal

condition tends towards
change, resulting in—

(1) New conditions;

(i) Movement;

(M) The j)rocesses of

behavior: trial, experi-

ment, investigation, etc.

Products of
Activity

New states.

'

Movement.
Response.
Regulation.

Adjustment.
Relative equilib-

rium.

Learning.
Orientation.

Data.
Concepts.

Explanation.

Theory.
Hypotheses.
Ideals.

Changes in the internal conditions

are produced eIso by external stim-

uli.

The responses of animals to the conditions in which they live are

of a composite character. Certain responses, such as the chirping re-

sponse of a coot within the egg, are inherited and are relatively auto-

matic in character; others are greatly modified by experience, as when
an animal "learns," or forms a habit by repeated responses.

The responses of animals to the conditions of existence are the

basis for any study of their relations, not only to other members of

their own .species, but to all elements, living or otherwise, of their com-

plete environment. It is from this standpoint that animals must be

considered in estimating their place in the economy of nature; that is,

in estimating how they influence one another in an association of ani-

mals living together in the same habitat, and in judging of their rela-

tion to the succession of animal communities, and even to man him-

self.

6. THE INTERRELATIONS OF ANIMALS

"A group or association of animals or plants is like a single organ-

ism in the fact that it brings to bear upon the outer world only the

surplus of forces remaining after all conflicts interior to itself have

been adjusted. Whatever expenditure of energy is necessary to main-

tain tlie existing internal Ix'ilance amounts to so much power locked

up, and rendered unavailable for external use."-—S. A. Forbes.

We have now seen the dependence of the animal upon its environ-

ment, as this forms the basis for an understanding of conditions in-

volved in the problem of viaiiilniaiicc or the upkeep of the animal.

The optimum conditions for prolonged maintenance produce the vital



13

and ecological optima. These conditions imply more than mere main-

tenance; they mean as well, a degree of favorable conditions which
permits the animal to exert an influence or stress upon its environment.

As Forbes has said, if all the energy available to the animal is utilized

internally there will be nothing left to influence the environment.

Metabolic changes show that large amounts of energy and substance

are used in maintenance. Under optimum conditions even greater

amounts must exist. An animal must not only be able to maintain

itself against other kinds of animals but even against its own kind, for

the overproduction of its own race will be practically self-destructive.

A good e.xample of this kind of influence is seen in the hordes of lem-

mings which migrate, even into the sea, when overproduction becomes
extreme.

The vital and ecological optima are thus to be looked upon as in-

ternally balanced, but externally, not as a state of balance or poise, but

as a condition in which the animal is exerting stress, pressure, or in-

fluence upon its environment, instead of being passive or inert. A
group of animals living together in any given condition such as an
association, is an assemblage of interacting organisms. The active,

free-moving animals collide with each other, with other kinds of ani-

mals, especially the relatively sedentary kinds, and with their environ-

ment of plants and the inorganic factors. The relatively sedentary

animals are correspondingly bombarded by all elements of their en-

vironment. The association, as a whole, is thus in a continuous proc-

ess of bombardment and response from every possible angle, and just

as the individual animal is stimulated and responds, so all the mem-
bers of any association are stimulated and respond in a similar man-
ner. It is by this form of activity that animals not only maintain

themselves but e.xert a radiating influence.

It will assist in realizing the constant pressure exerted by animals

if we compare their activity to the flow of a stream. The pressure ex-

erted by the stream mav be realized if by a dam or similar means the

current is resisted. Think for a moment of the amount of energy
which would be transformed in an efifort to prevent animals (or

plants) from taking possession of a favorable habitat. Imagine an
area lo feet square and think of the efifort it would require to prevent

animals permanently from invading and establishing themselves in this

habitat if no barriers were interposed, and if the means of destruction

of the invaders were not so drastic that they materially changed the

character of the habitat. Increase the size of the area and the diffi-

culties will increase in geometrical ratio, and the utter futility of such

an undertaking will soon be realized. The spreading processes of the

gypsy moth in Massachusetts, and of tlie San Jose scale and the cotton



14

boll weevil, show us in terms of human experience something of the

energy expended by these radiating animal activities even when there

are strong human economic inducements against sucli mvasions.

When a balanced condition, or relative equilibrium, in nature is

referred to, we must not assume that all balances are alike, for some

are disturbed with little effort and others are exceedingly difficult to

change. This distincticjn is an important one. Once the balance is

disturbed, the process of readjustment begins. This is a phase of the

l)alancing of a complex of forces. Just what stages this process will

pass through will depend, to an important degree, upon the extent of

the disturljance. Slight disturbances are taking place all the time and

grade imperceptibly into the normal process of maintenance, as when

a tree dies in the forest and its neighbors or suppressed trees expand

and take possession of the vacancy thus formed. Disturbances of a

greater degree, on the other hand, may only be adjusted by a long

cumulative process. This change can progress no faster than the rate

at which its slowest member can advance. Thus a forest association

of animals may be destroyed by a fire so severe that all the litter and

humus of the forest floor is burned. The animals which live in the

moist humic layer as a habitat, such as many land snails, diplopods,

and certain insects, can not maintain themselves upon a mineral soil,

rock, or clay. As such a forest area becomes reforested, these animals

can only find the optimum conditions when the slow process of

luunus formation reaches a certain degree of cuinulative development.

Under such circumstances this later stage must be preceded by ante-

cedent processes, and restoration of the balance is long delayed. Some
adjustments take place so cjuickly that little can be learned of the

stages through which they pass. There are, however, many slow proc-

esses which afford an abundance of time for study ; in fact some are

too slow to study during a lifetime. The processes which are moder-

ately slow are often particularly illuminating because all stages are

frequently so well preserved that comparison is a very useful method

of study; the slowness of a process has a certain resolving power, as

it were, recalling the influence of a prism upon a beam of white light,

which reveals many characteristics obscure to direct vision. A study

of the processes of adjustment among animals is a study of an im-

portant phase of the problem of maintenance. The continued process

of response will, if circumstances permit, lead to a condition of rela-

tive adjustment, or to a balancing among all the factors in operation.

7. KCOLOGICAL UNITS FOR STUDY

In the study of animal responses many different units are avail-

able, and a brief consideration of these will aid in an understanding of



15

the methods which are useful. Because the animal body has been

found to be composed of a single cell or a multitude of cells, a com-
mon belief has grown up that the cell is the natural unit for study.

This opinion seems to be due to overlooking the fact that there is just

as much reason for considering the zvholc animal as the unit. The
unicellular animals are whole anhnals as truly as they are cells, and
in multicellular animals the activity of single cells means little inde-

pendently of the animal as a whole. It thus seems that ecologically at

least the smallest valuable unit for study is the indk'idual animal. The
responses of the individual, as a kind of animal, to its condition of

existence form the basis for what may be called indiindiuil ecology.

Animals which are related by descent from common ancestors, as a

community of social animals (e. g., an ant colony), or taxonomic
units, such as genera, families, orders, etc. (e. g., fish, birds, catfishes,

and salamanders), are also units which may be studied ecologically.

Some of these hereditary units are, ecologically, fairly homogeneous,
as, for instance, when a taxonomic unit is equally distinct ecologically

:

e. g., the woodpeckers with their arboreal habits. In other cases the

taxonpmic unit contains animals of great ecological diversity, as in the

case of beetles, which possess almost unlimited ecological diversity, in-

cluding littoral, aquatic, subterranean, and arboreal habitats, and para-

sitic, herbivorous, and predaceous habits. The study of ecology, upon
the basis of such a unit, may be called aggregate ecology. Still another

unit is available, based upon the animals which live together in a given

combination of environmental conditions, as in a pond, on the shore

of the sea, in a cave, within the bodies of animals, on the floor of the

forest, or in the tree tops, etc. The animals found living together in

such conditions form an animal association or a social community, and
the study of the responses of such a community is the province of

ussociational ecology.

8. the; animai, association

In the study of the animal association as a unit, we consider it as

an agent, whose modes of activity, or responses, are of primary inter-

est. We desire to know the kinds of animals which compose the com-
munity, the optimum anil limiting influences which control its activity,

the character of its responses, and the orderly sequence of changes in

the environment to which it is responding.

The maintenance of an association depends upon the maintenance

of the individual members which compose it, just as the maintenance

of the entire animal depends upon the activities of the cells. There is

the same Iiasis for speaking of tlie responses of the association as there



16

is for speaking of the responses of the individual. The association can

continue to exist indefinitely only in such environments as possess, in

available form, substance antl energy for its individual members. The
activities of the individuals transform energy and substance, produc-

ing growth, development, multiplication, and behavior. The persist-

ence of an association in a given habitat brings about the formation of

certain waste products, which if not changed or transformed at a cer-

tain rate, or transported from the environment in some way, tend to

limit the optimum activity of the individuals and of the association.

In the association, as in the individual, there must be an internal rela-

tive balance before there can be such a surpkis of energy that the asso-

ciation can radiate or exert outward stress or pressure. An association

which is only maintaining itself is not at an optimum, for in this latter

condition there is a surplus of energy, and the activity, rate of multi-

plication, and favorable development under normal conditions are fa-

vorable to the extension of the association. The pressure which such

an association exerts is shown by the progressive extension of its range

of influence. By the active movements of the animals, by the activity

of the environment, or by both together, they tend to invade other

habitats and areas, and in such of these as afford favorable conditions

they tend to survive and extend the area of the association. From the

standpoint of the association the behavior of these active pioneering

animals corresponds to the trial activities in the behavior of the indi-

vidual animal. These activities are not different in kind from those

whicii are involved in normal maintenance. They are those which
form the initial stages in the establishment and extension of the asso-

ciation in a new locality or the re-establishment in an old one, and thus

lead to a sequence or succession of associations. Ecological succession

thus consists in an orderly sequence or series of associations which
occur successively and form a genetic series.

9. ASSOCIATIONAL SUCCESSION

A succession of associations takes place either through the trans-

formation of older ones, or through the origin of a new one on a

surface which has been newly formed and has had no population. A
favorable habitat without a population of animals is comparable in

some respects to a vacuum ; it exists as a condition of unstable equi-

librium which tends to change toward a more stable state. The active

life of animals tends to lead them into all possible habitats, and where
they find the conditions favorable for existence they tend to survive

and thus bring about the establishment of an association. Each asso-

ciation, like the individual animal, has a certain amount of unitv and



17

tends to maintain or perpetuate itself. But tlie stability of associations

is only relative, and some are much more stable than others. Naturally

the unstable ones are those which show succession most readily. Thus
if we destroy a few trees in a hardwood forest and produce a glade, a

large number of the characteristic animals of the dense forest will dis-

appear and be replaced by animals which normally frequent open
places; then in a few years sprout-growth and young and suppressed

trees will change the conditions so much that the kind of forest ani-

mals which were eliminated for a time will begin to return ; and when
the new grt)wth is replaced by the mature forest the animals of the

mature forest will return and a new ecjuilibrium will be formed. In

such a forested region the glade is to be looked upon as an unstable

condition, which through a succession of associations will later arrive

at a relatively stable conilition, whicli is able to perpetuate itself indefi-

nitely under existing conditions. Such an association is considered a

cliviax, or the culmination of a series of successions under existing

conditions. The succession of associations leading to a climax repre-

sents the process of adjustment to the conditions of stress, and the

climax represents a condition of relative et|uilibrium. Climax associa-

tions are large units, and are the resultants of certain climatic, geolog-

ical, physiographic, and biological conditions.

The Dynamic Relations of the Environment

i. introductory

In the preceding section we ha\e seen that to understand animals

we must consider them as active living agents which are constantly

changing and responding to their environment. That the environment
of animals should also be studied as an actk'cly cimnging medium has

not been as clearly recognized by students of plants and animals as one
might anticipate from its importance. Some students feel that the

study and understanding of the environment is not a part of zoology,

or at least not an essential part. Furthermore, to some of these stu-

dents at least, the envirt)nment seems largely chaotic, a confused un-

wieldy mass with no e\itlent favoraljle point of attack. This view is

quite natural to those w lio h.ive had no training and practical experi-

ence in recognizing the "orderly sequence" or laws of environmental

changes, and [larticularly to those wiio do not feel that environmental

relations are an essential i)art of their subject. By many such students

the environment is viewed in a manner comparable to the prevailing

chaotic views on weather before meteorology became a science, or on
taxonomy before IJnn:cus, or on geology before Lyell. If one has seri-

ous doubts on this point, he need only turn to the standard treatises



18

on zoology and search for a comprehensive and ade(|uate recognition

and utilization of the orderly and regulatory character of the environ-

ment as an essential part of the subject.

The fallacy of this position has been well expressed as follows by
Brooks ('99) : "I shall try to show that life is response to the order of

nature. . . . But if it be admitted, it follows that biology is the study

of response, and that the study of that order of nature to which re-

sponse is made is as well within its province as the study of the living

organism which responds, for all the knowledge we can get of both

these aspects of nature is needed as a preparation for the study of

that relation between them which constitute life." Later he says : "But
if we stop there, neglecting the relation of the living being to its en-

vironment, our study is not biology or the science of life." No one
seems to have attempted to refute this; naturally an easier path is fol-

lowed—to ignore it. Perhaps up to the time of the present generation

there has been some excuse for this confusion ; but now the respon-

sibility does not rest upon students of the physical and vegetational en-

vironment Ijut upon students of animals, because the former students

have arranged their scientific data in a manner which clearly shows
the orderly lawful sequence of changes in environmental activities.

This should form the basis for a study of the corresponding series of

changes which take place within the animal, and also be the basis for

a study of the reciprocal responses taking place between the animal and
the environment.

In this section an outline will be given of some of the most impor-

tant phases of environmental changes in inland areas viewed as lawful

and orderly, particularly those changes which influence animal hab-

itats.

2. THr; DYNAMIC AND GENETIC STANDPOINT

Since Lyell taught the scientific world that a study of processes

now in operation is the kev to an understanding of the present as well

as of the past, the process method has been slowly but inevitably pene-

trating to the utmost subdivisions of inquiry. With the progressive

appreciation and use of this method its efficiency has been increased.

Its progress has been the most rapid where the principles of its appli-

cation have been most clearlv understood. As models become known
in each field of work others will find the method much easier to apply,

and for this reason it is desirable that such examples become fairly

numerous and wide-spread.

In the application of the process method to an imperfectly under-

stood subject, and particularly to a complex one, it is desirable to con-

sider the subject as a unit or entity. This unit may then be regarded

as an agent whose process of activity is to be studied, for the activity



19

of an agent gives us a process. Thus an organism, a plant society, or

an animal community is a very complex unit or agent, which largely

through chemical energy, under conditions of a normal environment,
responds in an orderly sequence or changes. The environment changes,

the internal conditions of the animal change, and so do the correspond-
ing responses on the part of the animal. When all of these changes
are studied as orderly processes we are able to see the advantage of

this method of study. It is desirable to investigate all phases of animal
responses in this manner, such as growth, development, heredity, etc.,

in order to determine the causes and conditions of this orderly se-

cfuence. As a rule our recognition of the orderly sequence or laws of

action or succession precedes our knowledge of the causes and condi-

tions of the sequence. This order of secjuence is thus of fundamental
importance and must be recognized before it can be investigated or

explained. This method of studying the activity of agents, the char-

acter of their processes, constitutes the dynamic standpoint.

When the dynamic relations of an agent have been investigated, the

orderly sequence of its responses established, and the causes and con-

ditions of its activity determined, it is then possible to explain fully the

origin or genesis of its activities. The genetic metlwd is the study of

origins in terms of tlic processes iiivok'ed, and therefore the classifica-

tion of facts genetically implies a knowledge of the processes involved

in their origin. There are thus many degrees or stages in the develop-

ment of a genetic classification, the first step of which is to determine

the orderly secjuence of changes. In a certain sense, in its broadest

application, the process method is universal and includes the genetic,

but until their mutual relations become clearly recognized and are gen-

erally understood both should be emphasized.

Particular attention should be called to the fact that the activity of

an agent results in a process, and processes give us the laws of change.

Many processes are reversible; that is a process may go forward in

one direction and then become reversed and proceed in the opposite

direction. Other processes are non-reversible, and operate in only one

direction, being in a sense orthogenetic, as in the later stages of the

ontogenetic process.

Let us summarize the main characteristics and principles involved

in the dynamic and genetic method. They have been well expressed

by Keyes ('98), and for my purpose are arranged as follows:

"A truly genetic scheme for the classification of natural phenomena
thus always has prominently presented its underlying principle of

cause and effect. ... To begin with, an adequate scheme should be

based directly upon . . . agencies. . . . All products must find accu-

rate expression in terms of the agencies. . . . The primary groupings



20

uf the . . . processes must be based, therefore, upon the manner in

which these agencies affect the . . . materials. . . . Constructive and
destructive agencies can be recognized only when the phenomena are

made the basis for the scheme. Processes are merely operative. If

coupleil with products at all, in classification, all must be regarded as

formative or constructive. The product's destruction, its loss of iden-

tity, is wholly immaterial. The action of agencies is merely to pro-

duce constant change."

Van Hise ('04) has formulated other principles of the process

method as follows

:

"The agent is the substance containing energy which it expends in

doing work upon other substances. The substance upon which work is

done may thereby receive energy, and thus become an agent which

does work upon other substances ; and so on indefinitely. Indeed, the

rule is that one process follows another in the sequence of events, until

the energy concernet! becomes so dispersed as lo be no longer trace-

able. Theoretically this goes on indefinitely. . . . We have seen that

the action of one or more agents through the exertion of force and the

expenditure of energy upon one or more substances is a geological

process. It is rare indeed, if it ever happens, that a single agent works
thrtnigh a single force upon a single substance. . . .If geology is to

be simplified, the processes must be analyzed and classified in terms of

energies, agents, and results. Each of the classes of energy and agent

should be taken up, and the different kinds of work done by it dis-

cussed. . . . The general work of each of tlie agents and the results

accomplished should be similarly considered. Not only so, but the

work of the different forms that each of the agents takes should be

separately treated. Thus, besides considering the work of water gen-

erally, tiie work which it does both running and standing must be

treated. The first involves the work of streams ; the second, the work
of lakes and oceans. This involves the treatment of streams as enti-

ties. . . . The treatment of the agents will be more satisfactory in pro-

portion as the work done by each of the forms of each of the agents

is explained under physical and chemical principles in the terms of

energy."

X'iewed from this standpoint it is remarkable how many of our

current zoological conceptions are essentially static, and how confused

are our conceptions of the process method. Physiology is supposed
to be devoted solely to processes, yet physiologists use the terms anab-

olism and katabolism, constructive and destructive influences, and,

likewise, zoologists frequently use the expressions "the friends" or

"the enemies" of animals—a dual terminology which has a certain



21

utility but wiiich exists mainly on account of the static conceptions of

organic relations.

The dynamic or process concept is a difficult one to attain, and to

apply in all cases, as any one will soon learn if he strives to do this

consistently; and vet as a scientific ideal there can be no doubt that it

lias the same superiority over the older static methods and point of

view that an explanation has over an empirical description.

3. DYNAMIC AND GENETIC CLASSIFICATION OF ENVIRONMENTS

In the natural liistory sciences we have two main sorts of classifi-

cati(ins of phenomena, those which we call "natural" and those wliich

we call "artificial." Natural classifications are those in which
the basal criteria are of origin, the method of processes or gen-

esis. A classification of lakes upon the basis of the processes idiich

operated in their origin—crustal movements of the earth, the mean-
ders of streams, the work of an ice-sheet, volcanic activity, etc.

—

would at the same time furnish an explanation of them in terms of
their origin. Artificial classifications are those in wiiich the criteria

are arbitrarily chosen. Any character may be made the basis for an

artificial classification. Thus lakes may I)e classified upon the basis of

their size, depth, color of the water, distance from cities, number ot

boats upon them, etc., Iiut such classification would not furnish the

basis for -a scientific explanation of lakes. The artificial is often useful

or convenient ior a speci.al purpose; the genetic is illuminating from
the standpoint of scientific interpretation. This method may be ap-

plied to any kind of environment, physical, physical and biological

combined, or solely biological. To tlie degree that the environment is

dominated by the phvsical conditions the laws of physical change and
physical genesis will preponderate in the origin of such environments,

and corresponding relations apply to biological environments.

The dependence of the genetic method upon causes and conditions

makes it impossible to divorce it from the local conditions. This is

at once the strength and weakness of this method, for it is particular,

and generalized averages mean little because origins are different un-

der diflferent conditions; this is the key to indi-i'ldnality. Thus streams

viewed as stages in the progressive transformation of a lifitiid medium
for life, may be formed in many diveise ways, and for this reason tlie

general principles of tiic method of genesis mav be expressed most
simplv in an ideal case. Genetic scries are nnending, thev extend into

tlio past ;md will continue in the future. The point of departure for

study must therefore be arbitrarily chf)sen. and the more nearly a nat

ur;d Iiasis ain be apiimximated tlie simpler its application becomes



22

For this reason a cycle will be followed here which begins with a con-

dition of stress, advances through the process of adjustinent to strain,

and reaches a condition of relative equilihriuni. The starting point in

such a cycle we will consider as the original conditions, and the later

activities as the derived ones. The original conditions we will assume

to be an uplifted undulating plain, composed of relatively homoge-
neous materials, in a humid climate, and covered by a varied vegetation

including trees. The elevated condition of the land produces a condi-

tion of unstable equilibriitni or stress for the rain falling upon its sur-

face ; and, furthermore, the vegetation will tend to spread over the en-

tire surface, and thus exert a certain pressure also These original

conditions are, therefore, unstable and destined to change, and mu-
tually to influence and regulate one another.

If we now imagine the rain "turned on" under such conditions,

what are the main processes which will operate ? The rain falling in a

depression will be supplemented by that which drains from the eleva-

tions; thus, through the agency of running water, a standing water

habitat will have its origin. With this concentration of water will

come also a burden of debris from the upland ; and in this way the

"constructive" and "destructive" processes will begin at the same time.

Plants will invade such a depression and add their remains. Some of

the depressions will overflow and the outflowing streams will cut down
the outlet to progressively lower levels, and ultimately drain the basin.

On the other hand, inwash and organic debris may together accumu-

late at such a rate as to raise the level of the basin above ground water

and thus transform the conditions to that of land. The progressive

stages of the process of degradation thus favor the transformation of

the depression and a progressive formati(~)n of lakes, which are con-

verted into ]xinds and swamps and ultimately, with drainage, to dry

land. For depressions we thus get a genetic series which we may call

the lake, pond, and swamp series. This does not classify the depres-

sion series according to size, depth, character of water, etc., as in an

artificial classification, but in the order of their development or genesis

through the agency of running water. Accompanying this sequence

there are of course changes in size, depth, etc., but these are subordi-

nated in the classification to the developmental sequence centering

about the process of the degradation, of the land by the agency of run-

ning water. This is therefore a classification of environments, not on
the basis of the product, as it might appear from calling it a depression

or standing-water series, but upon the basis of the activity or proc-

esses of the dominant agent.

We will assume that all the lakes, ponds, and swamps, due to the

original relief of the land, become drained and constructively con-



23

verted into streams or dry land. Let us consider the streams, particu-

larly those which did not develop from the lake, pond, and swamp se-

ries, in order to consider them in their simpler conditions of develop-

ment.

The first shower on the new land surface, or the beginning of a

cycle, forms an extensive ramification of small streamlets, their den-

dritic branches flowing down all slopes. With the confluence of the

smaller branches the progressively larger trunks are formed, and with

their increase in volume, cutting progresses; but all traces of this

stream itself tend to vanish soon after the shower is over, although

some water may linger in pools in the deeper depressions. These con-

ditions form an initial stage in the development of the activity of run-

ning water as an animal habitat. These temporary streams are rain

waters intermingled with dust from the air and soil from the ground.

Since, viewed chemically, such waters have not existed as a liquid long

enough to dissolve much gaseous and solid material, they represent a

relatively original condition, or an initial stage in the chemical devel-

opment of the stream as a medium for living animals. Again and
again these showers are repeated, and where there is a slight variation

in the hardness of the substratum small pools are formed on the softer

materials, where erosion is more rapid. In these pools it is possible

for some aquatic or amphibious animals, of marked powers of disper-

sal, to become lodged, or even entrapped, as in the case of animals

which migrate up the stream during its temporary flow ; such pools,

in fact, may be reached even by individuals from the ground water.

Finally these temporary streams cut down to ground-water level

and become permanent. Such a stream then, in addition to the fresh

rain-water which it receives with each shower, has a permanent supply

of ground water. This water, having filtered through the soil, con-

tains both gas, particularly COo, and minerals, and thus as a solution

differs much from rain water. The composition of ground water

varies much with the chemical differences of the substratum. Such
water generally contains enough substance in solution to be a favor-

able medium for plant growth, such as algre—aquatic pioneers which

arc comparable to the lichens in their invasion upon bare rock. But
the temporary flow of water is still dominant, and will remain so until

the supply of permanent ground water is of such a volume that, hav-

ing a good current, it rushes over the obstacles in its path ; then a per-

manent Ijrook has been evolved, and a permanent rapid-water habitat

has originated.

As the erosion of the stream advances, organic debris not only

multiplies indigenously in the water, but it is also wa.shed and blown
in, and through its decay the composition of the water is changed.



24

particularly in the amount of COo present. This gas causes the water

to take into solution a greater amount of lime ; and at the same time

the agitation to which it is subjected while dashing over obstacles or

flowing over falls increases the amount of oxygen present, a process

further aided by the oxygen set free in it by water plants. Carbonic

acid, moreover, is set free by the rapids and falls. It is thus very

evident that the chemical processes are undergoing an important devel-

opment as the stream progresses, since there are going on both the

process of gaseous eciuilibrium with the air, and an increase of the sol-

ids in solution. The stream is progressively becoming a more favor-

able or enriched culture medium for organisms. The rapidly flowing

water which characterizes the brook is the predominant physical fea-

ture of this environment, the stretches of relatively quiet water which

form the pools, between the more rapidlv flowing parts, anticipating

the kind of conditions which are destined to increase with the trans-

formation of the brook conditions into those of a creek. With the

progress of development in drainage a brook is progressively trans-

formed by the processes of erosion into a creek. Here the rapid-water

conditions are more nearly equaled by a corresponding enlargement of

the pool or the quieter stretches of water, where the finer sediments

are deposited and the animals dwelling on the surface film or in the

mud and sand, find suitable conditions. The falls and rapids which

characterize the brook are exceptional in the creek, but may linger

where the rate of change has been very slow on account of the resist-

ance of the substratum. The alternation of rapid and slower water,

which characterizes the creek stage, with the preponderance of the

relatively rapidlv flowing water, is gradually transformed into that of

a river, where the water flows at a slower rate and rapids and falls

have as a rule become extinct, and where a condition of relative chem-
ical equilibrium has also been reached. Here the burden of coarse

debris is at a minimum, and the surface, sides, and bottom of the

stream, have become differentiated as relatively distinct habitats. Witii

progressive approach toward baselevel all conditions of the environ-

ment tend to become more stable and equalized until the stream erodes

to tide level, becomes brackish and finally as salt as the sea itself, and
reaches an c([uilibrium determined by the dominant animal environ-

ment upon the earth—that of the sea.

We have now outlined the developmental sequence of wet depres-

sions, the lake-pond-swanip series, and the running water, the brook-

creek-river series, these two series including the main inland animal

environments in a lir[uid medium in a humid climate. We have yet to

consider the animal environments of land rmimals proper, those which
live in the gaseous medium of air. The complexity of conditions upon



25

land is much greater than that in water, either fresh or salt. In othe.'

words the land habitats are the most complex on earth. For simplicity

in handling this involved i)roblem, an ideal series will also be followed,

and instead of attempting to discuss all the principles involved, only

such will be mentioned as may be illustrated by a single example. This
will serve to show the application of the method. We shall consider

the process of degradation of the land, such as might be developed

during a topographic cycle of erosion, and as applied to a snow-capped
conical mountain in a temperate humid region.

Let us consider the series of processes which operate upon such a

mountain. The snow and rain which fall upon it are in unstable equi-

librium, the snow creeps or plunges down the slopes, and the water

flows down. In the zone of ice and snow physical and mechanical

changes preponderate ; but at lower altitudes, with the melting of the

snow and ice, on account of the higher temperature, chemical changes

become more prominent and supplement the mechanical work of run-

ning water. Here, also, plants and animals become an important factor

in modifying the processes of change by hastening or retarding the

processes of degradation. We thus see that on different parts of a

mountain there are important modifications in the processes of degra-

dation. The same general processes which operate to form lakes, ponds,

swamps, brooks, creeks, and rivers, are also at the same time producing

changes in the land hal)itats. The entire surface of such a moimtain is

undergoing change, but because of the concentration of degradative

progress near its base, particularly on account of the concentration of

the drainage there, ravines and valleys develop here more rapidly and
converge toward the main divide, the mountain top. As these ravines

and valleys enlarge, the mountain is lowered; and ultimately all is re-

duced to a plain, and to baselevel. The condition of stress which existed

upon the slopes of such a mountain as degradation progressed, became
relatively adjusted at that place, but where the degraded materials

were deposited a stress was hccouiiiuj ciinnilalk'c, and it is this ever

changing adjustment of stresses which makes natural processes unend-

With the degradation of the mountain, progressively higher zones

are lowered; the snow cap disaiipears; the region abo\e the tree limit,

and later the lower parts, are spread over a large area, and the moun-
tainous character is largely gone. In this manner and at the same time

as the land is degraded to a lowland by running water, in the water

itself a series of habitats is developing, and thus all the cmiroumcnt
is being trausfoniied. aloiuj relatively distinct htit tmitiially interde-

pendent lines, to-a'ard the same t/eneral direction or condition—a rela-



26

tive equilibrium resulting from the balancing of all stresses near sea

level.

In the preceding discussion no emphasis has been placed on the fact

that degradation of the land is only a part of a large cycle of activity,

and that the deposition of the degraded materials may be a cause of so

much stress as to initiate an elevation of the land. If the heavy solu-

ble materials from the land are washed into the sea and only lighter

materials remain behind, the increased stress resulting between the sea

and the land will tend to elevate the lighter areas until an equilibrium

is established between the heavy sea and the lighter land; therefore,

some crustal movements, at least, may be complementary phases of the

degradation of the land. The elevations and depressions of the sur-

face of the land with regard to the sea level may thus initiate new
cycles of transformation in all environments. These processes do not

need amplification here, although they should be noted ; but this lack

of amplification does not imply a minor influence of this factor. Still

another cycle may be initiated by the processes of vulcanism, a factor

the influence of which is easily overlooked in large parts of the world

but in others is very prominent. Only one more comprehensive physi-

cal factor will be mentioned ; that due to alterations in the atmos-

phere—climatic changes. Although the temperate humid climate has

been made the basis for the preceding discussion, it must be remem-
bered not only that there are other kinds of climates, but that these

undergo transformation or changes from such extremes as the cold

arctic deserts on the one hand, to the dry hot deserts on the other.

Within this great amplitude of climatic possibility is found one of the

greatest causes both of complexity in land environment and of many
local differences in the transformation of habitats.

To simplify this sketch of the operation of the physical features of

the environment the organic factors have been neglected, and these

should now be considered. On account of the ultimate dependence of

animals for food upon vegetation, many intimate relations exist be-

tween plants and animals ; furthermore, in addition to the food rela-

tions there are many other important ones, such as the physical and
chemical influence of the vegetation upon the soil, its influence upon
the temperature and humidity of the air and on light ; and, finally,

there is qualification of these influences by the difi^erent kinds of vege-

tation. A vegetational cover of grass has a verv different effect from
one of shrubs or a forest cover; conifers and hard-wood forests differ

in effect also ; and the succession of plant societies varies, not only with

different kinds of vegetation but also in difi^erent climates, and with

different physiographic conditions. As Cowles ('ii) has shown, there

are several cycles or series of successions of vegetation. Many of



27

these changes are dependent upon physical conditions which are

equally potent in their influence upon animals. Thus physical and
vegetational changes in comhination influence animals directly and in-

directly, and in the conditions due to this fact we find the basis for the

important control which vegetation exerts upon animals.

Animals themselves form an important part of their own environ-

ment, not only in their relation to their own kind, as mates or as prog-

eny, but also as members of an animal community whose members
must adjust their activities to one another through symbiotic, competi-

tive, or predatory relations. If any animal becomes abnormally abun-

dant, that is, more numerous than the conditions can support, this

number in itself becomes a weakness, through the positive attraction

of the organisms (plant and animal) which are able to prey upon it,

and soon the normal abundance is restored. For example, in a conif-

erous forest, bark-beetles (Scolytoidca) may increase to such an extent

that the forest is largely destroyed, and a succession is produced in

the vegetation as the conifers are replaced by a growth of aspen and

birch. As a result of this destruction of the kind of food and habitat

essential for the next generation of beetles, a proper habitat is lacking,

and the restoration of the normal number of beetles is hastened. This

same example also shows how one kind of animal may influence the

character of a whole community by its control over the vegetation.

The influence of man must be looked on from the same standpoint

as one views the activity of any other animal; as that of a member of

an animal communitv. He hastens and retards the changes in his en-

vironment as do other animals. In general his early methods are pred-

atory; he reaps where he does not sow; but later the milder competi-

tive and symbiotic relations and the constructive or productive aspect

become more prominent. Civilization is an attempt to make the en-

vironment "to order," liut as yet man has not learned how to produce

a permanent "optimum" along the lines of an ecological community.
.\s has already been said, to understand man we should view him as

an integral part of an ecological communitv, as one member of a biotic

communitv of plants and animals, or at least of an animal community
which includes all animals that are influenced by man—and not con-

sider him, as some students do, as a distinct entity w^ith little regard

to his animal and plant associates.

The main features of the preceding discussion may be summarized
as in the following table.



28

Table 2.

—

The Oeni;sis and Formation op Inland Habitats in a Humid Climate
AND THE Dynamic Status op the Processes

Dynamic status.
Phases in the formation of inland environ-

ments.

I. Unstable eqiiilil)riimi—condition

of stress or pressure

Original cnnditions ; elevated land area, or
new land surface, or beginning of new
cycle.

II. Process of adjustment to stress

or strain.

Process of formation of habitats; all hab-
its are constiiictive.



29

The preceding discussion is based upon the conditions of a humid
climate, but the semi-arid and the arid chmates should also be touched

on. In time, as ecological studies are extended to all kinds of land

areas, it will be possible to formulate all of the general principles of

the origin or process of development of land habitats; but at present

vast areas of the land have never been observed by a zoologist from a

modern ecological standi)oint. Most of the ecological studies of ani-

mals have been carried on in a humid climate, only slight attention

having been given to the ecological relations existing in an arid cli-

mate, and still less to those in alpine and polar regions. After the

humid regions have been better studied, the arid regions will probably

be the next to be carefully investigated. The plant ecologists, by their

studies in these regions, have already furnished important facts pre-

paring the way for the animal ecologist, because they investigate both

the physical and vegetational conditions upon the prairies and plains

of the West. If the regions of progressively increasing aridity are

examined, there will be found to be a corresponding series of changes

in the animal habitats. The standing-water series of habitats found in

such a series, in contrast with those of humid regions with fresh-water

lakes, ponds, and swamps in addition to the temporary fresh waters,

are alkaline and salt waters, and we find an extensive series ranging

from Great Salt Lake, Salton Sea, and Devil's Lake, to strong briny

pools and alkaline mud flats. These are, of course, as capable of a

genetic treatment as are the corresponding fresh-water bodies of hu-

mid areas. The stream series is also present in the arid region, but it

exists under conditions quite different from those in humid areas. The
through-flowing streams are relatively independent of local conditions

because their main supply of water is from the mountain ; but they are

nevertheless much modified l)y the character and amount of the burdei.

which they carry during the time of high water, and they tend to be-

come clogged at low water stages. The chemical composition of such

waters is (|uite different frijm that of regions continually leached by
rains. The small streams flowing from the mountains, whose dimin-

ishing volume does not allow them to traverse the arid regions, suc-

cumb, and disappear in the dry earth—examples of a second degree of

dominance of the desert or plains. Rut the truly characteristic streams

of the arid regi(jns are those primarily dependent upon the desert con-

ditions. Such streams are well within the arid regions and are domi-
nated wholly by them. They are solely of a temporary character, and
correspond to the initial stage of stream develoj)ment, the temporary
stream, in a humid climate. In an arid climate, however, development

does not proceed beyond this early stage, and the degradation and



30

baseleveling of the land is due to the combined influence of water and
the wind.

On land, the movements of the soil by the wind, as in the sand-

dune regions of true deserts, show us a characteristic condition ; in a

more humid climate, however, the dunes would tend to become an-

chored by vegetation. Other soils than sand are also blown about. The
extreme of dry desert conditions must be looked upon as the ultimate

or climax condition, a condition of relative equilibrium, under present

climatic conditions, for certain regions. A slight departure from these

extreme conditions is seen in such localities as receive most abundant
showers during the growing season for vegetation. These are able to

influence the development of the drainage only in a minor way, but

they moisten a shallow surface layer of soil and permit the growth
of short grasses, such as the bufifalo-grass (Schantz, 'ii 140). Very
recently another important source of water in the arid regions has

come to be recognized. This, McGee has shown to be the subsurface

or artesian waters which come up from below ; and this is an important

supplementary source of moisture in extensive areas in the arid West
(McGee, '13), where the evaporation is large. It is not unlikely that

even in humid regions where the soils are very sandy, as upon the

Coastal Plain, and where the strata dip in such a manner as to favor

an underflow of water, this supply may be of considerable importance

to the biota. With a greater rainfall during the growing season, per-

mitting a relative humidity greater than on the short-grass area of the

plains, a deeper-rooted vegetational cover gives us the long prairie

grasses of the eastern prairie.

As soon as the physical conditions permit a growth of vegetation

this material becomes an environmental factor which reflexly modifies

the physical conditions of the air, the soil, and the animal halDitat. This

is shown to a marked degree in the humid area of the southeastern

United States, where the rainfall, greater than tliat on the arid plains

and prairies, favors the development of a forest cover. Such a forest

not only tends to retard evaporation but also acts as a sponge and by
its vegetable debris and loose soil retards the run-off. In this manner
not only are land habitats influenced, but this conservation of moisture

tends to prolong the duration of temporary streams, and to stabilize

the flow of permanent ones; and, further, through the same influence,

the ground-water level declines slowly, and bodies of standing water

are also influenced. Thus all the more important habitats are to some
degree regulated and made more stable by a forest cover.

The foregoing discussion and examples, selected from the activi-

ties of animals and changes in their environments, are varied enough to



31

show how diverse are the applications of the process method to inves-

tigation. The general idea is easily grasped, but to make the dynamic
method a regular habitual procedure in investigation is truly difficult,

so difficult, indeed, that there is reasonaI)le ground for doubting if this

method can be mastered without a practical application of it to a con-

crete problem, at the same time giving special attention to the method
of procedure.

REFERENCES TO LITERATURE

Adams, C. C.

'04. On the analogy between the departure from optimum vital

conditions and departure from geographical life centers.

Science, n. s., 19:210-211.

'13. Guide to the study of animal ecology. 183 pp. New York.
(This book contains nunicious references to the literature bearing upon the

subject of this article.)

Bancroft, W. D.

'11. A universal law. Science, n. s., 23; 159-179.

Blackman, F. F.

'05. Optima and limiting factdrs. Ann. B(it. 19:281-295.

Blackman, F. F. and Smith, A. M.

'11. Experimental researches on vegetable assimilation and res-

piration. IX. On assimilation in submerged water-plants,

and its relation to the concentration of carbon dioxide and

other factors. Proc. Royal Society, B., 83: 3S9-412, 1910.

Brooks, W. K.

'99. The foundations of zoology. 339 pp. New York.

Cowles, H. C.

'11. The causes of vegetative cycles. Bot. Gaz., 51:161-183;
also, Ann. Assoc. Amer. Gcogr., i : 1-20. 1912.



32

Jennings, H. S.

'06. Behavior of the lower organisms. 366 pp. New York.

Keyes, C. R.

'98. The genetic classification of geological phenomena. Journ.

Geol., 6:809-815.

McGee, W J

'13. Field records relating to suljsoil water. U. S. Dept. Agr.,

Bur. Soils, Bull. 93. 40 pp.

Powell, J. W.
'95. Physiographic processes. Nat. Geogr. Monographs, i : 1-32.

Schantz, H. L.

'i [. Natural vegetation as an indicator of the capabilities of land

for crop production in the Great Plains area. U. S. Dept.

Agr., Bur. Plant Industry, Bull. No. 201. 100 pp.

Van Hise, C. R.

'04. The problems of geology. Journ. Geol., 12:589—616.

The New York State College of Forestry, Syracuse, N. Y.



ORIGINAL FOREST AND PRAIRIE AREA IN ILLINOIS

(after brendel and barrows
)