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TWENTIETH CENTURY TEXT-BOOKS

EDITED BY
A. F. NIGHTINGALE, Pu. D.

SUPERINTENDENT OF HIGH SCHOOLS, CHICAGO

“HALSHHOIHD ANUVET Aq Ydvisojoyd ‘“dnois Joliqud ‘puvls] suave UO (372% LDLOIOLIDINDYT) SIUBLOWAOD PIIVBJ-poy]

TWENTIETH CENTURY TEXT-BOOKS

ANIMAL LIFE

A FIRST BOOK OF ZOOLOGY

BY
DAVID STARR JORDAN, Pu. D., LL. D.
PRESIDENT OF LELAND STANFORD JUNIOR UNIVERSITY
AND
VERNON L. KELLOGG, M. S.

PROFESSOR IN LELAND STANFORD JUNIOR UNIVERSITY

NEW YORK
D. APPLETON AND COMPANY
1900

CopyRicuHtT, 1900
By D. APPLETON AND COMPANY

PREFACE

THE authors present this book as an elementary ac-
count of animal ecology—that is, of the relations of ani-
mals to their surroundings and of the responsive adapt-
ing or fitting of the life of animals to these surroundings.
The book treats of animals from the point of view of the
observer and student of animal life who wishes to know
why animals are in structure and habits as they are.
The beginning student should know that the whole life
of animals, that all the variety of animal form and habit,
is an expression of the fitness of animals ‘to the varied
circumstances and conditions of their living, and that
this adapting and fitting of their life to the conditions
of living come about inevitably and naturally, and that
it can be readily studied and largely understood. The
ways and course of this fitting are the greatest facts of
life excepting the fact of life itself. In this kind of
study of animals every observation of a fact in animal
structure or behavior leads to a search for the signifi-
cance, or meaning in the life of the animal, of this fact.
The veriest beginner can be, and ought to be, an independ-
ent observer and thinker. It is the phase of the study of
zoology which appeals most strongly to the beginning
student, the phase which treats of the why and how of
animal form and habit. At the same time this phase is
that to which the attention of the most advanced mod-
ern scholars of biology is rightly and chiefly turned. The

v

a ANIMAL LIFE

point of view which the zodlogical beginner should take
is the point of view that the best and most enlightened
zoological scholar takes. With this belief in mind the
authors have tried to put into simple form the principal
facts and approved hypotheses upon which the modern
conceptions of animal life are based.

It is unnecessary to say that this book depends for its
best use on a basis of personal observational work by the
student in laboratory and field. Without independent
personal work of the student little can be learned about
animals and their life that will stick. But present-day
teachers of biology are too well informed to make a dis-
cussion of the methods of their work necessary here. As
a matter of fact, the methods of the teacher depend too
nearly absolutely on his training and individual initiative
to make worth while any attempt by the authors to point
out the place of this book in elementary zodlogical teach-
ing. That the phase of study it attempts to represent
should have a place in such teaching is, of course, firmly
believed by them.

The obligations of the authors for the use of certain
illustrations are acknowledged in proper place. Where no
credit is otherwise given, the drawings have been made by
Miss Mary H. Wellman or by Mr. James Carter Beard, and
the photographs have been made by the authors or under
their direction.

DaAviID STARR JORDAN,
VERNON LyMAN KELLOGG.

SranrorD University, July, 1900.

BETS

CONTENTS

CHAPTER PAGE
I.—THE LIFE OF THE SIMPLEST ANIMALS . : ; ; j 1
The simplest animals, or Protozoa, 1.—The animal cell, 2.—
What the primitive cell can do, 5.—Ameeba, 5.—Parameecium, 9.
—Vorticella, 12.—Marine Protozoa, 15.—Globigerine and Radio-
laria, 16.—Antiquity of the Protozoa, 20.—The primitive form,
20.—The primitive but successful life, 21.

II.—THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS . F mer yen |

Colonial Protozoa, 24.—Gonium, 25.—Pandorina, 26.—Eudo-
rina, 27.—Volvox, 28.—Steps toward complexity, 30.—Individual
or colony, 31.—Sponges, 32.—Polyps, corals, and jelly-fishes, 37.
—Hydra, 37.—Differentiation of the body cells, 41.—Medusz or
jelly-fishes, 41.—Corals, 43.—Colonial jelly-fishes, 45.—Increase
in the degree of complexity, 48.

III.—THE MULTIPLICATION OF ANIMALS AND SEX ‘A ie . 50

All life from life, 50—Spontaneous generation, 51.—The
simplest method of multiplication, 53.—Slightly complex methods
of multiplication, 54.—Differentiation of the reproductive cells, 55.
—Sex, or male and female, 57.—The object of sex, 57.—Sex di-
morphism, 58.—The number of young, 61.

IV.—FUNCTION AND STRUCTURE . “ : ; : ; . 68

Organs and functions, 63.—Differentiation of structure, 64.—
Anatomy and physiology, 64.—The animal body a machine, 65.
—The specialization of organs, 66.—The alimentary canal, 66.—
Stable and variable characteristics of an organ, 73.—Stable and
variable characteristics of the alimentary canal, 73.—The mutual
relation of function and structure, 77.

V.—THE LIFE CYCLE . : = 3 78

Birth, growth and development, and death, 78.—Life cycle of
simplest animals, 78.—The egg, 79.—Embryonic and post-em-
bryonic development, 80.—Continuity of development, 83.—De-
velopment after the gastrula stage, 84.—Divergence of develop-

42534 |

vill ANIMAL LIFE

CHAPTER
ment, 84.—The laws or general facts of development, 86.—The
significance of the facts of development, 89.—Metamorphosis,
90.—Metamorphosis among insects, 90.—Metamorphosis of the
toad, 94.—Metamorphosis among other animals, 96.—Duration of
life, 101.—Death, 103.

VI.—THE PRIMARY CONDITIONS OF ANIMAL LIFE. ; ;

_ Primary conditions and special conditions, 106.—Food, 106.—
Oxygen, 107.—Temperature, pressure, and other conditions, 108.
—Ditference between animals and plants, 111.—Living organic
matter and inorganic matter, 112.

VIJ.—THE CROWD OF ANIMALS AND THE STRUGGLE FOR EXIST-
ENCE .

The crowd of animals, 114.—The Pe for existence, 116.
—Selection by Nature, 117.—Adjustment to surroundings a re-
sult of natural selection, 120.—Artificial selection, 120.—Depend-
ence of species on species, 121.

VIIIL.—ApDaApPratIONsS. p _ . z x

Origin of adaptations, 123. = Ghasatoalion of thetic 1238.
—Adaptations for securing food, 125.—Adaptations for self-de-
fense, 128.—Adaptations for rivalry, 135.—Adaptations for the
defense of the young, 137.—Adaptations concerned with sur-
roundings in life, 143.—Degree of structural change in adapta-
tions, 146.—Vestigial organs, 147.

IX.—ANIMAL COMMUNITIES AND SOCIAL LIFE 3 i .

Man not the only social animal, 149.—The honey-bee, 149.—
The ants, 155.—Other communal insects, 158.—Gregariousness
and mutual aid, 163.—Division of labor and basis of communal
life, 168.—Advantages of communal life, 170.

X.—COMMENSALISM AND SYMBIOSIS . ; = os . x

Association between animals of different species, 172.—Com-
mensalism, 173.—Symbiosis, 175.

XI.—PARASITISM AND DEGENERATION . . i

Relation of parasite and host, 179.—Kinds of parasitism, 180.
—The simple structure of parasites, 181.—Gregarina, 182.—The
tape-worm and other flat-worms, 183.—Trichina and other round-
worms, 184.—Sacculina, 187.—Parasitic insects, 188.—Parasitic
vertebrates, 193.—Degeneration through quiescence, 193.—De-
generation through other causes, 197.—Immediate causes of de-
generation, 198.—Advantages and disadvantages of parasitism
and degeneration, 198.—Human degeneration, 200.

PAGE

106

114

123

149

172

179

CONTENTS ix

CHAPTER PAGE
XIJ.— PROTECTIVE RESEMBLANCES AND MIMICRY . Z 3 - 201

Protective resemblance defined, 201.—General protective or
aggressive resemblance, 202.—Special protective resemblance,
207.— Warning colors and terrifying appearances, 212.—Alluring
colorotion, 216.—Mimicry, 218.—Protective resemblances and
mimicry most common among insects, 221.—No volition in mim-
icry, 222.—Color: its utility and beauty, 222.

XIII.—THE SPECIAL SENSES . A ; : % . 224

Importance of the special senses, 224. _-Dittculty of the study
of the special senses, 224.—Special senses of the simplest ani-
mals, 225.—The sense of touch, 226.—The sense of taste, 228.—
The sense of smell, 229.—The sense of hearing, 232.—Sound-mak-
ing, 235 —The sense of sight, 237.

XIV.—INstTINCT AND REASON . * . 240

Trritability, 240.—Nerve cells er pica 240.—The brain or
sensorium, 241.—Reflex action, 241.—Instinct, 242.—Classifica-
tion of instincts, 248.—Feeding, 244.—Self-defense, 245.—Play,
247.—Climate, 248.—Environment, 248.—Courtship, 248.—Repro-
duction, 249.—Care of the young, 250.—Variability of instincts,
251.—Reason, 251.—Mind, 255.

XV.—HoMES AND DOMESTIC HABITS . P ; : : . 257

Importanee of care of the young, 257.—Care of the young and
communal life, 257.—The invertebrates (except spiders and in-
sects), 258.—Spiders, 259.—Insects, 262.—The vertebrates, 264.

XVI.—GEOGRAPHICAL DISTRIBUTION OF ANIMALS . ; . 272

Geographical distribution, 272.—Laws of ideabunion) 274.—
Species debarred by barriers, 274.—Species debarred by inability
to maintain their ground, 275.—Species altered by adaptation to
new conditions, 276.—Effect of barriers, 283.—Relation of species
to habitat, 288.—Character of barriers to distribution, 288.—Bar-
riers affecting fresh-water animals, 294—Modes of distribution,
296.—Fauna and faunal areas, 296.—Realms of animal life, 297.—
Subordinate realms or provinces, 303.—Faunal areas of the sea,
804.

CLASSIFICATION OF ANIMALS - : : : : ~ 2 BOT

GLOSSARY . 3 - : : 5 : r . 38138

CHAPTER I

THE LIFE OF THE SIMPLEST ANIMALS

1. The simplest animals, or Protozoa.—The simplest ani-
mals are those whose bodies are simplest in structure and
which do the things done by all living animals, such as
eating, breathing, moving, feeling, and reproducing in the
most primitive way. The body of a horse, made up of
various organs and tissues, is complexly formed, and the
various organs of the body perform the various kinds of
work for which they are fitted in a complex way. The
simplest animals are all very small, and almost all live in
the water; some kinds in fresh water and many kinds in
the ocean. Some live in damp sand or moss, and still others
are parasites in the bodies of other animals. They are not
familiarly known to us; we can not see them with the
unaided eye, and yet there are thousands of different kinds
of them, and they may be found wherever there is water.

In a glass of water taken from a stagnant pool there
is a host of animals. There may be a few water beetles
or water bugs swimming violently about, animals half an
inch long, with head and eyes and oar-like legs; or there
may be a little fish, or some tadpoles and wrigglers. These
are evidently not the simplest animals. There will be
many very small active animals barely visible to the un-
aided eyes. These, too, are animals of considerable com-
plexity. But if a single drop of the water be placed

2 1

9 ANIMAL LIFE

on a glass slip or in a watch glass and examined with a
compound microscope, there will be seen a number of ex-
tremely small creatures which swim about in the water-drop
by means of fine hairs, or crawl slowly on the surface of the
glass. These are among our simplest animals. There are,
as already said, many kinds of these “simplest animals,”
although, perhaps strictly speaking, only one kind can be
called simplest. Some of these kinds are spherical in
shape, some elliptical or football-shaped, some conical, some
flattened. Some have many fine, minute hairs projecting
from the surface; some have a few longer, stronger hairs
that lash back and forth in the water, and some have no
hairs at all. There are many kinds and they differ in size,
shape, body covering, manner of movement, and habit of
food-getting. And some are truly simpler than others.
But all agree in one thing—which is a very important
thing—and that is in being composed in the simplest way
possible among animals.

2. The animal cell.—The whole body of any one of the
simplest animals or Protozoa is composed for the animal’s
whole lifetime of but a single cell. The bodies of all other
animals are composed of many cells. The cell may be
called the unit of animal (or plant) structure. The body
of a horse is complexly composed of organs and tissues.
Each of these organs and tissues is in turn composed of a
large number of these structural units called cells. These
cells are of great variety in shape and size and general
character. The cells which compose muscular tissue are
very different from the cells which compose the brain.
And both of these kinds of cells are very different from
the simple primitive, undifferentiated kind of cell seen in
the body of a protozoan, or in the earliest embryonic
stages of a many-celled animal.

The animal cell is rarely typically cellular in character
—that is, it is rarely in the condition of a tiny sac or box
of symmetrical shape. Plant cells are often of this char-

THE LIFE OF THE SIMPLEST ANIMALS 3

acter. The primitive animal cell (Fig. 1) consists of a
small mass of a viscid, nearly colorless, substance called
protoplasm. This protoplasm is differentiated to form two
parts or regions of the cell, an inner denser mass called the
nucleus, and an outer, clearer, inclosing mass called the
cytoplasm. 'There may be more than
one nucleus in a cell. Sometimes
the cell is inclosed by a cell wall
which may be simply a tougher outer
layer of the cytoplasm, or may be a
thin membrane secreted by the pro-
toplasm. In addition to the proto-
plasm, which is the fundamental and
essential cell substance, the cell may yy¢. 1.—Blood cell of acrab
contain certain so-called cell prod- — (after Harcker). Show-
ucts, substances produced by the life oS cae ee
processes of the protoplasm. The circular spot) and gran-
cell may thus contain water, oils, Soe sie per tea
resin, starch grains, pigment gran-

ules, or other substances. These substances are held in
the protoplasm as liquid drops or solid particles.

The protoplasm itself of the cell shows an obvious
division into parts, so that certain parts of it, especially
parts in the nucleus, have received names. The nucleus
usually has a thin protoplasmic membrane surrounding it,
which is called the nuclear membrane. There appear to be
fine threads or rods in the nucleus which are evidently
different from the rest of the nuclear protoplasm. These
rods are called chromosomes. The cell is, indeed, not so
simple as the words “structural unit” might imply, but
science has not yet so well analyzed its parts as to warrant
the transfer of the name structural unit to any single part
of the cell—that is, to any lesser or simpler part of the
animal body than the cell as a whole.

The protoplasm, which is the essential substance of the
cell and hence of the whole animal body, is a substance

4 ANIMAL LIFE

of a very complex chemical and physical constitution. Its.
chemical structure is so complex that no chemist has yet
been able to analyze it, and as the further the attempts at
analysis reach the more complex and baffling the substance
is found to be, it is not improbable that it may never be
analyzed. It is a compound of numerous substances, some
of these composing substances being themselves extremely
complex. The most important thing we know about the
chemical constitution of protoplasm is that there are al-
ways present in it certain complex albuminous substances
which are never found in inorganic bodies. It is on the
presence of these albuminous substances that the power of
performing the processes of life depends. Protoplasm is the
primitive basic life substance, but it is the presence of these
complex albuminous compounds that makes protoplasm the
life substance. A student of protoplasm and the funda-
mental life processes, Dr. Davenport, has said, “Just as
the geologist is forced by the facts to assume a vast but
not infinite time for earth building, so the biologist has to
recognize an almost unlimited complexity in the constitu-
tion of the protoplasm.” *

* The physical structure of protoplasm has been much studied,
but even with the improved microscopes and other instruments neces-
sary for the study of minute structure, naturalists are still very far
from understanding the physical constitution of this substance. While
the appearance of protoplasm under the microscope is pretty generally
agreed on among naturalists, the interpretation of the kind of structure
which is indicated by this appearance is not at all well agreed on.
Protoplasm appears as a mesh work composed of fine granules sus-
pended in a clearer substance, the spaces of the mesh work being com-
posed of a third still clearer substance. Some naturalists believe, from
this appearance, that protoplasm is composed of a clear viscous sub-
tance, in which are imbedded many fine granules of denser substance,
and numerous large globules of a clearer, more liquid substance. Other
naturalists believe that the fine spots which appear to be granules are
simply cross sections of fine threads of dense protoplasm which lie
coiled and tangled in the thinner, clearer protoplasm, And, finally,

THE LIFE OF THE SIMPLEST ANIMALS 5

3. What the primitive cell can do.—The body of one of
the minute animals in the water-drop is a single cell. The
body is not composed of organs of different parts, as in the
body of the horse. There is no heart, no stomach ; there
are no muscles, no nerves. And yet the protozoan is a liv-
ing animal as truly as is the horse, and it breathes and eats
and moves and feels and produces young as truly as does
the horse. It performs all the processes necessary for the
life of an animal. The single cell, the single minute speck
of protoplasm, has the power of doing, in a very simple and
primitive way, all those things which are necessary for
life, and which are done in the case of other animals by
the various organs of the body.

4. Ameba.—The simple and primitive life of these
Protozoa can be best understood by the observation of
living individuals. In the slime and sediment at the
bottom of stagnant pools lives a certain specially interest-
ing kind of protozoan, the Ameba (Fig. 2). Of all the
simplest animals this is as simple or primitive as any. The
minute viscous particle of protoplasm which forms its
body is irregular in outline, and its outline or shape slowly
but constantly changes. It may contract into a tiny ball ;
it may become almost star-shaped ; it may become elongate
or flattened; short, blunt, finger-like projections called
pseudopods extend from the central body mass, and these
projections are constantly changing, slowly pushing out or

others believe that protoplasm exists as a foam work; that it is a vis-
cous liquid containing many fine globules (the granule-appearing spots)
of a liquid of different density and numerous larger globules of a liquid
of still other density. It is a foam in which the bubbles are not filled
with air, but with liquids of different density. This last theory of the
structure of protoplasm is the one accepted by a majority of modern
naturalists, although the other theories have numerous believers. But
just as with what little we know of the chemical constitution of proto-
plasm, the little we know of its physical structure throws almost no
light on the remarkable properties of this fundamental life substance.

6 ANIMAL LIFE

drawingin. The single protoplasmic cell which makes up
the body of the Am@ba has no fixed outline; it is a cell
without a wall. The substance of the cell or body is proto-
plasm, semiliquid and colorless. The changes in form of
the body are the moving of the Ameba. By close watching
it may be seen that the Ameda changes its position on the
glass slip. Although provided with no legs or wings or

Fie. 2.—An Amebda, showing different shapes assumed by it when crawling.
—After VERWORN.

scales or hooks—that is, with no special organs of locomo-
tion—the Amewba moves. There are no muscles in this tiny
body; muscles are composed of many contractile cells
massed together, and the Amewéa is but one cell. But it is
a contractile cell; it can do what the muscles of the com-
plex animals do.

If one of the finger-like projections of the Ameba, or,
indeed, if any part of its body comes in contact with some
other microscopic animal or plant or some small fragment
of a larger form, the soft body of the Ameba will be seen

THE LIFE OF THE SIMPLEST ANIMALS +

to press against it, and soon the plant or animal or organic
particle becomes sunken in the protoplasm of the formless
body and entirely inclosed in it (Fig. 3). The absorbed
particle soon wholly or partly disappears. This is the
manner in which the Ameda eats. It has no mouth or

Fie. 3.—Ameba eating a microscopic one-celled plant.—After VERWORN.

stomach. Any part of its body mass can take in and digest
food. The viscous, membraneless body simply flows about
the food and absorbs it. Such of the food particles as can
not be digested are thrust out of the body.

The Ameba breathes. Though we can not readily ob-
serve this act of respiration, it is true that the Amwda takes
into its body through any part of its surface oxygen from
the air which is mixed with water, and it gives off from any
part of its body carbonic-acid gas. Although the Ameba
has no lungs or gills or other special organs of respiration,
it breathes in oxygen and gives out carbonic-acid gas, which
is just what the horse does with its elaborately developed
organs of respiration.

If the Ameda, in moving slowly about, comes into con-
tact with a sand grain or other foreign particle not suitable
for food, the soft body slowly recoils and flows—for the
movement is really a flowing of the thickly fluid protoplasm
—so as to leave the sand grain at one side. The Ameba
feels. It shows the effects of stimulation. Its movements
can be changed, stopped, or induced by mechanical or
chemical stimuli or by changes in temperature. The

8 ANIMAL LIFE

Ameba is irritable; it possesses irritability, which is sensa-
tion in its simplest degree.

If food is abundant the Ameba soon increases in size.
The bulk of its body is bound to increase if new substance

Fie. 4.—Ameba polypodia in six successive stages of division. The dark, white-
margined spot in the interior is the nucleus.—After F. E. Scuunzez.

is constantly assimilated and added to it. The Ameda
grows. But there seem to be some fixed limits to the
extent of this increase in size. No Ama@ba becomes large.
A remarkable phenomenon always occurs to prevent this.

THE LIFE OF THE SIMPLEST ANIMALS 9

An Ameba which has grown for some time contracts all
its finger-like processes, and its body becomes constricted.
This constriction or fissure increases inward, so that the
body is soon divided fairly in two (Fig. 4). The body,
being an animal cell, possesses a nucleus imbedded in the
body protoplasm or cytoplasm. When the body begins to
divide, the nucleus begins to divide also, and becomes en-
tirely divided before the fission of the cytoplasm is com-
plete. There are now two Ameba, each half the size of
the original one; each, indeed, being actually one half of
the original one. This splitting of the body of the Ameéa,
which is called fission, is the process of reproduction. The
original Ame@ba is the parent; the two halves of the parent
are the young. Each of the young possesses all of the
characteristics and powers of the parent; each can move,
eat, feel, grow, and reproduce by fission. It is very evident
that this is so, for any part of the body or the whole body
was used in performing these functions, and the young are
simply two parts of the parent’s body. But if there be any
doubt about the matter, observation of the behavior of the
young or new Amebe will soon remove it. Each puts out
pseudopods, moves, ingests food particles, avoids sand
grains, contracts if the water is heated, grows, and finally
divides in two.

5. Paramecium.—Another protozoan which is common
in stagnant pools and can be readily obtained and observed
is Paramecium (Fig. 5). The body of the Paramecium is
much larger than that of the Ame@da, being nearly one fourth
of a millimeter in length, and is of fixed shape. It is elon-
gate, elliptical, and flattened, and when examined under the
microscope seems to be a very complexly formed little mass.
The body of the Paramecium is indeed less primitive than
that of the Amebda, and yet it is still but a single cell.
The protoplasm of the body is very soft within and dense
on the outside, and it is covered externally by a thin mem-
brane. The body is covered with short fine hairs or cilia,

10 ANIMAL LIFE

which are fine processes of the dense protoplasm of the
surface. There is on one side an oblique shallow groove
that leads to a small, funnel-shaped depression in the body
which serves as a primitive sort of mouth
or opening for the ingress of food.
The Paramecium swims about in the
water by vibrating the cilia which coy-
er the body, and brings food to the
mouth opening by producing tiny cur-
rents in the water by means of the
_ cilia in the oblique groove. The food,
which consists of other living Proto-
zoa, is taken into the body mass only
through the funnel-shaped opening, and
that part of it which is undigested is
thrust out always through a particular
part of the body surface. (The taking
in and ejecting of foreign particles can
be seen by putting a little powdered
carmine in the water.) Within the
body there are two nuclei and two so-
called pulsating vacuoles. These pul-
Fig. 5.—Parameciumau- gating vacuoles (Ame@eba has one) seem
Jeepmcogeel eit 4 4 to aid in discharging waste products
contractile vacuole,and from the body. When the Parame-
in the center is one of . ° 2
wear re cium touches some foreign substance or
is otherwise irritated it swims away,
and it shoots out from the surface of its body some fine
long threads which when at rest are probably coiled up in
little sacs on the surface of the body. When the Para-
mecium has taken in enough food and grown so that it
has reached the limit of its size, it divides transversely into
halves as the Ameda does. Both nuclei divide first, and
then the cytoplasm constricts and divides (Fig. 6). Thus
two new Paramecia are formed. One of them has to de-
velop a new mouth opening and groove, so that there is in

THE LIFE OF THE SIMPLEST ANIMALS 11

the case of the reproduction of Paramecium the beginnings
of developmental changes during the course of the growth
of the young. The young Ame@be have only to add sub-
stance to their bodies, to grow larger, in order to be exactly
like their parent.

The new Paramecia attain full size and then divide,
each into two. And so on for many generations. But it
has been discovered that this simplest kind of reproduction
can not go on indefinitely. After a number of generations
the Paramecia, instead of simply dividing in two, come
together in pairs, and a part of
one of the nuclei of each mem-
ber of a pair passes into the
body of and fuses with a part

Fie. 6.—Paramecium putorinum
dividing. The two nuclei be-
come very elongate before di- Fig. 7.—Paramecium caudatum ; two indi-
viding.—After Birscu.t. viduals separating after conjugation.

of one of the nuclei of the other member of the pair. In
the meantime the second nucleus in each Paramecium has
broken up into small pieces and disappeared. The new
nucleus composed of parts of the nuclei from two animals
divides, giving each animal two nuclei just as it had before
this extraordinary process, which is called conjugation,
began (Fig. 7). Each Paramecium, with its nuclei com-
posed of parts of the nuclei from two distinct individuals,

12 ANIMAL LIFE

now simply divides in two, and a large number of genera-
tions by simple fission follow. |

Paramecium in the character of its body and in the
manner of the performance of its life processes is distinctly
less simple than the Ameda, but its body is composed of a
single structural unit, a single cell, and it is truly one of
the “ simplest animals.”

6. Vorticella—Another interesting and readily found
protozoan is Vorticella (Fig.8). While the Ame@ba can crawl
and Paramecium swim, Vorticella, except when very young,

Fie. 8.—Vorticella microstoma (after STE). A, small, free-swimming individuals
conjugating with a large, stalked individual; B, a stalked individual dividing
longitudinally ; C, after division is completed one part severs itself from the
other, forms a ring of cilia, and swims away.

is attached by tiny stems to dead leaves or sticks in the
water, and can change its position only to a limited extent.

THE LIFE OF THE SIMPLEST ANIMALS 13

The body is pear-shaped or bell-shaped, with a mouth
opening at the broad end, and a delicate stem at the
narrow end. This stem is either hard and stiff, or is
flexible and capable of being suddenly contracted in a
close spiral. In the body mass there is one pulsating
vacuole and one nucleus. Usually many Vorticelle@ are
found together on a common stalk, thus forming a proto-
zoan colony.

The life processes of Vorticella are of the simple kind
already observed in Ameba and Paramecium. Vorticella
shows, however, some modifications of the process of repro-
duction which are interesting. The plane of division of
Vorticella is parallel to the long axis of the pear-shaped
body, so that when fission is complete there are two Vorti-
celle on a single stalk. One of the two becomes detached,
and by means of a circle of fine hairs or cilia which appear
around its basal end leads a free swimming life for a short
time. Finally it settles down and develops a stalk. Vorti-
cella shows two kinds of fission—one the usual division
into equal parts, and another division into unequal parts.
In this latter kind, called reproduction or multiplication
by budding, a small part of the parent body separates,
develops a basal circle of cilia, and swims away. The pro-
cess of conjugation also takes place among the Vorti-
cella, but they are never two equal forms which conju-
gate, but always one of the ordinary stalked forms and
one of the small free-swimming forms produced by
budding.

Here, then, in the life of Vorticella, are new modifica-
tions of the life processes ; but, after all, these life processes
are very simply performed, and the body is like the body of
the Ameba, a single cell. Vorticella.is plainly one of “ the
simplest animals.”

7. Gregarina.—A fourth kind of protozoan to which we
can profitably give some special attention is Gregarina
(Fig. 9), the various species of which live in the alimentary

14 ANIMAL LIFE

canal* of crayfishes and centipeds and certain insects.
Gregarina is a parasite, living at the expense of the host
in whose body it lies. It has no need to swim about quickly,

Fie. 9.—Gregarinide. A, a Gregarinid (Actinocephalus oligacanthus) from the intes-
tine of an insect (after Stein); B and C, spore forming by a Gregarinid (Coc-
cidium oviforme) from the liver of a guinea-pig (after LeEucKART); D, E, and
F, successive stages in the conjugation and spore forming of Gregarina poly-
morphea (after KOELLIKER).

and hence has no swimming cilia like Paramecium and
the young Vorticella. It does need to cling to the inner
wall of the alimentary canal of its host, and the body of
some species is provided with hooks for that purpose. The

* Specimens of Gregarina can be abundantly found in the alimen-
tary canal of meal worms, the larve of the black beetle (Tenebrio moli-
tor), common in granaries, mills, and brans. “Snip off with small
scissors both ends of a larva, seize the protruding (white) intestine with
forceps, draw it out, and tease a portion in normal salt solution (water
will do) on a slide. Cover, find with the low power (minute, oblong,
transparent bodies), and study with any higher objective to suit,”—
MuRBACH,

THE LIFE OF THE SIMPLEST ANIMALS 15

food of Gregarina is the liquid food of the host as it exists
in the intestine, and which is simply absorbed anywhere
through the surface of the body of the parasite. There is
no mouth opening nor fixed point of ejection of waste
material, nor is there any contractile vacuole in the body.

In the method of multiplication or reproduction Gre-
garina shows an interesting difference from Ameba and
Paramecium and Vorticella. When the Gregarina is
ready to multiply, its body, which in most species is rather
elongate and flattened, contracts into a ball-shaped mass
and becomes encysted—that is, becomes inclosed in a tough,
membranous coat. This may in turn be covered externally
by a jelly-like substance. The nucleus and the protoplasm
of the body inside of the coat now divide into many small
parts called spores, each spore consisting of a bit of the
cytoplasm inclosing a small part of the original nucleus,
Later the tough outer wall of the cyst breaks and the
spores fall out, each to grow and develop into a new Gre-
garina. In some species there are fine ducts or canals
leading from the center of the cyst through the wall to the
outside, and through these canals the spores issue. Some-
times two Gregarine come together before encystation and
become inclosed in a common wall, the two thus forming a
single cyst. This isa kind of conjugation. In some spe-
cies each of the young or new Gregarine coming from the
spores immediately divides by fission to form two indi-
viduals.

8. Marine Protozoa.—If called upon to name the char-
acteristic animals of the ocean, we answer readily with the
names of the better-known ocean fishes, like the herring and
cod, which we know to live there in enormous numbers; the
seals and sea lions, the whales and porpoises, those fish-like
animals which are really more like land animals than like
the true fishes; and the jelly-fishes and corals and star-fishes
which abound along the ocean’s edge. But in naming only
these we should be omitting certain animals which in point

16 ANIMAL LIFE

of abundance of individuals vastly outnumber all other
animals, and which in point of importance in helping main-
tain the complex and varied life of the ocean distinctly out-
class all other marine forms. These animals are the marine
Protozoa, those of the “ simplest animals ” which live in the
ocean.

Although the water at the surface of the ocean appears
clear, and on superficial examination devoid of life, yet a
drop of this water taken from certain ocean regions exam-
ined under the microscope reveals the fact that this water
is inhabited by Protozoa. Not only is the water at the
very surface of the ocean the home of the simplest animals,
but they can be found in all the water from the surface to
a great depth beneath it. In a pint of this ocean water
from the surface or near it there may be millions of these
animals. In the oceans of the world the number of them
is inconceivable. Dr. W. K. Brooks says that the “ basis
of all the life in the modern ocean is found in the micro-
organisms of the surface.” By micro-organisms he means
the one-celled animals and the one-celled plants. For
the simplest plants are, like the simplest animals, one-
celled. ‘“ Modern microscopical research,” he says, “ has
shown that these simple plants, and the Globigerine and
Radiolaria [kinds of Protozoa] which feed upon them, are
so abundant and prolific that they meet all demands and
supply the food for all the animals of the ocean.”

9. The Globigerine and Radiolaria—The Globigerine
(Fig. 10) and Radiolaria (Fig. 11) are among the most in-
teresting of all the simplest animals. Their simple one-
celled body is surrounded by a microscopic shell, which
among the Globigerine is usually made of lime (calcium
carbonate), in the case of Radiolaria of silica. These minute
shells present a great variety of shape and pattern, many
being of the most exquisite symmetry and beauty. The
shells are usually perforated by many small holes, through
which project long, delicate, protoplasmic threads. These

THE LIFE OF THE SIMPLEST ANIMALS 17

fine threads interlace when they touch each other, thus
forming a sort of protoplasmic network outside of the shell.
In some cases there is a complete layer of protoplasm—
part of the body protoplasm of the protozoan —surround-

Fig. 10.—Polystomelia strigillata, one of the Globigerine.—After Max SoHuLTzE.

ing the cell externally. The Radiolaria, whose shells are

made of silica, possess also a perforated membranous sac

called the central capsule, which lies imbedded in the

protoplasm, dividing it into two portions, one within and
3

18 ANIMAL. LIFE

one outside of the capsule. In the protoplasm inside of
the capsule lies the nucleus or nuclei; and from the proto-
plasm outside of the capsule rise the numerous fine, thread-
like pseudopods which project through the apertures in the
shell, and enable the animal to swim and to get food.

Most of the myriads of the simplest animals which
swarm in the surface waters of the ocean belong to a few
kinds of these shell-bearing Globigerine and Radiolaria.
Large areas of the bottom of the Atlantic Ocean are coy-
ered with a slimy gray mud, often of great thickness, which
is called globigerina-ooze, because it is made up chiefly of
the microscopic shells of Globigerine. As death comes to
the minute protoplasmic animals their hard shells sink
slowly to the bottom, and accumulate in such vast quanti-
ties as to form a thick layer on the ocean floor. Nor is it
only in present times and in the oceans we know that the
Globigerine have flourished. All over the world there are
thick rock strata which are composed chiefly of the fos-
silized shells of these simplest animals. Where the strata
are made up exclusively of these shells the rock is chalk.
Thus are composed the great chalk cliffs of Kent, which
gave to England the early name of Albion, and the chalk
beds of France and Spain and Greece. The existence of
these chalk strata means that where now is land, in earlier
geologic times were oceans, and that in the oceans Globi-
gerine lived in countless numbers. Dying, their shells
accumulated to form thick layers on the sea bottom. In
later geologic ages this sea bottom has been uplifted and
is now land, far perhaps from any ocean. The chalk strata
of the plains of the United States, like those in Kansas, are
more than a thousand miles from the sea, and yet they are
mainly composed of the fossilized shells of marine Pro-
tozoa. Indeed, we are acquainted with more than twice as
many fossil species of Globigerine as species living at the
present time. The ancestors of these Globigerine, from
which the present Globigerine differ but little, can be

THE LIFE OF THE SIMPLEST ANIMALS 19

traced far back in the geologic history of the world. It is
an ancient type of animal structure.

The Radiolaria, too, which live abundantly in the pres-
ent oceans, especially in the marine waters of the tropical
and temperate zones, are found as fossils in the rocks from
the time of the coal age on. The siliceous shells of the

Fie. 11.—Heliosphera actinota (after HAECKEL); a radiolarian with symmetrical
shell.

Radiolaria sinking to the sea bottom and accumulating
there in great masses form a radiolaria-ooze similar to the
globigerinz-ooze ; and just as with the Globigerine, the
remains of the ancient Radiolaria formed thick layers on
the floor of the ancient oceans, which have since been up-
lifted and now form certain rock strata. That kind of
rock called Tripoli, found in Sicily, and the Barbados
earth from the island of Barbados, both of which are used

20 ANIMAL LIFE

as polishing powder, are composed almost exclusively of
the siliceous shells of ancient and long-extinct Radiolaria.

10. Antiquity of the Protozoa—All the animals of the
ocean depend upon the marine Protozoa (and the marine
Protophyta, or one-celled plants) for food. Either they
prey upon these one-celled organisms directly, or they prey
upon animals which do prey on these simplest animals.
The great zodlogist already quoted says: “The food sup-
ply of marine animals consists of a few species of micro-
scopic organisms which are inexhaustible and the only
source of food for all the inhabitants of the ocean. The
supply is primeval as well as inexhaustible, and all the life
of the ocean has gradually taken shape in direct depend-
ence upon it.” That is, the marine simplest animals are
the only marine animals which live independently; they
alone can live or could have lived in earlier ages without
depending on other animals. They must therefore be the
oldest of marine animals. By oldest we mean that their
kind appeared earliest in the history of the world. As it
is certain that marine life is older than terrestrial life—that
is, that the first animals lived in the ocean—it is obvious
that the marine Protozoa are the most ancient of animals.
This is an important and interesting fact. Zodlogists try
to find out the relationships and the degrees of antiquity
or modernness of the various kinds of animals. We have
seen that the Protozoa, those animals which have the sim-
plest body structure and perform the necessary life pro-
cesses in the simplest way, are the oldest, the first animals.
This is just what we would expect.

11. The primitive form.—We find among the simplest
animals a considerable variety of shape and some manifest
variation in habit. But the points of resemblance are far
more pronounced than the points of difference, and are of
fundamental importance. The composition of the body of
one cell, as opposed to the many-celled structure of the
bodies of all other animals, is the fact to be most distinctly

THE LIFE OF THE SIMPLEST ANIMALS 21

emphasized. The shape of this one-celled body varies.
With the most primitive or simplest of the “simplest ani-
mals,” like Ameba, for example, there is no “distinction
of ends, sides, or surfaces, such as we are familiar with in
in the higher animals. Anterior and posterior ends, right
and left sides, dorsal and ventral surfaces are terms which
have no meaning in reference to an Ameba, for any part
of the animal may go first in locomotion, and when crawl-
ing the animal moves along on whatever part of its
surface happens to be in contact with foreign bodies.”
The one shape most often seen among the Protozoa, or
most nearly fairly to be called the typical shape, is the
spherical or subspherical shape. Why this is so is readily
seen. Most of the Protozoa are aquatic and free swim-
ming. They live in a medium, the water, which supports
or presses on the body equally on all sides, and the body is
not forced to assume any particular form by the environ-
ment. The body rests suspended in the water with any
part of its surface uppermost or any part undermost. As
any part of the surface serves equally well in many of the
Protozoa for breathing or eating or excreting, it is obvious
that the spherical form is the simplest and most conven-
ient shape for such a body. It is interesting to note that
the spherical form is the common shape of the egg cell of
the higher animals. Each one of the higher, multicellular
animals begins life (as we shall find it explained in another
chapter of this book) as a single cell, the egg cell, and
these egg cells are usually spherical in shape. The full
significance of this we need not now attempt to under-
stand, but it is interesting to note that normally the whole
body of the simplest animals is a single spherical cell, and
that every one of the higher animals, however complex
it may become by growth and development, begins life as a
single spherical cell.

12. The primitive but successful life—Living consists of
the performing of certain so-called life processes, such as

29 ANIMAL LIFE

eating, breathing, feeling, and multiplying. These pro-
cesses are performed among the higher animals by various
organs, special parts of the body, each of which is fitted to
do some one kind of work, to perform some one of these
processes. There is a division or assignment of labor here
among different parts of the body. Such a division of
labor, and special fitting of different parts of the body for
special kinds of work does not exist, or exists only in
slightest degree among the simplest animals. The Ameba
eats or feels or moves with any part of its body; all of the
body exposed to the air (air held in the water) breathes;
the whole body mass takes part in the process of repro-
duction.

Only very small organisms can live in this simplest way.
So all of the Protozoa are minute. When the only part of
the body which can absorb oxygen is the simple external
surface of a spherical body, the mass of that body must be
very small..- With any increase in size of the animal the
mass of the body increases as the cube of the diameter,
while the surface increases only as the square of the diam-
eter. Therefore: the part of the body (inside) which re-
quires to be provided with oxygen increases more rapidly
than the part (the outside) which absorbs oxygen. Thus
this need of oxygen alone is sufficient to determine the
limit of size which can be attained by the spherical or sub-
spherical Protozoa.

That the simplest animals, despite the lack of. organs
and the primitive way of performing the life processes, live
successfully is evident from their existence in such ex-
traordinary numbers. They outnumber all other animals.
Although serving as food for hosts of ocean animals, the .
marine Protozoa are the most abundant in individuals of
all living animals. The conditions of life in the surface
waters of the ocean are easy, and a simple structure and
simple method of performance of the life processes are
wholly adequate for successful life under these conditions.

THE LIFE OF THE SIMPLEST ANIMALS 93

That the character of the body structure of the Protozoa
has changed but little since early geologic times is ex-
plained by the even, unchanging character of their sur-
roundings. ‘The oceans of former ages have undoubtedly
been essentially like the oceans of to-day—not in extent
and position, but in their character of place of habitation
for animals. The environment is so simple and uniform
that there is little demand for diversity of habits and conse-
quent diversity of body structure. Where life is easy there
is no necessity for complex structure or complicated habits
of living. So the simplest animals, unseen by us, and so
inferior to us in elaborateness of body structure and habit,
swarm in countless hordes in all the oceans and rivers and
lakes, and live successfully their simple lives.

CHAPTER II
THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS

13. Colonial Protozoa.— When one of the simplest animals
multiplies by fission, the halves of the one-celled body sepa-
rate wholly from each other, move apart, and pursue their
lives independently. The original parent cell divides to
form two cells, which exist thereafter wholly apart from
each other. There are, however, certain simple animals
which are classed with the Protozoa, which show an inter-
esting and important difference from the great majority of
the simplest animals. These are the so-called colony-form-
ing or colonial Protozoa.

These colonial Protozoa belong to a group of organisms
called the * Volvocine. The simplest of the Volvocine are
single cells, which live wholly independently and are in
structure and habit essentially like the other Protozoa we
have studied. They have, however, imbedded in the one-
celled body a bit of chlorophyll, the green substance which
gives the color to green plants and is so important in their
physiology. In this respect they differ from the other
Protozoa. Among the other Volvocine, however, a few or
many cells live together, forming a small colony—that is,

* These colonial organisms, the Volvocine, are the objects of some
contention between botanists and zodlogists. The botanists call them
plants because they possess a cellulose membrane and green chroma-
tophores, and exhibit the metabolism characteristic of most plants ; but
most zodlogists consider them to be animals belonging to the order
Flagellata of the Protozoa. In the latest authoritative text-book of
zoology, that of Parker and Haswell (1897), they are so classed.

24

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS) 95

there is formed a group of a few or many cells, each cell
having the structure of the simpler unicellular forms.
These cells are held together in a gelatinous envelope, and
the mass is usually spherical in shape. In most of the
colonies each of the cells possesses two or three long, pro-
toplasmic, whiplash-like hairs, called flagella, and by the
lashing of these flagella in the water the whole group swims
about.

14. Gonium.—If when one of the simplest animals di-
vided to form two daughter cells, these two cells did not
move apart, but remained
side by side and each di-
vided to form two more,
and each of these divided
to form two more, and
these eight divided each
into two, each cell com-
plete and independent but
all remaining together
in a group—if this pro-
cess should take place we
should have produced a
group or colony of sixteen
cells, each cell a complete
animal capable of living
independently like the
other simplest animals,
but all holding together
to form a tiny, flat, plate- Fig. 12.—Gonium pectorale (after Stem). A,
like colony. Now, this is ee above; B, colony seen
precisely what takes place :
in the case of those colonial Protozoa belonging to the genus
Gonium (Fig. 12). When the mother cell of Gonium di-
vides, the daughter cells do not swim apart, but remain
side by side, and by repeated fission, until there are sixteen
cells side by side, the colony is formed. Each cell of the

26 ANIMAL LIFE

colony eats and breathes and feels for itself; each can and
does perform all the processes necessary to keep it alive.
When ready to multiply, the sixteen cells of the Goniwm
colony separate, and each cell becomes the ancestor of a
new colony.

15. Pandorina.—Another colony usually composed of six-
teen cells is Pandorina, but the cells are arranged to form
a spherical instead of a plate-like colony (Fig. 13). In Pan-
dorina morum the colony consists of sixteen ovoid cells in
a spherical jelly-like mass. Each cell has two flagella, and
by the lashing of all the flagella the whole colony moves
through the water. Food is taken by any of the cells, is
assimilated, and the cells increase in size. When Pan-
dorina is ready to multiply, each cell divides repeatedly
until it has formed sixteen daughter cells. The inclosing
gelatinous mass which holds the colony together dissolves,
and the daughter colonies be-
come free and swim apart.
Each colony soon grows to the
size of the original colony.
This kind of multiplication or
reproduction may be continued
for several generations. But
it does not go on indefinitely.
After a number of these gener-
ations have been produced by
simple division, the cells of a
colony divide each into eight
Fie. 13.—Pandorina sp. (from Na- instead of sixteen daughter

ture). The cells composing the cells. The daughter cells are
fers aah Ga avide not all of the same size, but
the difference is hardly notice-

able. The eight cells resulting from the repeated division
of one of the original cells separate and swim about inde-
pendently by means of their flagella. If one of these cells
comes near a similar free-swimming cell from another

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 97

colony, the two cells conjugate (Fig. 14)—that is, fuse to
form a single cell. This new cell formed by the fusion of
two, develops a tough enveloping membrane (of cellulose)

and passes into what is called
the “resting stage.” That is,
the cell remains dormant for a
shorter or longer time. It may
thus tide over a drought or a
winter. It may become dry or
be frozen, yet when suitable
conditions of moisture or tem-
perature are again present the
outer wall breaks and the pro-
toplasm issues as a large free-
swimming cell, which soon di-
vides into sixteen daughter
cells which constitute a new
colony.

16. Eudorina,— Another colo-
nial protozoan which much re-
sembles Pandorina, but differs
from it in one interesting and
suggestive thing, is Hudorina.
In Ludorina elegans (Fig. 15)
the colony is spherical and is
composed of sixteen or thirty-
two cells. Each of these cells
can become the parent of a new
colony by simple repeated divi-
sion. But this simple mode of
reproduction, just as with Pan-
dorina, can not persist indefi-
nitely. There must be conjuga-
tion. But the process of mul-

Fig. 14.— Pandorina morum (after
GoEBEL). Three stages in the
conjugation and formation of the
resting spore. A, two cells just
fused; B, the two cells completely
fused, but with flagella still per-
sisting ; C, the resting spore.

tiplication, which includes conjugation, is different from
that process in Pandorina, in that in Hudorina the conju-

98 ANIMAL LIFE

gating cells are of two distinctly different kinds. When
this kind of multiplication is to take place in the case of
Eudorina elegans, to choose a common species, some of
the cells of a colony divide into sixteen or thirty-two

minute elongated cells, each
Va provided with two flagella.

These small cells escape

Fig. 15.—Eudorina elegans. A, a mature colony (from Nature); B, formation of
the two kinds of reproductive cells.

from the envelope of the parent cell, remaining for some
time united in small bundles. Other cells of the colony
do not divide, but increase slightly in size and become
spherical in shape. When a bundle of the small cells
comes into contact with some of these large spherical
cells the bundle breaks up, and conjugation takes place
between the small flagellated free-swimming cells and the
large non-flagellate spherical cells. Each new cell formed
by the fusion of one of the small and one of the large cells
develops a cellulose wall and assumes a resting stage.
After a time from each of these resting spores a new colony
of sixteen or thirty-two cells is formed by direct, repeated
division.

17. Volvox.—Another interesting colonial protozoan is
Volvoz. The large spherical colonies of Volvox globator

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS) 99

(Fig. 16) are composed of several thousand cells, arranged
in a single peripheral layer about the hollow center of

the ball.
two long flagella which pro-
ject out into the water. The
lashing of the thousands of
the flagella give the ball-
like colony a rotary motion.
The cells are held together
by a jelly-like intercellular
substance and are connect-
ed with each other by fine
protoplasmic threads which
extend from the body pro-
toplasm of one cell to the
cells surrounding it. When
the colony is full grown and
ready to reproduce itself
certain cells of the colony
undergo great changes.
Some of them increase in
size enormously, having re-
serve food material stored
in them, and they may be
called the egg cells of the
colony. Reproduction may
now occur by simple divi-
sion of one of these great
egg cells into many small
cells, all held together in a
common envelope. These
form a daughter colony
which escapes from the

The cells are ovoid, and each is provided with

Fie. 16.—A, Volvox minor, entire colony

(from Nature). B,C, and D, reproductive
cells of Volvox globator.

mother colony and by growth and further division comes to
be a new full-sized colony. Or reproduction may occur in
another, more complex, way. Certain cells of the colony

30 ANIMAL LIFE

divide into bundles of very small, slender cells, each of
which is provided with flagella. The remaining cells of
the colony (that is, those which have not swollen into egg
cells or divided into many—sixty-four to one hundred and
twenty-eight—minute, flagellate cells) remain unchanged for
a while and finally die. They take absolutely no part in
reproducing the colony. One of the minute free-swim-
ming cells fuses with one of the enormous egg cells, the
new cell thus formed being a resting spore. From this
resting spore a new colony develops by repeated division.
18. Steps toward complexity.— Within the group of Vol-
vocine there are plainly several steps on the way from
simplicity of structure to complexity of structure. Gonium,
Pandorina, Eudorina, and Volvox form a series proceeding
from the simplest animals toward the complex animals.
In Gonium the cells composing the colony are all alike in
structure, and each one is capable of performing all the
processes or functions of life. In Pandorina and Hudorina
the cells are at first alike, but there is, as the time for
reproduction approaches, a differentiation of structure ;
the cells of the colony, all of which take part in the process
of reproduction, come to be in certain generations of two
kinds—an inactive large kind which may be called the egg
cells, and a small, active, free-swimming kind which seeks
out and conjugates with, or, we may say, fertilizes the egg
cells. In Volvox there is a new differentiation. Only cer-
tain particular and relatively few cells take part in repro-
ducing the colony; most of the cells have given up the
power or function of reproduction. These cells, when the
time of multiplication comes, simply support the special
reproductive cells. They continue to waft the great colony
through the water by lashing their flagella; they continue
to take in food from the outside. The reproductive cells
devote themselves wholly to the business of producing new
colonies, of perpetuating the species. And this matter of
reproduction is less simple than in the other Volvocine.

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 8]

At least there is much more difference between the two
kinds of reproductive cells. The egg cells are compara-
tively enormous, and they are stored with a mass of food
material. The fertilizing cells are very small, but very
active and very different from the egg cells. We have in
Volvox the beginnings of a distinct division of labor and
an accompanying differentiation of structure. Certain
cells of the colony do certain things, and are modified in
structure to fit them specially for their particular duties.
The steps from the simplest structure toward a complex
structure are plainly visible.

19. Individual or colony.—Is the Gonium colony, the
Pandorina colony, or the Volvoz colony a group of several or
many distinct organisms, or is it to be considered as a sin-
gle organism? With Gonium, which we may call the sim-
plest of these colonial organisms, the colony is composed
of a few wholly similar cells or one-celled animals, each
fully capable of performing all the life processes, each
wholly competent to lead an independent life. In fact,
each does, for part of its life, live independently, as we
have already described. In the case of Pandorina and Eu-
dorina, while all the cells are for most of the lifetime of the
colony alike and each is capable of living independently,
at the time of reproduction the cells become of two kinds.
A difference of structure is apparent, and for the perpetua-
tion of the species the co-operation of these different kinds
of cells is necessary. That is, it is impossible for a single
one of the members of the colony to reproduce the colony,
except for a limited number of generations. With Volvox
this giving up of independence on the part of the individual
members of the colony is more marked. There is a real in-
terdependence among the thousands of cells of the colony.
The function of reproduction rests with a few particular
cells, and for the perpetuation of the species there is demand-
ed a co-operation of two distinct kinds of reproductive cells.
The great majority of the cells take no part in reproduc-

39 ANIMAL LIFE

tion. They can perform all the other life processes ; they
move the colony by lashing the water with their flagella;
they take in food and assimilate it; they can feel. All the
cells of the great colony, too, are intimately connected by
means of protoplasmic threads. The protoplasm of one
cell can mingle with that of another cell; food can go
from cell to cell. The question whether the Volvox colony
is a group of distinct organisms or is a single organism
made up of cells among which there is a simple but obvi-
ous difference in structure and function ; in other words,
whether Volvox is a colony of one-celled animals, of Pro-
tozoa, or is a multicellular animal, one of the Metazoa (for
so all the many-celled animals are called), is a difficult one
to decide. Most zodlogists class the Volvocine with the
Protozoa—that is, they incline to consider Goniwm, Pan-
dorina, Volvox, and the other Volvocine as groups or col-
onies of one-celled animals.

20, Sponges.—If the Volvocine be considered to belong
to the Protozoa, the sponges are the simplest of all the
many-celled animals. Sponges are not free-swimming ani-
mals, except for a short time in their young stage, but are
fixed, like plants. They live attached to some solid sub-
stance on the sea bottom. They resemble plants, too, in
the way in which the body is modified during growth by
the environment. If the rock to which the young sponge
is attached is rough and uneven, the body of the sponge
will grow so as to fit the unevenness ; if the rock surface is
smooth, the body of the sponge will be more regular. Thus
a sponge may be said to have no fixed shape of body ; indi-
viduals of the same species of sponge differ much in form.
The typical form of the sponges is that of a short cylinder
or vase attached by one end and with the upper free end
open (Fig. 17). Many individuals of one kind usually live
together in a close group or colony, and they may be so
attached to each other as to appear like a branching plant.
This branching may be very diffuse, and the branches

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 33

may become so interwoven with each other as to form a
very complex group. A sponge is composed of many cells
arranged in three layers—that is, the body of a sponge is a
cylinder closed at one end whose wall is composed of three

layers of cells. The outer layer of
cells is called the ectoderm, and the
cells composing it are flat and are
all closely attached to each other.
The inner layer is called the endo-
derm, and its cells are thicker than
those of the ectoderm ; they are
also closely attached to each other.
Sometimes they are provided with
flagella like the flagellate Protozoa.
The flagella are, however, not for the
purpose of locomotion, but for creat-
ing currents in the water, which
bathes the interior of the open cylin-
drical body. The middle layer,
called the mesoderm, is composed of
numerous separate cells lying in a
jelly-like matrix. From these meso-
derm cells fine needles or spicules
of lime or silica often project out
through the ectoderm. These mi-
hute sponge spicules are of a great
variety of shapes, and they form a
sort of skeleton for the support of
the soft body mass. All over the
outer surface of the body are scat-
tered fine openings or pores, which
lead through the walls of the body

\s

yy i
OW

-_
iT

~~ <>
- 5
Sl

y
|

Fie. 17.—One of the simplest
sponges, Calcolynthus pri-
migenius (after HAECKEL).
A part of the outer wall is
cut away to show the in-
side.

into the inner cavity. This cavity is of course also con-
nected with the outside by the large opening at the free or

apical end of the body.

There is hardly any differentiation of parts among the
4

34 ANIMAL LIFE

sponges. Asin the Protozoa, there are no special organs .
for the performance of special functions. The sponge
feeds by creating, with its flagella, water currents which

Fie. 18.—One of the simple sponges,
Prophysema primordiale (after
HAECKEL). The body is represented
as cut in two longitudinally. The
large cells of the inner layer are the
egg cells.

flow in through the many fine
pores of the body and out from
the inner body cavity through
the large opening at the free
end of the body. These cur-
rents of water bear fine parti-
cles of organic matter which
are taken up by the cells lining
the pores and body cavity, and
assimilated. There are no
special organs of digestion.
Each cell takes up food and
digests it. The water cur-
rents also bring air to these
same cells, and thus the sponge
breathes. Although the
sponge as a whole can not
move, does not possess the
power of locomotion, yet the
protoplasm of the cells has
the power of contracting, just
as with the Protozoa, and the
pores can be opened or closed
by this cellular movement.
Practically, thus, the only
movements the sponge can
make are the movements made
by the individual cells.
Reproduction is accom-
plished by a process of divi-

sion, or by a process of conjugation and subsequent division.
In its simplest way multiplication takes place by a group
of cells separating from the body of the parent sponge,

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 35

becoming inclosed in a common capsular envelope, and by
repeated division and consequent increase in number of
cells becoming a new sponge. This is reproduction by
“budding.” The “ buds,” or small groups of cells which
separate from the parent sponge, are called gemmules.
Reproduction in the more complex way occurs as follows:
Some of the free ameeboid cells of the mesoderm (the mid-
dle one of the three layers of the body wall) become en-
larged and spherical in form. These are the egg cells.
Other mesodermic cells divide into many small cells, which
are oval with a long, tapering, tail-like projection. These
cells are active, being able to swim by the lashing of the
tapering tail. These are the fertilizing cells. The two
kinds of reproductive cells may be formed in one sponge ;
if so, they are formed at different times. Or one sponge
may produce only egg cells, another only fertilizing or,
as they are called, sperm cells. Conjugation takes place
between a sperm cell and an egg cell. That is, one of the
small active sperm cells finds one of the large, spherical,
inactive cells and penetrates into the protoplasm of its
body. The two cells fuse and form a single cell, which
may be called the fertilized or impregnated egg. This fer-
tilized egg, remaining in the body mass of the parent
sponge, divides repeatedly, the new cells formed by this
division remaining together. The young or embryo sponge
finally escapes from the body of the parent sponge, and
lives for a short time as an active free-swimming animal.
Its body consists of an oval mass of cells, of which those on
one side are provided with cilia or swimming hairs. The
cells of the body continue to divide and to grow, and the
body shape gradually changes. The young sponge finally
becomes attached to some rock, the body assumes the typi-
cal cylindrical shape, an aperture appears at the free end,
and small perforations appear on the surface. The sponge
becomes full grown.

It is unfortunate that most of us do not live on the

36 ANIMAL LIFE

seashore, and hence can not observe the structure and life
history of the living ocean sponges. There are, however,
among the thousand and more kinds of sponges a few
kinds which live in fresh water, and these are so widely
spread over the earth that examples of them can be found
in almost any region. They belong to the genus Spongilla,
and thirty or more species or kinds of Spongilla are known.
In standing or slowly flowing water, Spongilla grows erect
and branching, like a shrub or miniature tree; in swift
water it grows low and spreading, forming a sort of mat
over the surface to which it is attached. Reproduction
takes place very actively by the process of budding. The
budded-off gemmules are spherical in shape, and the cells
of each gemmule are inclosed in an envelope composed of
siliceous spicules of peculiar shape. These gemmules are
formed in the body substance of the parent sponge toward
the end of the year, and are set free by the decaying of
that part of the body of the parent sponge in which they
lie. They sink to the bottom of the pond or brook, and
lie there dormant until the following spring. Then they
develop rapidly by repeated. division of the cells and
growth.

It is impossible here to tell anything of the many and
interesting kinds of sponges which inhabit the ocean. The
“sponge ” of the bathroom is simply the skeleton of a large
sponge or group of sponges. The skeleton here is not
composed of lime or silica, but of a tough, horny substance,
which is secreted by cells of the mesodermal layer of the
body wall of the sponge. This substance is called spongin,
and is a substance allied to silk in its chemical composi-
tion. All the commercial sponges, the spongin skeletons,
belong to one genus—Spongia. These sponges grow espe-
cially abundantly in the Mediterranean and Red Seas, and
in the Atlantic Ocean off the Florida reefs, and on the
shores of the Bahama Islands. The sponges are pulled
up by divers, or by means of hooks or dredges. The

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 37

living matter soon dies and decays, leaving the horny
skeleton, which when cleaned and trimmed is ready for
use.

The most beautiful sponges are those with siliceous
skeletons. The fine needles or threads of glass, arranged
often in delicate and intricate pattern, make these sponges
objects of real beauty.

21. Polyps, corals, and jelly-fishes—The general or typ-
ical plan of body structure of those animals which come
next in degree of complexity to the sponges can be best
understood by imagining the typical cylindrical body of a
sponge modified in the following way: The middle one
of the three layers of the body wall not to be composed
of cells in a gelatinous mass, but to be simply a thin non-
cellular membrane; the body wall to be pierced by no
fine openings or pores, so that the interior cavity of the
body is connected with the outside only by the single
large opening at the free end, and this opening to be sur-
rounded by a circlet of arm-like processes or tentacles,
continuations of the body wall and similarly composed.
Such a body structure is the general or fundamental one
for all polyps, corals, sea-anemones, and jelly-fishes. The
variety in shape and the superficial modifications of this
type-plan are many and striking; but, after all, the type-
plan is recognizable throughout the whole of this great
group of animals. Perhaps the simplest representative of
the group is a tiny polyp which grows abundantly in the
fresh-water streams and pools, and can be readily obtained
for observation. It is called Hydra.

22. Hydra.—The body of Hydra (Fig. 19), which is
very small and appears to the unaided eye as a tiny white
or greenish gelatinous particle attached to some submerged
stone or bit of wood or aquatic plant, is a simple cylinder
attached by one end to the stone or weed. The other free
end is contracted so as to be conical, and it is narrowly
open. Around the opening are six or eight small waving

38 ANIMAL LIFE

tentacles. The wall of the cylinder is composed of an
outer and an inner layer of cells and a thin non-cellular
membranous layer between them. The tentacles are hol-
low and are simple expansions of the body wall. The cells
of the outer layer, or ectoderm, are not all alike. Some
are smaller than the others and appear to be crowded in

Mi)

Fie. 19.—The fresh-water polyp, Hydra vulgaris. A, in expanded condition, and
in contracted condition; B, cross section of body, showing the two layers of
cells which make up the body wall.

between the bases or inner ends of the larger ones. The
inner ends of the large cells are extended as narrow-pointed
prolongations directed at right angles with the rest of the
cell. These processes are very contractile and are called
muscle processes. Each one is simply a continuation of
the protoplasm of the cell body, which is especially con-
tractile. Some of the smaller ectoderm cells are very

a

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS) 39

irregular in shape and possess specially large nuclei. These
cells are more irritable or sensitive than the others and
are called nerve cells. The ectoderm cells of the base or
foot of the Hydra are peculiarly granular, and secrete a
sticky substance by which the Hydra holds fast to the
stone or weed on which it is found. These cells are called
gland cells. Imbedded in many of the larger ectoderm
cells, especially those of the tentacles, are small oval sacs,
in each of which lies folded or coiled a fine long thread.
When the tentacles touch one of the small animals which
serve Hydra as food, these fine threads shoot out.from
their sacs and so poison or sting the prey that it is
paralyzed. The tentacles then contract and bend inward,
forcing the captured animal into the mouth opening
in the center of the circle of tentacles. Through the
mouth opening the prey enters the body cavity of Hydra
and is digested by the cells lining this cavity. These cells
belonging to the inner layer of the body wall or endoderm
are mostly large, and each contains one or more contractile
vacuoles. From the free ends—the ends which are next to
the body cavity—of these cells project pseudopods or fine
flagella. These projections are constantly changing: now
two or three short, blunt pseudopods are projecting into
the body cavity ; now they are withdrawn, and a few fine,
long flagella are projected. In addition to these cells there
are in the endoderm, especially abundant near the mouth
opening and wholly lacking in the tentacles and at the
base of the body, many long, narrow, granular cells. They
are gland cells which secrete a digestive fluid. The food
captured by the tentacles and taken in through the
mouth opening disintegrates in the body cavity, or diges-
tive cavity, as it may be called. The digestive fluid se-
creted by the gland cells of the endoderm acts upon it,
so that it becomes broken into small parts. These par-
ticles are probably seized by the pseudopods of the other
endoderm cells and are taken into the body protoplasm

40 ANIMAL LIFE

of these cells. The ectoderm cells do not take food
directly, but receive nourishment only through the endo-
derm cells.

Hydra is not permanently attached. It holds firmly
to the submerged stone or weed by means of the sticky
secretion from the ectodermal gland cells of its base, but it
~ can loosen itself, and by a slow creeping or gliding move
along the surface of the stone to another spot. Even when
attached, the form of the body changes; it extends itself
longitudinally, or it contracts into a compact globular mass.
The tentacles move about in the water, and are continually
contracting or extending.

Like Volvox and the sponges, those other slightly com-
plex animals we have already considered, Hydra has two
methods of multiplication. In the simpler way, there
appears on the outer surface of the body a little bud which
is composed, at first, of ectoderm cells alone; but soon it is
evident that it is a budding, or outpushing, of the whole
body wall, ectoderm, endoderm, and middle membrane. In
a few hours the bud has six or eight tiny, blunt tentacles,
a mouth opening appears at the free end, and the little
Hydra breaks off from the parent body and leads an inde-
pendent existence. In the more complex way, two kinds of |
special reproductive cells are produced by each individual,
viz., large, inactive, spherical egg cells, and small, active
sperm cells, each with an oval part or head (consisting of
the nucleus) and a slender, tapering tail-like part (consist-
ing of the cytoplasm). The egg cell lies inclosed in a layer
of thin, surrounding cells, which compose a capsule for it.
When the egg cell is ready for fertilization this capsule
breaks, and one of the active sperm cells finds its way to
and fuses with the egg cell. The fertilized egg cell now
divides into several cells, which remain together. The
outer ones form a hard capsule, and thus protected the
embryo falls to the bottom, and after lying dormant for
awhile develops into a Hydra.

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 4{

23. Differentiation of the body cells—In Hydra we have
the beginnings of complexity of structure carried a step
further than in the sponges. The division of labor among
the cells composing the body is more pronounced, and the
structural modification of the different cells to enable them
better to perform their special duties is obvious. Some of
the cells of the body specially devote themselves to food-
taking; some specially to the digestion of the food; some
are specially contractile, and on them the movements of
the body depend, while others are specially irritable or
sensitive, and on them the body depends for knowledge of
the contact of prey or enemies. In the lasso cells—those
with the stinging threads—there is a very wide departure
from the simple primitive type of cells. There is in Hydra
a manifest differentiation of the cells into various kinds of
cells. The beginnings of distinct tissues and organs are
foreshadowed.

The individuals of Hydra live, usually, distinct from
each other. There is no tree-like colony, as with the sponges.
But most of the other polyps do live in this colonial manner.
The new polyps which develop as buds from the body of
the parent do not separate from the parent, but remain
attached by their bases. They, in turn, produce new
polyps which remain attached, so that in time a branching,
tree-like colony is formed.

24. Meduse or jelly-fishes—Most of the other polyps
differ from Hydra also in producing, in addition to ordi-
nary polyp buds, buds which develop into bell-shaped struc-
tures called meduse@ (Fig. 20). These medusz consist of a
soft gelatinous bell- or umbrella-shaped body, with a short
clapper or stem which has an opening at its free end.
From the edge of the bell or umbrella four pairs of tenta-
cles arise. The meduse usually separate from the parent
polyp and live an independent, free-swimming life. These
are the beautiful animals commonly known as jelly-fishes.
The medusz or jelly-fishes produce special reproductive

49 ANIMAL LIFE

cells, a single medusa producing only one kind of such cells
—that is, producing either egg cells alone or sperm cells
alone. The active sperm cells produced by one medusa
find their way to an egg cell producing medusa, and fuse
with or fertilize these egg cells. The
fertilized egg develops into a small,
oval, free-swimming embryo called a
planula, which finally attaches itself
to a stone or bit of wood or seaweed,
and grows to be a simple cylindrical
polyp attached at its base and with
mouth and tentacles at its free end.
This polyp gives rise by budding to
new polyps, which remain attached
to it, and gradually a new tree-like
colony is formed, From this polyp
or this colony new meduse bud off,
swim away, and finally produce new
polyps. Thus there is in the life of
the polyps what is called an alterna-
tion of generations. ‘There are two kinds of individuals
which evidently belong to the same species of animal, or,
put in another way, one kind of animal has two distinct
forms. This appearance of one kind of animal in two
forms is called dimorphism. We shall see later that one
kind of animal may appear in more than two forms; such
a condition is called polymorphism. In alternation of gen-
erations we have the polyp animal appearing in one genera-
tion as a fixed cylindrical polyp, while in the next generation
it is a free-swimming, umbrella-shaped medusa or jelly-fish.

The polyps which are dimorphic—that is, have a polyp
form of individual and a medusa form of individual—show
more differentiation in structure than the simple Hydra.
This further differentiation is especially apparent in the
meduse or jelly-fishes. Here the nerve cells are aggregated
in little groups arranged along the edge of the umbrella

Fig. 20.—A medusa, Hucope.
—After HAECKEL.

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 43

to form distinct sense organs. The muscle processes are
better developed, and the digestive cavity is differentiated
into central and peripheral portions. In these dimorphic
polyps the fixed polyp individuals reproduce by the simple
way of budding, while the medusa individuals reproduce
by producing special reproductive cells of two kinds, which
must fuse to form a cell capable of developing into a new
polyp.

25. Corals—There are many kinds of polyps and jelly-
fishes, and they present a great variety of shape and size
and general appearance. Many polyps exist only in the
true polyp form, never producing meduse. Others have

Fig. 21.—A polyp, or sea-anemone (Metridium dianthus).

only the medusa form. Some live in colonies, and others
are always solitary. The animals we know as corals are
polyps which live in enormous colonies, and which exist
only in the true polyp form, not producing meduse. They

Fie. 22.—Coral island (atoll), looking seaward, showing line of breakers.

ic H
Fig. 23.—Coral island, view across lagoon,

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 45

form a firm skeleton of lime (calcium carbonate), and after
their death these skeletons persist, and because of their
abundance and close massing form great reefs or banks and
islands. Coral islands occur only in the warmer oceans.
In the Atlantic they are found along the coasts of southern
Florida, Brazil, and the West Indies; in the Pacific and
Indian Oceans there are great coral reefs on the coast of
Australia, Madagascar, and elsewhere, and certain large

Fie. 24.—Organ-pipe coral.

groups of inhabited islands like the Fiji, Society, and
Friendly Islands are composed exclusively of coral islands.
More than two thousand kinds of living corals are known,
and their skeletons offer much variety in structure and
appearance. Brain coral, organ-pipe coral (Fig. 24), the
well-known red coral from Italy and Sicily, used as jewelry,
and the sea pens and sea fans are among the better known
and more beautiful kinds of coral skeletons.

26. Colonial jelly-fishes— While many of the medusz or
jelly-fishes are another form of individual of a true fixed
polyp, many of the larger and more beautiful jelly-fishes do
not exist in any other form. Some of these larger jelly-
fishes are several feet in diameter, and when cast up on the
beach form a great shapeless mass of soft, jelly-like sub-

AG ANIMAL LIFE

stance. The bodies of all jelly-fishes are soft and gelatinous,
the body substance containing hardly one per cent of solid
matter. It is mostly water. Many jelly-fishes are beauti-
fully and strikingly colored, and as they swim slowly about
near the surface of the ocean, lazily opening and shutting
their iridescent, umbrella-like bodies, they are among the
most beautiful of marine organisms. When one of the
jelly-fishes is taken from the water, however, it quickly loses
its brilliant colors, and dries away to a snapeless, shrivel-
ing, sticky mass.

Some of the most beautiful of the jelly-fishes belong
to a group called the Siphonophora. These jelly-fishes are
elongate and tube-like rather than umbrella- or bell-shaped,
and they are polymorphic—that is, there are several. dif-
ferent forms of individuals belonging to a single kind
or species. The Siphonophora are all free-swimming, but
nevertheless form small colonies. In the Mediterranean
Sea and in other southern ocean waters the surface may be
covered for great areas by these brilliantly colored jelly-fish
colonies, each of which looks, as a celebrated German natu-
ralist has said, like a swimming flower cluster whose parts,
flowers, stems, and leaves seem to be made of transparent
crystal, but which possess the life and soul of an animal.
An abundant species of these Siphonophora (Fig. 25) is com-
posed of a slender, flexible, floating, central stem several feet
long, to which are attached thousands of medusa and polyp
individuals representing several different kinds of forms,
each kind of individual being specially modified or adapted
to perform some one duty. The central stem is a greatly
elongated polyp individual, whose upper end is dilated and
filled with air to form a float. This individual holds up
the whole colony. Grouped around this central stem just
below the float are many bell-shaped bodies which alter-
nately open and close, and by thus drawing in and expelling
water from their cavities impel the whole colony through
the water. These bell-shaped structures are attached me-

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 47

dusa individuals, whose
business it is to be the
locomotive organs for the
colony. These medusze

are without tentacles, and

take no food and produce
no young. ‘They have
given up the power of
performing these other
life processes, and devote
themselves wholly to the
business of locomotion.
From the lower end of the
central stem rises a host of
structures, among which
several distinct kinds are
readily perceived. One
kind is composed of a pear-
shaped hollow body open
at its free end, and bear-
ing a long tentacle which

is furnished with numer-—

ous groups of stinging
cells. These are the polyp
individuals whose especial
business it is to capture
and sting prey and to eat
it. These individuals are
the food- getters for the
colony. Scattered among
these stinging, feeding
polyps, are numerous
smaller individuals with
oval, closed body, each
bearing a long, slender
thread. These threads

Fie. 25.—A colonial jelly-fish, Physophora
(after HAECKEL). At the top is the float
polyp, around its stem the swimming
meduse, and below are the feeding, feel-
ing, protecting, and reproducing polyps
and meduse.

48 ANIMAL LIFE

are very sensitive, and the polyps bearing them have for
special function that of feeling or being sensible of stimuli
from without. They are the sense organs or sense indi-
viduals of the colony. Finally, there are two other. kinds
of structures or individuals which produce the special
reproductive cells for the perpetuation of the species.
These are the modified medusa individuals, and one kind,
larger than the other, produces the active sperm cells,
while the other produces the inactive egg cells.

27. Increase in the degree of complexity.—In the corals,
sea-anemones, and jelly-fishes there is plainly much more
of a division of labor among the various parts of an indi-
vidual and much more modification of these parts—that is,
much more structural complexity than among the sponges
and Hydra. And these, in their turn, are more complex than
are the colonial Protozoa, the Volvocine. There is a great
difference in degree of complexity among the slightly com-
plex animals. But the various groups of these animals
which we have studied can all be arranged roughly in a
series beginning with the least complex among them and
ascending to the most complex. And in this series the
gradual increase in complexity, and in the always accom-
panying division of labor among the different parts, is
beautifully shown.

From an animal composed of many structurally simi-
lar cells, each cell capable of performing all the life pro-
cesses, we pass to an animal composed of cells of a few
different kinds, of slight structural diversity. Each kind
of cell devotes itself especially to a certain few life pro-
cesses or functions. Next we find an animal in which the
- cells of one kind are specially aggregated to form a single
part of the body which is specially devoted to the perform-
ance of a single function. This diversity among the cells
increases, this aggregation of similar cells to form special
parts or organs increases, and the division of labor or
assignment of special functions to special organs becomes

a a ro

THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 49

more and more pronounced. Among the more complex
polyps and jelly-fishes the contractile cells form distinct
muscle fibers and muscles; the sensitive cells form dis-
tinct nerve cells and nerve fibers which are arranged in a
primitive nervous system; the digestive cavity becomes
complex and composed of different portions ; the reproduc-
tive cells are formed by special organs, and the distinction
between the egg cells and the sperm cells—that is, be-
tween the female reproductive elements and the male
reproductive elements—becomes more pronounced.

We have followed this increase or development of struc-
tural and physiological complexity from simplest animals
to fairly complex ones. The principle of this development
of complexity is evident. It will not be profitable to at-
tempt to follow in detail this development among the
higher animals. The complex animals are complex be-
cause their life processes are performed by special parts of
their body, which parts are specially modified so as to perform
these processes well. The animals which are more complex
than those we have studied differ from these simply in the
degree of complexity attained. In order to understand
this better we shall not further consider special groups of
animals, but’ special processes or functions, and attempt to
see how the modification and increase in complexity of
structure goes hand in hand with the increase of elaborate-
ness or complexity in the performance of function.

CHAPTER III
THE MULTIPLICATION OF ANIMALS AND SEX

28. All life from life—On the performance of the func-
tion of reproduction or multiplication depends the exist-
ence or perpetuation of the species. Although an animal
may take food and perform all the functions necessary to
its own life, it does not fulfill the demands of successful
existence unless it reproduces itself. Some individuals of
every species must produce offspring or the species becomes
extinct. We have seen in our study of the simple animals
that the function of reproduction is the first function to
become differentiated in the ascent from simplest animals
to complex animals. The first division of labor among the
cells composing the bodies of the slightly complex animals
and the first structural differences among the cells are
connected with the performance of the function of repro-
duction or multiplication.

We are so familiar with the fact that a new kitten
comes into the world only through being born, as the off-
spring of parents of its kind, that we shall likely not appre-
ciate at first the full significance of the statement that all
life comes from life; that all organisms are produced by
other organisms. Nor shall we at first appreciate the im-
portance of the statement. This is a generalization of
modern times. It has always been easy to see that cats
and horses and chickens and the other animals we famil-
iarly know give birth to young or new animals of their
own kind; or, put conversely, that young or new cats and
horses and chickens come into existence only as the off-

50

a

THE MULTIPLICATION OF ANIMALS AND SEX 5]

spring of parents of their kind. And in these latter days of
microscopes and mechanical aids to observation it is even
easy to see that the smaller animals, the microscopic organ-
isms, come into existence only as they are produced by the
division of other similar animals, which we may call their
parents. But in the days of the earlier naturalists the
life of the microscopic organisms, and even that of many
of the larger but unfamiliar animals, was shrouded in
mystery. And what seem to us ridiculous beliefs were
held regarding the origin of new individuals.

29. Spontaneous generation. The ancients believed that
many animals were spontaneously generated. The early
naturalists thought that flies arose by spontaneous genera-
tion from the decaying matter of dead animals; from a
dead horse come myriads of maggots which change into
flesh flies. Frogs and many insects were thought to be
generated spontaneously from mud. Eels were thought to
arise from the slime rubbed from the skin of fishes. Aris-
totle, the Greek philosopher, who was the greatest of the
ancient naturalists, expresses these beliefs in his books. It
was not until the middle of the seventeenth century—
Aristotle lived three hundred and fifty years before the
birth of Christ—that these beliefs were attacked and be-
gan to be given up. In the beginning of the seventeenth
century William Harvey, an English naturalist, declared
that every animal comes from an egg, but he said that the
egg might “ proceed from parents or arise spontaneously or
out of putrefaction.” In the middle of the same century
Redi proved that the maggots in decaying meat which pro-
duce the flesh flies develop from eggs laid on the meat by
flies of the same kind. Other zodlogists of this time were
active in investigating the origin of new individuals. And
all their discoveries tended to weaken the belief in the
theory of spontaneous generation.

Finally, the adherents of this theory were forced to
restrict their belief in spontaneous generation to the case

59 ANIMAL LIFE

of a few kinds of animals, like parasites and the animalcules
of stagnant water. It was maintained that parasites arose
spontaneously from the matter of the living animal in
which they lay. Many parasites have so complicated and
extraordinary a life history that it was only after long and
careful study that the truth regarding their origin was dis-
covered. But in the case of every parasite whose life his-
tory is known the young are offspring of parents, of other
individuals of their kind. No case of spontaneous genera-
tion among parasites is known. The same is true of the
animalcules of stagnant water. If some water in which
there are apparently no living organisms, however minute,
be allowed to stand for a few days, it will come to be
swarming with microscopic plants and animals. Any or-
ganic liquid, as a broth or a vegetable infusion exposed for
a short time, becomes foul through the presence of innumer-
able bacteria, infusoria, and other one-celled animals and
plants, or rather through the changes produced by their
life processes. But it has been certainly proved that these
organisms are not spontaneously produced by the water or
organic liquid. A few of them enter the water from the
air, in which there are always greater or less numbers of
spores of microscopic organisms. These spores (embryo or-
ganisms in the resting stage) germinate quickly when they
fall into water or some organic liquid, and the rapid suc-
cession of generations soon gives rise to the hosts of bacteria
and Protozoa which infest all standing water. If all the
active organisms and inactive spores in a glass of water are
killed by boiling the water, “ sterilizing ” it, as it is called,
and this sterilized water or organic liquid be put into a
sterilized glass, and this glass be so well closed that germs
or spores can not pass from the air without into the steril-
ized liquid, no living animals will ever appear in it. It is
now known that flesh will not decay or liquids ferment
except through the presence of living animals or plants.
To sum up, we may say that we know of no instance of the

ee

THE MULTIPLICATION OF ANIMALS AND SEX 53

spontaneous generation of organisms, and that all the ani-
mals whose life history we know are produced from other
animals of the same kind. “ Omne vivum ex vivo,’ All life
from life.

Fie. 26.—The multiplication of Ameba by simple fission.

30. The simplest method of multiplication —In our study
of the simplest and the slightly complex animals we became
acquainted with the simplest methods of multiplication
and with methods which are more complex. The method

BA ANIMAL LIFE

of simple fission or splitting—binary fission it is often called,
because the division is always in two—by which the body
of the parent becomes divided into two equal parts—into
halves—is the simplest method of multiplication. This is
the only method of Ameba (Fig. 26) and of many other of
the simplest animals. In this kind of reproduction it is
hardly exact to speak of parent and children. The chil-
dren, the new Amebe, are simply the parent cut into
halves. The parent persists; it does not produce off-
spring and die. Its whole body continues to live. The
new Amebe take in and assimilate food and add new mat-
ter to the original matter of the parent body; then each
of them divides in two. The grandparent’s body is now
divided into four parts, one fourth of it forming one half
of each of the bodies of the four grandchildren. The pro-
cess of assimilation, growth, and subsequent division takes
place again, and again, and again. Each time there is given
to the new Ameba an ever-lessening part of the actual
body substance of the original ancestor. Thus an Ameba
never dies a natural death, or, as has been said, “no Ameba
ever lost an ancestor by death.” It may be killed outright,
but in that case it leaves no descendants. If it is not killed
before it produces new Ame@be it never dies, although it
ceases to exist as a single individual. The Amebda and
other simple animals which multiply by direct binary
fission may be said to be immortal, and the ‘‘ immortality
of the Protozoa” is a phrase which you will be sure to meet
if you begin to read the writings of the modern philosoph-
ical zodlogists.

31. Slightly complex methods of multiplication Most of
the Protozoa multiply or reproduce themselves in two
ways—by simple fission and by conjugation. Parame-
cium, for example, reproduces itself for many generations
by fission, but a generation finally appears in which a dif-
ferent method of reproduction is followed. Two individu-
als come together and each exchanges with the other a part

THE MULTIPLICATION OF ANIMALS AND SEX 55

of its nucleus. Then the two individuals separate and
each divides into two. The result of this conjugation is
to give to the new Paramecia produced by the conjugat-
ing individuals a body which contains part of the body
substance of two distinct individuals. The new Parame-
cia are not simply halves of a single parent; they are parts
of two parents. If the two conjugating individuals differ
at all—and they always do differ, because no two individual
animals, although belonging to the same species, are exactly
alike—the new individual, made up of parts of each of them,
will differ from both. We shall, as we study further, see
that Nature seems intent on making every new individual
differ slightly from the individual which produces it; and
the method of multiplication or the production of new indi-
viduals which Nature has adopted to produce the result is
the method which we have seen exhibited in its simplest
form among the simplest animals—the method of having
two individuals take part in the production of a new one.
The further study of multiplication among animals is the
study of the development and elaboration of this method.

32. Differentiation of the reproductive cells—Among the
colonial Protozoa the first differentiation of the cells or
members composing the colony is the differentiation into
two kinds of reproductive cells. Reproduction by simple
division, without preceding conjugation, can and does take
place, to a certain extent, among all the colonial Protozoa.
Indeed, this simple method of multiplication, or some modi-
fication of it, like budding, persists among many of the com-
plex animals, as the sponges, the polyps, and even higher
and more complex forms. But such a method of single-
parent reproduction can not be used alone by a species for
many generations, and those animals which possess the
power of multiplication in this way always exhibit also the
other more complex kind of multiplication, the method of
double-parent reproduction. Conjugation takes place be-
tween different members of a single colony of one of the

56 ANIMAL LIFE

colonial Protozoa, or between members of different colonies
of the same species. These conjugating individuals in the
simpler kinds of colonies, like Goniwm, are similar; in
Pandorina they appear to be slightly different, and in Ludo-
rina and Volvox the conjugating cells are very different from
each other (Figs. 15 and 16). One kind of cell, which is
called the egg cell, is large, spherical, and inactive, while
the other kind, the sperm cell, is small, with ovoid head
and tapering tail, and free-swimming. In the simpler colo-
nial Protozoa all the cells of the body take part in repro-
duction, but in Volvox only certain cells perform this func-
tion, and the other cells of the body die. Or we may say
that the body of Volvoz dies after it has produced special
reproductive cells which shall fulfill the function of multi-
plication.

Beginning with the more complex Volvocine, which we
may call either the most complex of the one-celled animals
or the simplest of the many-celled animals, all the complex
animals show this distinct differentiation between the re-
productive cells and the cells of the rest of the body. Of
course, we find, as soon as we go up at all far in the scale of
the animal world, that there is a great deal of differentia-
tion among the cells of the body: the cells which have to
do with the assimilation of food are of one kind; those on
which depend the motions of the body are of another kind;
those which take oxygen and those which excrete waste
matter are of other kinds. But the first of this cell differ-
entiation, as we have already often repeated, is that shown
by the reproductive cells; and with the very first of this
differentiation between ropeadaakive cells and the other
body cells appears a differentiation of the reproductive
cells into two kinds. These two kinds, among all animals,
are always essentially similar to the two kinds shown by
Volvox and the simplest of the many-celled animals—namely,
large, inactive, spherical egg cells, and small, active, elon-
gate or “ tailed ” sperm cells.

THE MULTIPLICATION OF ANIMALS AND SEX 57

33. Sex, or male and female.—In the slightly complex
animals one individual produces both egg cells and sperm
cells. But in the Siphonophora, or colonial jelly-fishes, stud-
ied in the last chapter, certain members of the colony pro-
duce only sperm cells, and certain other members of the
colony produce only egg cells. If the Siphonophora be
considered an individual organism and not a colony com-
posed of many individuals, then, of course, it is like the .
others of the slightly complex animals in this respect. But
as soon as we rise higher in the scale of animal life, as soon
as we study the more complex animals, we find that the
egg cells and sperm cells are almost always produced by
different individuals. Those individuals which produce
egg cells are called female, and those which produce sperm
cells are called male. There are two sexes. Male and
female are terms usually applied only to individuals, but
it is evidently fair to call the egg cells the female reproduc-
tive cells, and the sperm cells the male reproductive cells.
A single individual of the simpler kinds of animals pro-
duces both male and female cells. But such an individual
can not be said to be either male or female; it is sexless—
that is, sex is something which appears only after a certain
degree of structural and physiological differentiation is
reached. It is true that even among many of the higher
or complex animals certain species are not represented by
male and female individuals, any individual of the species
being able to produce both male and female cells. But this
is the exception.

34. The object of sex.—Among almost all the complex
animals it is necessary that there be a conjugation of male
and female reproductive cells in order that a new individual
may be produced. This necessity first appears, we remem-
ber, among very simple animals. This intermixing of body
substance from two distinct individuals, and the develop-
ment therefrom of the new individual, is a phenomenon
which takes place through the whole scale of animal life.

58 ANIMAL LIFE

The object of this intermixing is the production of va-
riation. Nature demands that the offspring shall differ
slightly from its parents. By having the beginnings of its
body, the single cell from which the whole body develops,
composed of parts of two different individuals, this differ-
ence, although slight and nearly imperceptible, is insured.
Sex is a provision of Nature to insure variation.

35. Sex dimorphism.—As we have seen, almost every
species of animal is represented by two kinds of individuals,
males and females. In the case of many animals, espe-

a

Fig. 27.—Bird of paradise, male.

cially the simpler ones, these two kinds of individuals do
not differ in appearance or in structure apart from the
organs concerned with multiplication. But with many
animals the sexes can be readily distinguished. The male
and female individuals often show marked differences,
especially in external structural characters. We can read-

THE MULTIPLICATION OF ANIMALS AND SEX 59

ily tell the peacock, with its splendidly ornamental tail
feathers, from the unadorned peafowl, or the horned ram
from the bleating ewe. There is here, plainly, a dimor-
phism—the existence of two kinds of individuals belonging
to a single species. This dimorphism is due to sex, and
the condition may be called sex dimorphism. Among some
animals this sex dimorphism, or difference between the
sexes, is carried to extraordinary extremes. This is espe-
cially true among polygamous animals, or those in which
the males mate with many females, and are forced to fight
for their possession. The male bird of paradise, with its
gorgeous display of brilliantly colored and fantastically
shaped feathers (Fig. 27), seems a wholly different kind of
bird from the modest brown female. The male golden and
silver pheasants, and allied species with their elaborate
plumage, are very unlike the dull-colored females. The
great, rough, warlike male fur seal, roaring like a lion, is
three times as large as the dainty, soft-furred female, which
bleats like a sheep.

‘Among some of the lower animals the differences be-
tween male and female are even greater. The males of
the common cankerworm moth (Fig. 28) have four wings;

Fie. 28.—Cankerworm moth; the winged male and wingless female.

the females are wingless, and several other insect species
show this same difference. Among certain species of white
ants the females grow to be five or six inches long, while
the males do not exceed half an inch in length. In the

60 ANIMAL LIFE

case of some of the parasitic worms which live in the bod-
ies of other animals, the male has an extraordinarily de-
graded, simple body, much smaller than that of the female
and differing greatly from that of the female in structure.
In some cases even—as, for example,
the worm which causes “ gapes ” in
chickens —the male lives parasiti-
cally on the female, being attached to
the body of the female for its whole
lifetime, and drawing its nourish-
ment from her blood (Fig. 29).

A condition known as partheno-
genesis is found among certain of
the complex animals. Although the
species is represented by individu-
als of both sexes, the female can
produce young from eggs which
have not been fertilized. For ex-
ample, the queen bee lays both fer-
tilized and unfertilized eggs. From
the fertilized eggs hatch the work-
ers, which are rudimentary females,
and other queens, which are fully-
Gcaoy vnc, developed females ; from the unfer-

causes the “gapes” in fowls. tilized eggs hatch only males—the
The male is attached to the drones. Many generations of plant
female, and lives as a para- .,
giteon tek: lice are produced each year parthe-
nogenetically — that is, by unferti-
lized females. But there is at least one generation each
year produced in the normal way from fertilized eggs.

Some of the complex animals are hermaphroditic—that
is, a single individual produces both egg cells and sperm
cells. The tapeworm and many allied worms show this
condition. This is the normal condition for the simplest
animals, as we have already learned, but it is an excep-
tional condition among the complex animals.

THE MULTIPLICATION OF ANIMALS AND SEX 6]

36. The number of young.—There is great variation in
the number of young produced by different species of ani-
mals. Among the animals we know familiarly, as the
mammals, which give birth to young alive, and the birds,
which lay eggs, it is the general rule that but few young
are produced at a time, and the young are born or eggs
are laid only once or perhaps a few times in a year. The
robin lays five or six eggs once a year ; a cow may produce
a calf each year. Rabbits and pigeons are more prolific,
each having several broods a year. But when we observe
the multiplication of some of the animals whose habits are
not so familiar to us, we find that the production of so few
young is the exceptional and not the usual habit. A lob-
ster lays ten thousand eggs at a time; a queen bee lays
about five million eggs in her life of four or five years. A
female white ant, which after it is full grown does nothing
but lie in a cell and lay eggs, produces eighty thousand
eggs a day steadily for several months. A large codfish
was found on dissection to contain about eight million
eggs.

If we search for some reason for this great difference in
fertility among different animals, we may find a promis-
ing clew by attending to the duration of life of animals,
and to the amount of care for the young exercised by the
parents. We find it to be the general rule that animals
which live many years, and which take care of their young,
produce but few young; while animals which live but a
short time, and which do not care for their young, are very
prolific. The codfish produces its millions of eggs; thou-
sands are eaten by sculpins and other predatory fishes be-
fore they are hatched, and other thousands of the defense-
less young fish are eaten long before attaining maturity.
Of the great number produced by the parent, a few only
reach maturity and produce new young. But the eggs of the
robin are hatched and protected, and the helpless fledglings
are fed and cared for until able to cope with their natural

62 ANIMAL LIFE

enemies. In the next year another brood is carefully reared,
and so on for the few years of the robin’s life.

Under normal conditions in any given locality the num-
ber of individuals of a certain species of animal remains
about the same. The fish which produces tens of thousands
of eggs and the bird which reproduces half a dozen eggs a
year maintain equally well their numbers. In one case a
few survive of many born; in the other many (relatively)
survive of the few born ; in both cases the species is effect-
ively maintained. In general, no agency for the perpetua-
tion of the species is so effective as that of care for the

young.

a

CHAPTER IV
FUNCTION AND STRUCTURE

37. Organs and functions.—An animal does certain things
which are necessary to life. It eats and digests food, it
breathes in air and takes oxygen from it and breathes out
carbonic-acid gas; it feels and has other sensations; it pro-
duces offspring, thus reproducing itself. These things are
done by the simplest animals as well as by the complex
animals. But while with the simplest animals the whole
body (which is but a single cell) takes part in doing each
of these things, among the complex animals only a part
of the body is concerned with any one of these things.
Only a part of the body has to do with the taking in of
oxygen. Another part has to do with the digestion of
food, and another with the business of locomotion. These
parts of the body, as we know, differ from each other, and
they differ because they have different things todo. These
different parts are called organs of the body, and the things
they do are called their functions. The nostrils, trachee,
and lungs are the organs which have for function the pro-
cess of respiration. The legs of a cat are the organs which
perform for it the function of locomotion. The structure
of one of the higher animals is complex because the body
is made up of many distinct organs having distinct func-
tions. The things done by one of the complex animals are
many; around each of the principal functions or necessary
processes, as a center, are grouped many minor accessory

functions, all helping to make more successful the accom-
63

64 ANIMAL LIFE

plishment of the principal functions. While many of the
lower animals have no eyes and no ears, and trust to more
primitive means to discover food or avoid enemies, the
higher animals have extraordinarily complex organs for
seeing and hearing, two functions which are accessory only
to such a principal function as food-taking.

38. Differentiation of structure—We have seen, in our
study of the slightly complex animals, how the body be-
comes more and more complex in proportion to the degree
in which the different life processes are divided or assigned
to different parts of it for performance. With the gradu-
ally increasing division of labor the body becomes less
homogeneous in structure; a differentiation of structure
becomes apparent and gradually increases. The extent of
the division of labor and the extent of the differentiation
of structure, or division of the body into distinct and dif-
ferent parts and organs, go hand in hand. An animal in
which the division of labor is carried to an extreme is an
animal in which complexity of structure is extreme.

39. Anatomy and physiology.—Zodlogy, or the study of
animals, is divided for convenience into several branches
or phases. The study of the classification of animals is
called systematic zodlogy; the study of the development
of animals from their beginning as a single cell to the time
of their birth is called animal embryology; the study of
the structure of animals is called animal anatomy, and the
study of the performance of their life processes or functions
is called physiology. Because the whole field of zodlogy is
so great, some zodlogists limit themselves exclusively to one
of these phases of zodlogical study, and those who do not
so definitely limit their study, at least give their special at-
tention to a single phase, although all try to keep in touch
with the state of knowledge in other phases. In earlier
days the study of the anatomy of animals and of their
physiology were held to be two very distinct lines of in-
vestigation, and the anatomists paid little attention to

a

FUNCTION AND STRUCTURE 65

physiology and the physiologists little to anatomy. But
we have seen how inseparably linked are structure and
function. The structure of an animal is as it is because
of the work it has to do, and the functions of an animal
are performed as they are performed because of the special
structural condition of the organs which perform them.
The study of the anatomy and the study of the physiology
of animals can not be separated. To understand aright
the structure of an animal it is necessary to know to
what use the structure is put; to understand aright the
processes of an animal it is necessary to know the struc-
ture on which the performance of the processes depends.
40. The animal body a machine.—The body of an animal
may be well compared ‘with some machine like a locomotive
engine. Indeed, the animal body is a machine. It is a
machine composed of many parts, each part doing some
particular kind of work for which a particular kind of
structure fits it; and all the parts are dependent on each
other and work together for the accomplishment of the
total business of the machine. The locomotive must be
provided with fuel, such as coal or wood or other readily
combustible substance, the consumption of which furnishes
the force or energy of the machine. The animal body
must be provided with fuel, which is called food, which
furnishes similarly the energy of the animal. Oxygen must
be provided for the combustion of the fuel in the locomo-
tive and the food in the body. The locomotive is com-
posed of special parts: the firebox for the reception and
combustion of fuel; the steam pipes for the carriage of
steam ; the wheels for locomotion; the smoke stack for
throwing off of waste. The animal body is similarly com-
posed of parts: the alimentary canal for the reception and
assimilation of food ; the excretory organs for the throwing
off of waste matter; the arteries and veins for the carriage
of the oxygen and food-holding blood; the legs or wings

for locomotion.
6

66 ANIMAL LIFE

The locomotive is an inorganic machine; the animal is
an organic machine. There is a great and real difference
between an organism, a living animal, and a locomotive, an
inorganic structure. But for a good understanding of the
relation between function and structure, and of the com-
position of the body of the complex animals, the compari-
son of the animal and locomotive is very instructive.

41. The specialization of organs.—The organ for the per-
formance of some definite function in one of the higher
animals may be very complex. The corresponding organ
in one of the lower animals for the performance of the
same function may be comparatively simple. For example,
the organ for the digestion of food is, in the case of the
polyp, a simple cylindrical cavity in the body into which
food enters through a large opening at the apical or free
end of the body. The digestive organ of a cow is a long
coiled tube, comprising many regions of distinct structural
and physiological character and altogether extremely com-
plicated. An organ in simple or primitive condition is
said to be generalized ; in complex or highly modified con-
dition it is said to be specialized. That is, an organ may
be modified and complexly developed to perform its func-
tion in a special way, in a way differing in many particu-
lars from the way the corresponding organ in some other
animal performs the same general function. The speciali-
zation of organs, or their modification to perform their
functions in special ways, is what makes animal bodies
complex, for specialization is almost always in the line of
complexity. Later we shall see more clearly how specializa-
tion is brought about. For the present we may study
one of the more important organs of the animal body for
the sake of having concrete examples of some of the gen-
eral statements made in this discussion of function and
structure.

‘42. The alimentary canal_—The organ which has to do
with the taking and digesting of food is called the ali-

FUNCTION AND STRUCTURE 67

mentary canal. In some of the higher animals this is a
very complex organ. In the cow, one of the cud-chewing
mammals or ruminants, it consists of several distinct por-
tions, which differ among themselves very much (Fig. 30).
First, there is the mouth, or opening for the entrance of
the food. The mouth is sup-
plied with teeth for biting
off and chewing the food,
with a tongue for manipu-
lating it, and with taste pa-
pille situated on the tongue
and palate for determining
the desirability of the food.
Into the mouth a peculiar
fluid (the saliva) is poured
by certain glands, organs ac-
cessory to the alimentary
canal. The herbage bitten
off, mixed with saliva, and
rolled by the tongue into a
ball, passes back through a
narrow tube, the esophagus,
and into a sac called the ru-

Fie. 30.—Alimentary canal of the ox

men, or paunch. Here it
lies until the cow ceases for
the while to take in food,
when it passes back again
through the cesophagus and

(after CoL1In and MULLER). @, rumen
(left hemsiphere) ; 5, ramen (right hem-
isphere) ; ¢c, insertion of cesophagus ; d,
reticulum ;' ¢, omasum; f, abomasum ;
g, duodenum; f and i, jejunum and
ileum; j, cecum; &, colon, with its

: : various convolutions ; 7, rectum.
into the mouth for mastica-

tion. After being masticated it again passes downward
through the esophagus, and enters this time another sac
called the reticulum, lying next to the rumen. From here
it passes into another sac-like portion of the alimentary
canal called the omasum, where it is strained through
numerous leaf-like folds which line the walls of this part
of the canal. From here the food passes into a fourth

68 ANIMAL LIFE

sac-like part of the canal, called the abomasum. Here
the process of digestion goes on. The four sacs—rumen,
reticulum, omasum, and abomasum—are called stomachs,
or they may be considered to be four chambers forming
one large stomach. In the abomasum, or digesting stom-
ach, digestive fluids are poured from glands lining its
walls, and the food becomes converted into a liquid called
chyle. The chyle passes from the stomach into a long,
narrow, tubular portion of the canal called the intestine.
The intestine is very long, and lies coiled in a large mass
in the body of the cow. The intestine is divided into
distinct regions, which vary in size and in the character
of the inner wall. These parts of the intestine have
names, as duodenum, jejunum, ileum, cecum, colon, etc.
Part of the intestine is lined inside with fine papilla,
which take up the chyle (the digested food) and pass it
through the walls of the intestine to other special organs,
which pass it on to the blood, with which it becomes mixed
and carried by an elaborate system of tubes to all parts of
the body. Part of the grass taken into the alimentary
canal by the cow can not be digested, and must be got rid
of. This passes on into a final posterior part of the intes-
tine called the rectum, and leaves the body through the
anus or posterior opening of the alimentary canal. The
whole canal is more than twenty times as long as the body
of the cow; it is composed of parts of different shape ; its
walls are supplied with muscles and blood-vessels ; the inner
lining is covered with folds, papillae, and gland cells. It is
altogether a highly specialized organ, a structurally com-
plex and elaborately functioning organ.

Let us now examine the alimentary canal, or organ of
digestion, in some of the simpler animals.

The Protozoa, or simplest animals, have no special organ
at all. When the surface of the body of an Ameba comes
into contact with an organic particle which will serve as
food, the surface becomes bent in at the point of its con-

FUNCTION AND STRUCTURE 69

tact with the food particle, and the body substance simply
incloses the food (Fig. 3). Food is taken in by the sur-
face. The whole outer surface of the body is the food-
taking organ. In the simplest many-celled animals, the
sponges, there is no special food-taking and digestive organ.
Each of the cells of the body takes in and assimilates food
for itself. The sponge is like a great group of Amebe
holding fast to each other, but each looking out for its own
necessities. Among the mM.

polyps, however, there

‘

~

is a definite organ of & VEGEN
digestion—that is, food Sh ve Kes ZN
is only taken and di- Be NN eee <2),
gested by certain parts ON ee A
of the body. The sim- \’ Bs OW,
ple polyp’s body (Fig. ra ty

Fe - al A 3 ea a]
31) is a cylinder or vase i é-
closed at one end and ie Se
open at the other end, \ jm

| [*\)
7S
be
une

and attached by the
closed end to a rock. \
The opening is usually |

|

Tec
: iv

=
rt
2

of less diameter than
the diameter of the

Fig. 31.—Obelia sp.,a simple polyp; vertical sec-

body, and it is sur- tion, highly magnified. m, mouth opening;
rounded by a number al. s., alimentary sac.— After PARKER and
HASWELL.

of tentacles, whose

function it is to seize the food and convey it to the mouth
opening. There are, of course, no teeth, no tongue, none
of the various parts which are in or are part of the mouth
of the higher animals. The polyp’s mouth is simply a
hole or opening into the inside of the body. This body
eavity, or simplest of all stomachs, is simply the cylindrical
or yase-shaped hollow space inclosed by the body wall.
This space extends also into the tentacles. There is no
other opening, no posterior or anal opening. We can not

10 ANIMAL LIFE

speak of an cesophagus or intestine in connection with this
most primitive of alimentary sacs. The cells which line
the sacs show some differentiation ; some are gland cells
and secrete digestive fluids; some are amoeboid and are
provided with pseudopods or flagella for seizing bits of
food. The food caught by the tentacles comes into the ali-
mentary sac through the opening or primitive mouth, and

Doin WF
yp) - 77
Bi

an

pe

A

Fie. 32.—Diagrammatic sketch of a flat- Fig. 33.—Sea-cucumber (Holothurian)
worm (Planaria), showing the dissected to show alimentary canal,
branched alimentary canal, al. c.— al. c.—After LEUCKART.

After Jisima and HaTsHER.

what of it is digestible is, by the aid of the gland cells and
the ameeboid cells, taken up and assimilated, while the rest
of it is carried out by water currents again through the
single opening.

In the flatworms (Fig. 32) like Planaria (small, thin,
flattened worms to be found in the mud at the bottom of
fresh-water ponds) the mouth opens into a short, narrow
tube which may be called an esophagus. The esophagus

FUNCTION AND STRUCTURE 71

connects the mouth with the rest of the alimentary canal,
which gives out many side branches or diverticula, which
are themselves branched, so that the
alimentary sac or stomach is a system
of ramifying tubes extending from a
central main tube to all parts of the
body of the worm. There is no
anal opening. In the round or thread
worms, of which the deadly Trichina
is an example, the alimentary canal
is a simple straight tube with both
anterior or mouth opening and pos-
terior or anal opening. In the sea-
urchins and sea-cucumbers (Fig. 33)
the alimentary canal is a simple tube
with two openings, but it is longer
than the body between mouth and
anus, and so is more or less bent or
coiled. In the earthworm the ali-
mentary canal (Fig. 34), although a
simple straight tube running through
the body, plainly shows a differentia-
tion into particular regions. Behind
the mouth opening the alimentary
tube is large and thick - walled and
is called the pharynx; behind the
pharynx it is narrower and is called
the esophagus. Behind the csopha-
gus it expands to form a rounded,
thin-walled chamber called the crop,
and just behind this there is another ee
rounded but very thick-walled cham- Fic. 34.—Earthworm dissected
ber called the gizzard. From the os nce erence ee
gizzard back the alimentary canal is

about uniform in size, being rather wide and having thick,
soft walls. This portion of it is called the intestine. The

\4
RETIN
= We —

=. #
--) >
t¢

a

=

Talal

vt

OS

al

"9 ANIMAL LIFE

posterior part of the intestine, called the rectum, leads to
the anal opening. ‘There is some differentiation of the
inner surface of the canal. In the great group of mol-
lusks, of which the common fresh-water clam or mussel is
an example, the alimentary canal (Fig. 35) shows much
variation. The microscopic plants, which are the food of
the mussel, are taken in through the mouth and pass into
a short csophagus, thence into a wide stomach and there
digested. Behind the stomach is a long, much-folded, nar-
row intestine which winds about through the fleshy “ foot ”
and finally reaches the surface of the body, and has an
anal opening at a point opposite the position of the mouth.

Among the insects there is a great range in degree of
complexity of the alimentary canal. The digestive organs
are, however, in most insects in a condition of high speciali-
zation. The mouth opening is provided with well-developed

Fig. 35.—Pond mussel dissected to show alimentary canal, al. c.—After HaATSHEK
and Corl.

biting and masticating or piercing and sucking mouth parts;
pharynx, cesophagus, stomach, and intestine are always dif-
ferentiated and sometimes greatly modified. In the com-
mon cockroach, for example (Fig. 36), the mouth has a
complicated food-getting apparatus, and the canal, which

FUNCTION AND STRUCTURE "3

is much longer than the body of the insect, and hence
much bent and coiled, consists of a pharynx, esophagus,
fore-stomach or proventriculus,
true digesting stomach or ven-
triculus, intestine, and rectum
which opens at the posterior
tip of the body. The inner
lining of the canal shows much
differentiation in the different
parts of the canal, and there
are numerous accessory glands
connected with various parts of
the canal.

Finally, among the highest
animals, the vertebrates, we
find still more elaborate special-
ization of the alimentary canal.
As an example the alimentary J
canal of a cow has already been +f
described in detail. SF

43. Stable and variable char- #16 36.—Cockroach dissected to show

Ne alimentary canal, a/. c.—After Hat-
acteristics of an organ.—In § gyex and Cont.
spite of all this variation in
the structure and general character of the alimentary
canal, there are certain characteristics which are features
of all alimentary canals. In the examination of an organ
we must ever distinguish between its so-called constant or
stable characteristics and its inconstant or variable charac-
teristics. The constant characteristics are the fundamen-
tally essential ones of the organ; the variable ones are the
special characteristics which adapt the organ for the pecul-
iar habits of the animal possessing it—habits which may
differ very much from those of some other animal of similar
size, similar distribution, similar abundance.

44. Stable and variable characteristics of the alimentary
canal.—A tiger or a lion has an alimentary canal not more

"4 ANIMAL LIFE

than three or four times the length of its body, while a
sheep has an alimentary canal twenty-eight times as long
as its body. The tiger is carnivorous; the sheep her-
bivorous. Associated with the different food habits of the
two animals is a striking difference in the alimentary
canals. Animals like the horse or cat, which chew their
food before swallowing it, have a slender cesophagus; ani-
mals like snakes which swallow their food whole have a
wide csophagus. Birds, that have no teeth and hence
can not masticate or grind their food in their mouths, usu-
ally have a special grinding stomach, the gizzard, for this
purpose. And so we might cite innumerable examples
of these inconstant or variable characteristics of the ali-
mentary canal. On the other hand, the alimentary canals
of all the many-celled animals except the lowest agree in
certain important characteristics. Each alimentary canal
has two openings, one for the ingress of food and one for
the exit of the indigestible portions of the matter taken in,
and the canal itself stretches through the body from mouth
to anus as a tube, now narrow, now wide, now suddenly
expanding into a sac or giving off lateral diverticula, but
always simply a lumen or hollow inclosed by a flexible mus-
cular wall. The inner lining of the wall is provided with
secreting and absorbing structures. Indeed, we can reduce
the essential characters of the alimentary canal to even
more simple features. The organ of digestion or assimila-
tion of all the many-celled animals is merely a surface with
which food is brought into contact, and which has the
power of digesting this food by means of digestive secre-
tions, and of absorbing the food when digested. This sur-
face is small or yreat in extent, depending upon the amount
of food necessary to the life of the animal and the difficulty
or readiness with which the food can be digested. This
surface might just as well be on the outside of the animal’s
body as on the inside, if it were convenient. In fact, it is
on the outside of some animals. Among the Protozoa the

FUNCTION AND STRUCTURE 15

digesting surface is simply the external surface of the body.
And not alone among the one-celled animals. Many of the
parasitic worms which live in the bodies of other animals,
and the larve or “ grubs” of many insects which lie in the
tissues of plants bathed by the sap, have no inner alimen-
tary canal, but take food through the outer surface of the
body. But in these cases the food is ready for immediate
absorption, so that no special treatment of it is necessary,
hence no complex structures are required.

Even were no such special treatment of the food neces-
sary in the case of the larger animals, it would still be im-

Fie. 37.—Diagram illustrating increase of volume and surface with increase of
diameter of sphere.

possible for the simple external surface of the body to serve
for food absorption, because of the well-known relation
between the surface and the mass of a solid body. When
a solid body in the form of a sphere increases in size, its
mass or volume increases as the cube of the diameter, while
the surface increases only as the square of the diameter
(Fig. 37). The external surface of minute animals a few
millimeters in diameter can take up enough food to supply
the whole body mass. But among large animals this food-
getting surface is increased as the square of the diameter of

76 ANIMAL LIFE

the body, while the volume or food-using surface of the
body is increased as the cube of its diameter. The food sup-
plying can not keep pace with the food using. Hence it is
absolutely essential that among large animals the food-tak-
ing surface be increased so that it will remain in the same
favorable proportion to the mass of the animal as is the
case among the minute animals, where the simple external
body surface is sufficient to obtain all the food necessary.
This increase of surface, without an accompanying increase
of size of the animal, is accomplished by having the digest-
ing and assimilating surface inside the body and by having
it greatly folded. The surface of the alimentary canal is,
after all, simply a bent-in continuation of the outer surface
of the body. It is open to the outside of the body by two
openings, and wholly closed (except by its porosity) to the
true inside of the body. By the bending and coiling of
the alimentary canal, and by the repeated folding of its
inner wall, the alimentary surface is greatly increased.
The necessity for this increase accounts largely for the
complexity of the alimentary canal.

But it is not alone this necessity for increased surface
that accounts for the great specialization of the alimentary
canal in such animals as the insects and the vertebrates.
The structural differences in different portions of the canal,
resulting in the differentiation of the canal into distinct
parts, or the differentiation of the whole organ into distinct
subordinate organs, each with a special work or function to
perform, are the result of the necessity for the special
manipulation of the special kinds of foods taken. Animals
which feed on other animals must have mouth structures
fit for seizing and rending their prey, and the alimentary
canal must be specially modified for the digestion of flesh.
Animals which feed on vegetable substances must have
special modifications of the alimentary canal quite different
from those of the carnivores. Some insects, like the mos-
quito, take only liquid food, the sap of plants, or the blood

FUNCTION AND STRUCTURE raf

of animals; others, like the weevils, feed on the hard, dry
substance of seeds and grains; others, like the grasshop-
pers and caterpillars, eat green leaves; and still others eat
other insects. The alimentary canal of each of these kinds
of insects differs more or less from that of the other kinds.
The specialization of the alimentary canal depends then
upon the necessity for a large food-digesting and absorbing
surface, and on the complex treatment of the food. The
character of this specialization in each case depends upon
the special kind or quality of food taken by the animal in
question.

45. The mutual relation of function and structure—The
structure of an animal depends upon the manner in which
the life processes or functions of the animal are performed.
If the functions are performed in a complex manner, the
structure of the body is complex ; if the functions are per-
formed in simple manner, the body will be simple in struc-
ture. With the increase in degree of the division of labor
among various parts of the body, there is an increase in
definiteness and extent of differentiation of structure.
Each part or organ of the body becomes more modified and
better fitted to perform its own special function. A pecul-
iar structural condition of any part of the body, or of the
whole body of any animal, is not to be looked on as a freak
of Nature, or as a wonder or marvel. Such a structure has
a significance which may be sought for. The unusual
structural condition is associated with some special habit
or manner of performance of a function. Function and
structure are always associated in Nature, and should always
be associated in our study of Nature.

CHAPTER V

THE LIFE CYCLE

46. Birth, growth and development, and death.—Certain
phenomena are familiar to us as occurring inevitably in the
life of every animal. Each individual is born in an imma-
ture or young condition ; it grows (that is, it increases in
size), and develops (that is, changes more or less in struc-
ture), and dies. ‘These phenomena occur in the succession
of birth, growth and development, and death. But before
any animal appears to us as an independent individual—
that is, outside the body of the mother and outside of an
egg (i. e., before birth or hatching, as we are accustomed to
call such appearance)—it has already undergone a longer
or shorter period of life. It has been a new living organ-
ism hours or days or months, perhaps, before its appear-
ance tous. This period of life has been passed inside an
egg, or as an egg or in the egg stage, as it is variously
termed. The life of an animal as a distinct organism be-
gins in an egg. And the true life cycle of an organism is
its life from egg through birth, growth and development,
and maturity to the time it produces new organisms in
the condition of eggs. The life cycle is from egg to egg.
Birth and growth, two of the phenomena readily apparent
to us in the life of every animal, are two phenomena in the
true life cycle. Death is a third inevitable phenomenon in
the life of each individual, but it is not a part of the cycle.
It is something outside.

4”. Life cycle of simplest animals——The simplest animals
have no true egg stage, nor perhaps have they any true

78

THE LIFE CYCLE 9

death. The new Amebe are from their beginning like the
full-grown Ameba, except as regards size. And the old
Ameba does not die, because its whole body continues to
live, although in two parts—the two new Amebe. The life
cycle of the simplest animals includes birth (usually by
simple fission of the body of the parent), growth, and some,
but usually very little, development, and finally the repro-
duction of new individuals, not by the formation of eggs,
but by direct division of the body.

48. The egg.—In our study of the multiplication of ani-
mals (Chapter IIL) we learned that it is the almost univer-

Fia. 38.—Eggs of different animals showing variety in external appearance. a, egg
of bird; 0, eggs of toad; ¢, egg of fish; d, egg of butterfly ; ¢, eggs of katydid
on leaf ; f,egg-case of skate.

sal rule among many-celled animals that each individual
begins life as a single cell, which has been produced by the

80 ANIMAL LIFE

fusion of two germ cells, a sperm cell from a male indi-
vidual of the species and an egg cell from a female indi-
vidual of the species. The single cell thus formed is called
the fertilized egg cell, and its subsequent development
results in the formation of a new individual of the same
species with its parents. Now, in the development of this
cell into a new animal, food is necessary, and sometimes a
certain amount of warmth. So with the fertilized egg cell
there is, in the case of all animals that lay eggs, a greater
or less amount of food matter—food yolk, it is called—gath-
ered about the germ cell, and both germ cell and food yolk
are inclosed in a soft or hard wall. Thus is composed the
egg as we know it. The hen’s egg is as large as it is be-
cause of the great amount of food yolk it contains. The
egg of a fish as large as a hen is much smaller than the
hen’s egg; it contains less food yolk. Eggs (Fig. 38) may
vary also in their external appearance, because of the dif-
ferent kinds of membrane or shells which may inclose and
protect them. Thus the frog’s eggs are inclosed in a thin
membrane and imbedded in a soft, jelly-like substance ;
the skate’s egg has a tough, dark-brown leathery inclosing
wall; the spiral egg of the bull-head sharks is leathery and
colored like the dark-olive seaweeds among which it lies;
and a bird’s egg has a hard shell of carbonate of lime. But
in each case there is the essential fertilized germ cell; in
this the eggs of hen and fish and butterfly and cray-fish and
worm are alike, however much they may differ in size and
external appearance. |

49. Embryonic and post-embryonic development.—Some
animals do not lay eggs, that is they do not deposit the fer-
tilized egg cell outside of the body, but allow the develop-
ment of the new individual to go on inside the body of the
mother for a longer or shorter period. All the mammals
and some other animals have this habit. When such an
animal issues from the body of the mother, it is said to be
born. When the developing animal issues from an egg

THE LIFE CYCLE 81

which has been deposited outside the body of the mother,
it is said to hatch. The animal at birth or at time of hatch-
ing is not yet fully developed. Only part of its development
or period of immaturity is passed within the egg or within
the body of the mother. That part of its life thus passed
within the egg or mother’s body is called the embryonic life
or embryonic stages of development; while that period of
development or immaturity from the time of birth or hatch-
ing until maturity is reached is called the post-embryonic
life or post-embryonic stages of development.

50. First stages in development.—The embryonic develop-
ment is from the beginning up to a certain point practically
identical for all many-celled animals—that is, there are cer-

Fig. 39.—First stages in embryonic development of the pond snail (Lymne@us). a,
egg cell; 0, first cleavage ; c, second cleavage ; d, third cleavage ; e, after numer-
ous cleavages ; f, blastula (in section); g, gastrula, just forming (in section) ;
h, gastrula, completed (in section).—After RAB.

tain principal or constant characteristics of the beginning
development which are present in the development of all
many-celled animals. The first stage or phenomenon of
development is the simple fission of the germ cell into
halves (Fig. 39, 0). These two daughter cells next divide so
that there are four cells (Fig. 39, c); each of these divides,

and this division is repeated until a greater or lesser num-
7

82 ANIMAL LIFE

ber (varying with the various species or groups of ani
mals) of cells is produced (Fig. 39, d). The phenomenon of
repeated division of the germ cell is called cleavage, and
this cleavage is the first stage of development in the case
of all many-celled animals. The first division of the germ
cell produces two equal cells, but in some of the later
divisions the new cells formed may not be equal. In some
animals all the cleavage cells are of equal size; in some
there are two sizes of cells. The germ or embryo animal
consists now of a mass of few or many undifferentiated
primitive cells lying together and usually forming a sphere
(Fig. 39, e), or perhaps separated and scattered through
the food yolk of the egg. The next stage of development
is this: the cleavage cells arrange themselves so as to form
a hollow sphere or ball, the cells lying side by side to form
the outer circumferential wall of this hollow sphere (Fig.
39, f). This is called the blastula or blastoderm stage of
development, and the embryo itself is called the blastula
or blastoderm. This stage also is common to all the many-
celled animals. The next stage in embryonic development
is formed by the bending inward of a part of the blasto-
derm cell layer, as shown in Fig. 39,g. This bending in
may produce a small depression or groove; but whatever the
shape or extent of the sunken-in part of the blastoderm, it
results in distinguishing the blastoderm layer into two
parts, a sunken-in portion called the endodlast and the
other unmodified portion called the ectoblast. Hndo- means
within, and the cells of the endoblast usually push so far
into the original blastoderm cavity as to come into contact
with the cells of the ectoblast and thus obliterate this cavity
(Fig. 39, 2). This third well-marked stage in the embry-
onic development is called the gastrula* stage, and it also

* This gastrula stage is not always formed by a bending in or in-
vagination of the blastoderm, but in some animals is formed by the
splitting off or delamination of cells from a definite limited region of

THE LIFE CYCLE 83

occurs in the development of all or nearly all many-celled
animals.

51. Continuity of development.—In the case of a few of
the simple many-celled animals the embryo hatches—that
is, issues from the egg at the time of or very soon after
reaching the gastrula stage. In the higher animals, how-
ever, development goes on within the egg or within the
body of the mother until the embryo becomes a complex
body, composed of many various tissues and organs. Al-
most all the development may take place within the egg,

a

Fie. 40.—Honey-bee. «@, adult worker ; b, young or larval worker.

so that when the young animal hatches there is necessary
little more than a rapid growth and increase of size to
make it a fully developed, mature animal. This is the case
with the birds: a chicken just hatched has most of the
tissues and organs of a full-grown fowl, and is simply a
little hen. But in the case of other animals the young
hatches from the egg before it has reached such an ad-
vanced stage of development; a young star-fish or young
crab or young honey-bee (Fig. 40) just hatched looks very
different from its parent. It has yet a great deal of devel-
opment to undergo before it reaches the structural condi-
tion of a fully developed and fully grown star-fish or crab
or bee. Thus the development of some animals is almost

the blastoderm. Our knowledge of gastrulation and the gastrula stage
- is yet far from complete,

84 ANIMAL LIFE

wholly embryonic development—that is, development with-
in the egg or in the body of the mother—while the devel-
opment of other animals is largely post-embryonic or larval
development, as it is often called. There is no important
difference between embryonic and post-embryonic develop-
ment. The development is continucus from egg cell to
mature animal, and whether inside or outside of an egg it
goes on regularly and uninterruptedly.

52. Development after the gastrula stage.—The cells which
compose the embryo in the cleavage stage and blastoderm
stage, and even in the gastrula stage, are all similar; there
is little or no differentiation shown among them. But from
the gastrula stage on development includes three important
things: the gradual differentiation of cells into various
kinds to form the various kinds of animal tissues; the
arrangement and grouping of these cells into organs and
body parts; and finally the developing of these organs
and body parts into the special condition characteristic of
the species of animal to which the developing individual
belongs. From the primitive undifferentiated cells of the
blastoderm, development leads to the special cell types of
muscle tissue, of bone tissue, of nerve tissue ; and from the
generalized condition of the embryo in its early stages de-
velopment leads to the specialized condition of the body of
the adult animal. Development is from the general to the
special, as was said years ago by the first great student of
development.

53. Divergence of development.—A star-fish, a beetle, a
dove, and a horse are all alike in their beginning-—that is,
the body of each is composed of a single cell, a single struc-
tural unit. And they are all alike, or very much alike,
through several stages of development; the body of each
is first a single cell, then a number of similar undifferen-
tiated cells, and then a hollow sphere consisting of a single
layer of similar undifferentiated cells. But soon in the
course of development the embryos begin to differ, and as

THE LIFE CYCLE 85

the young animals get further and further along in the
course of their development, they become more and more
different until each finally reaches its fully developed ma-
ture form, showing all the great structural differences be-
tween the star-fish and the dove, the beetle and the horse.
That is, all animals begin development alike, but gradually
diverge from each other during the course of development.

There are some extremely interesting and significant
things about this divergence to which attention should be
given. While all animals are alike structurally * at the
beginning of development, so far as we can see, they do not
all differ at the time of the first divergence in development.
This first divergence is only to be noted between two kinds
of animals which belong to different great groups or classes.
But two animals of different kinds, both belonging to some
one great group, do not show differences until later in their
development. This can best be understood by an example.
All the butterflies and beetles and grasshoppers and flies
belong to the great group of animals called Insecta, or in-
sects. There are many different kinds of insects, and these
kinds can be arranged in subordinate groups, such as the
Diptera, or flies, the Lepidoptera, or butterflies and moths,
and soon. But all have certain structural characteristics
in common, so that they are comprised in one great group
or class—the Insecta. Another great group of animals is
known as the Vertebrata, or back-boned animals.. The class
Vertebrata includes the fishes, the batrachians, the reptiles,
the birds, and the mammals, each composing a subordinate
group, but all characterized by the possession of a back-

* They are alike structurally, when we consider the cell as the unit
of animal structure. That the egg cells of different animals may dif-
fer in their fine or ultimate structure, seems certain. For each one of
these egg cells is destined to become some one kind of animal, and no
other; each is, indeed, an individual in simplest, least developed con-
dition of some one kind of animal, and we must believe that difference
in kind of animals depends upon difference in structure in the egg itself.

86 ANIMAL LIFE

bone, or, more accurately speaking, of a notochord, a back-
bone-like structure. Now, an insect and a vertebrate di-
verge very soon in their development from each other; but
two insects, such as a beetle and a honey-bee, or any two
vertebrates, such as a frog and a pigeon, do not diverge
from each other so soon. That is, all vertebrate animals
diverge in one direction from the other great groups, but
all the members of the great group keep together for some
time longer. Then the subordinate groups of the Verte-
brata, such as the fishes, the birds, and the others diverge,
and still later the different kinds of animals in each of
these groups diverge from each other. In the illustration
(Fig. 41) on the opposite page will be seen pictures of the
embryos of various vertebrate animals shown as they appear
at different stages or times in the course of development.
The embryos of a fish, a salamander, a tortoise, a bird, and
a mammal, representing the five principal groups of the
Vertebrata, are shown. In the upper row the embryos are
in the earliest of all the stages figured, and they are very
much alike. There are no distinctive characteristics of
fish or bird. Yet there are distinctive characteristics of
the great class Vertebrata. Any of these embryos could
readily be distinguished from an embryonic insect or worm
or sea-urchin. In the second row there is beginning to be
manifest a divergence among the different embryos, al-
though it would still be a difficult matter to distinguish
certainly which was the young fish and which the young
salamander, or which the young tortoise and which the
young bird. In the bottom row, showing the animals in a
later stage of development, the divergence has proceeded
so far that it is now plain which is a fish, which batrachian,
which reptile, which bird, and which mammal.

54. The laws or general facts of development.—That the
course of development of any animal from its beginning to
fully developed adult form is fixed and certain is readily
seen. Every rabbit develops in the same way; every grass-

= Salamander x

S) Jortoise Chick Rabbit

Fig. 41.—Different vertebrate animal in successive embryonic stages. I, first
or earliest of the stages figured ; II, second of the stages; III, third or
latest of the stages.—After HAECKEL,

88 ANIMAL LIFE

hopper goes through the same developmental changes from
single egg cell to the full-grown active hopper as every
other grasshopper of the same kind—that is, development
takes place according to certain natural laws, the laws of
animal development. These laws may be roughly stated as
follows: All many-celled animals begin life as a single cell,
the fertilized egg cell; each animal goes through a certain
orderly series of developmental changes which, accom-
panied by growth, leads the animal to change from single
cell to the many-celled, complex form characteristic of the
species to which the animal belongs; this development is
from simple to complex structural condition; the develop-
ment is the same for all individuals of one species. While
all animals begin development similarly, the course of devel-
opment in the different groups soon diverges, the diver-
gence being of the nature of a branching, like that shown
in the growth of a tree. In the free tips of the smallest
branches we have represented the various species of ani-
mals in their fully developed condition, all standing clearly
apart from each other. But in tracing back the develop-
ment of any kind of animal, we soon come to a point where
it very much resembles or becomes apparently identical
with some other kind of animal, and going further back we
find it resembling other animals in their young condition,
and so on until we come to that first stage of development,
that trunk stage, where all animals are structurally alike.
To be sure, any animal at any stage in its existence differs
absolutely from any other kind of animal, in that it can
develop into only its own kind of animal. There is some-
thing inherent in each developing animal that gives it an
identity of its own. Although in its young stages it may
be indistinguishable from some other kind of animal in its
young stages, it is sure to come out, when fully developed,
an individual of the same kind as its parents were or are.
The young fish and the young salamander in the upper
row in Fig. 41 are indistinguishably alike, but one embryo

THE LIFE CYCLE 89

is sure to develop into a fish and the other into a sala-
mander. This certainty of an embryo to become an indi-
vidual of a certain kind is called the law of heredity.
Viewed in the light of development, there must be as great
a difference between one egg and another as between one
animal and another, for the greater difference is included
in the less.

55. The significance of the facts of development.—The sig-
nificance of the developmental phenomena is a matter
about which naturalists have yet very much to learn. It is
believed, however, by practically all naturalists that many
of the various stages in the development of an animal cor-
respond to or repeat the structural condition of the ani-
mal’s ancestors. Naturalists believe that all backboned or
vertebrate animals are related to each other through being
descended from a common ancestor, the first or oldest
backboned animal. In fact, it is because all these back-
boned animals—the fishes, the batrachians, the reptiles, the
birds, and the mammals—have descended from a common
ancestor that they all have a backbone. It is believed that
the descendants of the first backboned animal have in the
course of many generations branched off little by little
from the original type until there came to exist very real
and obvious differences among the backboned animals—dif-
ferences which among the living backboned animals are
familiar to all of us. The course of development of an
individual animal is believed to be a very rapid and evi-
dently much condensed and changed, recapitulation of the
history which the species or kind of animal to which the
developing individual belongs has passed through in the
course of its descent through a long series of gradually chang-
ing ancestors. If this is true, then we can readily under-
stand why the fish and the salamander and tortoise and
bird and rabbit are all alike in their earlier stages of devel-
opment, and gradually come to differ more and more as
they pass through later and later developmental stages.

90 ANIMAL. LIFE

56. Metamorphosis.— W hile a young robin when it hatches
from the egg or a young kitten at birth resembles its par-
ents, a young star-fish or a young crab or a young butterfly
when hatched does not at all resemble its parents. And
while the young robin after hatching becomes a fully grown
robin simply by growing larger and undergoing compara-
tively slight developmental changes, the young star-fish or
young butterfly not only grows larger, but undergoes some
very striking developmental changes; the body changes
very much in appearance. Marked changes in the body of
an animal during post-embryonic or larval development
constitute what is called metamorphic development, or the
animal is said to undergo or to show metamorphosis in its
development. Metamorphosis is one of the most interest-
ing features in the life history or development of animals,
and it can be, at least as far as its external aspects are con-
cerned, very readily observed and studied.

57. Metamorphosis among insects.— All the butterflies and
moths show metamorphosis in their development. So do
many other insects, as the ants, bees, and wasps, and all the
flies and beetles. On the other hand, many insects do not
show metamorphosis, but, like the birds, are hatched from
the egg in a condition plainly resembling the parents. A
grasshopper (Fig. 42) is a convenient example of an insect
without metamorphosis, or rather, as there are, after all,
a few easily perceived changes in its post-embryonic devel-
opment, of an insect with an “incomplete metamorpho-
sis.” The eggs of grasshoppers are laid in little packets
of several score half an inch below the surface of the
ground. When the young grasshopper hatches from the
egg it is of course very small, but it is plainly recognizable
as a grasshopper. But in one important character it dif-
fers from the adult, and that is in its lack of wings. The
adult grasshopper has two pairs of wings; the just hatched
young or larval grasshopper has no wings at all. The
young grasshopper feeds voraciously and grows rapidly.

THE LIFE CYCLE 91

In a few days it molts, or casts its outer skin (not the
true skin, but a thin, firm covering or outer body wall com-
posed of a substance called chitin, which is secreted by the
cells of the true skin). In this second larval stage there
can be seen the rudiments of four wings, in the condition

2s Ps . x x b Vie Z Oe sh 7}
wey |
RET ONG TILE

AIGOO

Fig. 42.—Post-embryonic development (incomplete metamorphosis) of the Rocky
Mountain locust (Meélanoplus spretus). a, b, ¢, d, e, and f, successive develop-
mental stages from just hatched to adult individual.—After EMERTON.

of tiny wing pads on the back of the middle part of the
body (the thorax). Soon the chitinous body covering is
shed again, and after this molt the wing pads are mark-
edly larger than before. Still another molt occurs, with
another increase in size of the developing wings, and after
a fifth and last molt the wings are fully developed, and

\

99 ANIMAL LIFE

the grasshopper is no longer in a larval or immature condi-
tion, but is full grown and adult.

For example of complete metamorphosis among insects
we may choose a butterfly, the large red-brown butterfly

Fie. 43.—Metamorphosis of monarch butterfly (Anosia plexippus). a, egg; 5, larva;
¢, pupa; @, imago or adult.

common in the United States and called the monarch or
milkweed butterfly (Anosia plexippus). The eggs (Fig.
43, a) of this butterfly are laid on the leaves of various kinds
of milkweed (Asclepias). The larval butterfly or butterfly
larva or caterpillar (as the first young stage of the butter-

THE LIFE CYCLE 93

flies and moths is usually called), which hatches from the
egg in three or four days, is a creature bearing little or no
resemblance to the beautiful winged imago (the adult but-
terfly). It is worm-like, and instead of having three pairs
of legs like the butterfly it has eight pairs; it has biting
jaws in its mouth with which it nips off bits of the green
milkweed leaves, instead of having a long, slender, sucking
proboscis for drinking flower nectar as the butterfly has.
The body of the crawl-
ing worm-like larva
(Fig. 43, 0) is greenish
yellow in color, with
broad rings or bands of
shining black. It has
no wings, of course. It
eats voraciously, grows
rapidly and molts. But
after the molting there
is no appearance of
rudimentary wings; it
is simply a larger worm-
like larva. It continues
to feed and grow, molt-
ing several times, until
after the fourth molt it
appears no longer as an
active, crawling, feed-
ing, worm-like larva, but as a quiescent, non-feeding pupa
or chrysalis (Fig. 43, c). The immature butterfly is now
greatly contracted, and the outer chitinous wall is very
thick and firm. It is bright green in color with golden dots.
It is fastened by one end to a leaf of the milkweed, where
it hangs immovable for from a few days to two weeks.
Finally, the chitin wall of the chrysalis splits, and there
issues the full-fledged, great, four-winged, red-brown butter-
fly (Fig. 43,d). Truly this is a metamorphosis, and a start-

my

Fig. 44.—Metamorphosis of mosquito (Cwlex).
a, larva; b, pupa.

94 ANIMAL LIFE

ling one. But we know that development in other animals
is a gradual and continuous process, and so it is in the
case of the butterfly.
The gradual chang-
ing is masked by the
outer covering of the
body in both larva
and pupa. It is only
at each molting or
throwing off of this
unchanging, unyield-
ing chitin armor that
we perceive how far
this change has gone.
The longest time of
concealment is that
during the pupal or
chrysalis stage, and
the results of the
changing or develop-
) _ ment when finally re-

eee er eee ee vealed by the split-
Fig. 45.—Larva of a butterfly just changing into ting of the pupal
ye big last larval molt). Photograph case are hence the

most striking.

58. Metamorphosis of the toad. Metamorphosis is found
in the development of numerous other animals, as well as
among the insects. Certain cases are familiar to all—the
metamorphosis of the frogs and toads (Fig. 46). The eggs
of the toad are arranged in long strings or ribbons in a
transparent jelly-like substance. These jelly ribbons with
the small, black, bead-like eggs in them are wound around
the stems of submerged plants or sticks near the shores of
the pond. From each egg hatches a tiny, wriggling tad-
pole, differing nearly as much from a full-grown toad as
a caterpillar differs from a butterfly. The tadpoles feed on

THE LIFE CYCLE 95

the microscopic plants to be found in the water, and swim
easily about by means of the long tail. The very young
tadpoles remain underneath the surface of the water all the
time, breathing the air which is mixed with water by means
of gills. But as they become older and larger they come
often to the surface of the water. Lungs are developing
inside the body, and the tadpole is beginning to breathe as
a land animal, although it still breathes partly by means of
gills, that is, as an aquatic animal. Soon it is apparent that
although the tadpole is steadily and rapidly growing larger,
its tail is growing shorter and smaller instead of larger. At
the same time, fore and hind legs bud out and rapidly take

Fia. 46.—Metamorphosis of the toad (partly after Gage). At left the strings of eggs,
in water the various tadpole or larval stages, and on bank the adult toads.

form and become functional. By the time that the tail
gets very short, indeed, the young toad is ready to leave the
water and live as a land animal. On land the toad lives, as

96 ANIMAL LIFE

we know, on insects and snails and worms. The metamor-
phosis of the toad is not so striking as that of the butter-
fly, but if the tadpole were inclosed in an unchanging
opaque body wall while it was losing its tail and getting its
legs, and this wall were to be shed after these changes were
made, would not the metamorphosis be nearly as extraordi-

Fie. 47.—Metamorphosis of sea-
urchin. Upper figure the adult,
lower figure the pluteus larva.

nary as in the case of
the butterfly? But in
the metamorphosis of
the toad we can see the
gradual and continuous
character of the change.

59. Metamorphosis among other animals——Many other
animals, besides insects and frogs and toads, undergo meta-
morphosis. The just-hatched sea-urchin does not resemble
a fully developed sea-urchin at all. It is a minute worm-
like creature, provided with cilia or vibratile hairs, by means
of which it swims freely about. It changes next into a curi-
ous bootjack-shaped body called the pluteus stage (Fig. 47).
In the pluteus a skeleton of lime is formed, and the final
true sea-urchin body begins to appear inside the pluteus,

THE LIFE CYCLE 97 |

developing and growing by using up the body substance of
the pluteus. Star-fishes, which are closely related to sea-
urchins, show a simi-
lar metamorphosis,
except that there is
no pluteus stage, the
true star-fish-shaped
body forming, with- |
in and at the expense
of the first larval
stage, the ciliated
free-swimming stage.

A young crab just
issued from the egg
(Fig. 48) is a very
different appearing
creature from the
adult or fully devel-
oped crab. The body
of the crab in its
first larval stage is
composed of a short,
globular portion, fur-
nished with conspicuous long spines and a relatively long,
jointed tail. This is called the zoéa stage. The zoéa
changes into a stage called the megalops, which has many
characteristics of the adult crab condition, but differs espe-
cially from it in the possession of a long, segmented tail,
and in having the front half of the body longer than wide.
The crab in the megalops stage looks very much like a
tiny lobster or shrimp, The tail soon disappears and the
body widens, and the final stage is reached.

In many families of fishes the changes which take place
in the course of the life cycle are almost as great as in the
case of the insect or the toad. In the lady-fish (Albula
vulpes) the very young (Fig. 49) are ribbon-like in form,

8

Fie. 48.—Metamorphosis of the crab. a, the zoéa
stage ; 6, the megalops; c, the adult.

98 - ANIMAL LIFE

with small heads and very loose texture of the tissues, the
body substance being jelly-like and transparent. As the fish
grows older the body becomes more compact, and therefore

4

Fie. 49.—Stages in the post-embryonic development of the lady-fish (Albula vulpes),
showing metamorphosis. —After C. H. GinBERT.

shorter and slimmer. After shrinking to the texture of an
ordinary fish, its growth in size begins normally, although

THE LIFE CYCLE 99

it has steadily increased in actual weight. Many herring,
eels, and other soft-bodied fishes pass through stages simi-
lar to those seen in the lady-fish. Another type of devel-
opment is illustrated in the sword-fish. The young has a
bony head, bristling with spines. As it grows older the
spines disappear, the skin grows smoother, and, finally, the
‘bones of the upper jaw grow together, forming a prolonged
sword, the teeth are lost and the fins become greatly modi-
fied. Fig. 50 shows three of these stages of growth. The

Fig. 50.—Three stages in the development of the sword-fish (Xiphias gladius).
a, very young; 0, older; c, adult.—Partly after LUTKEN.

flounder or flat-fish (Fig. 51) when full grown lies flat on
one side when swimming or when resting in the sand on
the bottom of the sea. The eyes are both on the upper
side of the body, and the lower side is blind and colorless.
When the flounder is hatched it is a transparent fish, broad
and flat, swimming vertically in the water, with an eye on
each side. As its development (Fig. 52) goes on it rests
itself obliquely on the bottom, the eye of the lower side
turns upward, and as growth proceeds it passes gradually

100 ANIMAL LIFE

around the forehead, its socket moving with it, until both
eyes and sockets are transferred by twisting of the skull to

Fig. 51.—The wide-eyed flounder (Platophrys lunatus). Adult, showing both eyes on
upper side of head.

the upper side. In some related forms or soles the small
eye passes through the head and not around it, appearing
finally in the same socket with the other eye.

Thus in almost all the great groups of animals we find
certain kinds which show metamorphosis in their post-
embryonic development. But metamorphosis is simply
development; its striking and extraordinary features are
usually due to the fact that the orderly, gradual course of
the development is revealed to us only occasionally, with
the result of giving the impression that the development is
proceeding by leaps and bounds from one strange stage to

Fia. 52.—Development of a flounder (after Emery). The eyes in the young flounder
are arranged normally, one on each side of head.

another. If metamorphosis is carefully studied it loses its
aspect of marvel, although never its great interest.

THE LIFE CYCLE 101

60. Duration of life—After an animal has completed its
development it has but one thing to do to complete its life
cycle, and that is the production of offspring. When it
has laid eggs or given birth to young, it has insured the
beginning of a new life cycle. Does it now die? Is the
business of its life accomplished ? There are many animals
which die immediately or very soon after laying eggs. The
May-flies—ephemeral insects which issue as winged adults
from ponds or lakes in which
they have spent from one to
three years as aquatic crawl-
ing or swimming larve, flutter
about for an evening, mate,
drop their packets of fertil-
ized eggs into the water, and
die before the sunrise — are
extreme examples of the nu-
merous kinds of animals
whose adult life lasts only long
enough for mating and egg-
laying. But elephants live for
two hundred years. Whales
probably live longer. <A horse
lives about forty years, and so
may a cat or toad. A sea-
anemone, which was kept in an aquarium, lived sixty-six
years. Cray-fishes may live twenty years. A queen bee
was kept in captivity for fifteen years. Most birds have
long lives—the small song birds from eight to eighteen
years, and the great eagles and vultures up to a hundred
years or more. On the other hand, among all the thou-
sands of species of insects, the individuals of very few in-
deed live more than a year; the adult life of most insects
being but a few days or weeks, or at best months. Even
among the higher animals, some are very short-lived.
In Japan is a small fish (So/awx) which probably lives

Fia. 53.—Metamorphosis of a barnacle
(Lepas). a, larva; 6, adult.

102 ANIMAL LIFE

but a year, ascending the rivers in numbers when young in
the spring, the whole mass of individuals dying in the fall
after spawning.

Naturalists have sought to discover the reason for these
extraordinary differences in the duration of life of different
animals, and while it can not be said that the reason or
reasons are wholly known, yet the probability is strong that
the duration of life is closely connected with, or dependent
upon, the conditions attending the production of offspring.
It is not sufficient, as we have learned from our study of
the multiplication of animals (Chapter III), that an adult
animal shall produce simply a single new individual of its
kind, or even only a few. It must produce many, or if it
produces comparatively few it must devote great care to
the rearing of these few, if the perpetuation of the species
is to be insured. Now, almost all long-lived animals are
species which produce but few offspring at a time, and
reproduce only at long intervals, while most short-lived ani-
mals produce a great many eggs, and these all at one time.
Birds are long-lived animals; as we know, most of them
lay eggs but once a year, and lay only a few eggs each time.
Many of the sea birds which swarm in countless numbers
on the rocky ocean islets and great sea cliffs lay only a
single egg once each year. And these birds, the guillemots
and murres and auks, are especially long-lived. Insects, on
the contrary, usually produce many eggs, and all of them
inashort time. The May-fly, with its one evening’s lifetime,
lets fall from its body two packets of eggs and then dies.
Thus the shortening of the*period of reproduction with the
production of a great many offspring seem to be always
associated with a short adult lifetime; while a long period
of reproduction with the production of few offspring at a
time and care of the offspring are associated with a long
adult lifetime.

There seems also to be some relation between the size
of animals and the length of life. As a general rule,

THE LIFE CYCLE 103

large animals are long-lived and small animals have short
lives.

61. Death At the end comes death. After the animal
has completed its life cycle, after it has done its share toward
insuring the perpetuation of its species, it dies. It may
meet a violent death, may be killed by accident or by ene-
mies, before the life cycle is completed. And this is the
fate of the vast majority of animals which are born or
hatched. Or death may come before the time for birth or
hatching. Of the millions of eggs laid by a fish, each egg
a new fish in simplest stage of development, how many or
rather how few come to maturity, how few complete the
cycle of life!

Of death we know the essential meaning. Life ceases
and can never be renewed in the body of the dead animal.
It is important that we include the words “can never be
renewed,” for to say simply that “life ceases,” that is, that
the performance of the life processes or functions ceases,
is not really death. It is easy to distinguish in most cases
between life and death, between a live animal and a dead
one, yet there are cases of apparent death or a semblance of
death which are very puzzling. The test of life is usually
taken to be the performance of life functions, the assimila-
tion of food and excretion of waste, the breathing in of oxy-
gen, and breathing out of carbonic-acid gas, movement,
feeling, etc. But some animals can actually suspend all
of these functions, or at least reduce them to such a mini-
mum that they can not be perceived by the strictest exami-
nation, and yet not be dead. That is, they can renew
again the performance of the life processes. Bears and
some other animals, among them many insects, spend the
winter ina state of death-like sleep. Perhaps it is but sleep ;
and yet hibernating insects can be frozen solid and remain
frozen for weeks and months, and still retain the power of
actively living again in the following spring. Even more
remarkable is the case of certain minute animals called o-

104. ANIMAL LIFE

tatoria and of others called Tardigrada, or bear-animalcules.
These bear-animalcules live in water. If the water dries
up, the animalcules dry up too; they shrivel up into form-
less little masses and become completely desiccated. They
are simply dried-up bits of organic matter, they are organic
dust. Now, if after a long time—years even—one of these
organic dust particles, one of these dried-up bear-animal-
cules is put into water, a strange thing happens. The body
swells and stretches out, the skin becomes smooth instead
of all wrinkled and folded, and the legs appear in normal
shape. The body is again as it was years before, and after
a quarter of an hour to several hours (depending on the
length of time the animal has lain dormant and dried) slow
movements of the body parts begin, and soon the animal-
cule crawls about, begins again its life where it had been
interrupted. Various other small animals, such as vinegar
eels and certain Protozoa, show similar powers. Certainly
here is an interesting problem in life and death.

When death comes to one of the animals with which
we are familiar, we are accustomed to think of its coming
to the whole body at some exact moment of time. As we
stand beside a pet which has been fatally injured, we wait
until suddenly we say, “It is dead.” As a matter of fact,
it is difficult to say when death occurs. Long after the
heart ceases to beat, other organs of the body are alive—
that is, are able to perform their special functions. The
muscles can contract for minutes or hours (for a short time
in warm-blooded, for a long time in cold-blooded animals)
after the animal ceases to breathe and its heart to beat.
Even longer live certain cells of the body, especially the
amceboid white blood-corpuscles. These cells, very like
the Ameba in character, live for days after the animal is,
as we say, dead. The cells which line the tracheal tube
leading to the lungs bear cilia or fine hairs which they
wave back and forth. They continue this movement for
days after the heart has ceased beating. Among cold-

THE LIFE CYCLE 108

blooded animals, like snakes and turtles, complete cessa-
tion of life functions comes very slowly, even after the
body has been literally cut to pieces. .

Thus it is essential in defining death to speak of a
complete and permanent cessation of the performance of
the life processes.

awe ee

i aa al

a ee eee

ee A eee
3

a
x

A grasshopper (Melanoplus differentialis) killed by disease caused by a
parasitic fungus. On golden-rod.

CHAPTER VI
THE PRIMARY CONDITIONS OF ANIMAL LIFE

62. Primary conditions and special conditions.—Certain
primary conditions are necessary for the existence of all
animals. We know that fishes can not live very long out
of water, and that birds can not live in water. These,
however, are special conditions which depend on the spe-
cial structure and habits of these two particular kinds of
backboned animals. But the necessity of a constant and
sufficient supply of air is a necessity common to both ; it is
one of the primary conditions of their life. All animals
must have air. Similarly both fishes and birds, and all
other animals as well, must have food. This is another one
of the primary conditions of animal life. That backboned
animals must find somehow a supply of salts or compounds
of lime to form into bones is a special condition peculiar
to these animals. Other animals having shells or teeth
composed of carbonate or phosphate of lime are subject to
the same special demand, but many animals have no hard
parts, and therefore need no lime. a

63. Food.—All the higher plants, those that are green
(chlorophyll-bearing), can make their living substance out
of inorganic matter alone—that is, use inorganic substances
as food. But animals can not do this. They must have
already formed organic matter for food. This organic mat-
ter may be the living or dead tissues of plants, or the living
or dead tissues of animals. For the life of animals it is
necessary that other organisms live, or have lived. It is
this need which primarily distinguishes an animal from a

106

THE PRIMARY CONDITIONS OF ANIMAL LIFE 107

plant. Animals can not exist without plants. The plants
furnish all animals with food, either directly or indirectly.
The amount of food and the kinds of food required by
various kinds of animals are special conditions depending
on the size, the degree of activity, the structural character
of the body, ete., of the animal in question. Those which
do the most need most. Those with warmest blood, great-
est activity, and most rapid change of tissues are most
dependent on abundance, regularity, and fitness of their
food. As we well know, an animal can live for a longer or
shorter time without food. Men have fasted for a month,
or even two months. Among cold-blooded animals, like the
‘ reptiles, the general habit of food taking is that of an occa-
sional gorging, succeeded by a long period of abstinence.
Many of the lower animals can go without food for surpris-
ingly long periods without loss of life. But the continued
lack of food results inevitably in death. Any animal may
be starved in time.

If water be held not to be included in the general con-
ception of food, then special mention must be made of the
necessity of water as one of the primary conditions of ani-
mal life. Protoplasm, the basis of life, is a fluid, although
thick and viscous. To be fluid its components must be
dissolved or suspended in water. In fact, all the truly
living substance in an animal’s body contains water. The
hard parts, as bones and teeth and nails, are not really liv-
ing. The water necessary for the animal may be derived
from the other food, all of which contains water in greater
or less quantity, or may be taken apart from the other food,
by drinking or by absorption through the skin. Sheep are
seldom seen to drink, for they find almost enough water in
their green food. Fur seals never drink, for they absorb
the water needed through pores in the skin.

64. Oxygen.—Animals must have air in order to live,
but the essential element of the air which they need is its
oxygen. For the metabolism of the body, for the chemical

108 ANIMAL LIFE

changes which take place in the body of every living ani-
mal, a supply of oxygen is required. This oxygen is de-
rived directly or indirectly from the air. The atmosphere
of the earth is composed of 79.02 parts of nitrogen (includ-
ing argon), .03 parts of carbonic acid, and 20.95 parts of
oxygen. Thus all the animals which live on land are en-
veloped by a substance containing nearly 21 per cent of
oxygen. But animals can live in an atmosphere containing
much less oxygen. Certain mammals, experimented on,
lived without difficulty in an atmosphere containing only
14 per cent of oxygen; when the oxygen was reduced to 7
per cent serious disturbances were caused in the animal’s
condition, and death by suffocation ensued when 3 per
cent of oxygen was left in the atmosphere. Animals which
live in water get their oxygen, not from the water itself
(water being composed of hydrogen and oxygen), but from
air which is mechanically mixed with the water. Fishes
breathe the air which is mixed with or dissolved in the
water. This scanty supply therefore constitutes their at-
mosphere, for in water from which all air is excluded no
animal can breathe. Whatever the habits of life of the
animal, whether it lives on the land, in the ground, or in
the water, it must have oxygen or die.

65. Temperature, pressure, and other conditions—Some
physiologists include among the primary or essential gen-
eral conditions of animal life such conditions as favorable
temperature and favorable pressure. It is known from ob-
servation and experiment that animals die when a too low
or a too high temperature prevails. The minimum or
maximum of temperature between which limits an animal
can live varies much among different kinds of animals. It
is familiar knowledge that many kinds of animals can be
frozen and yet not be killed. Insects and other small ani-
mals may lie frozen through a winter and resume active
life again in the spring. An experimenter kept certain
fish frozen in blocks of ice at a temperature of —15° C.

THE PRIMARY CONDITIONS OF ANIMAL LIFE 109

for some time and then gradually thawed them out un-
hurt. Only very hardy kinds adapted to the cold would,
however, survive such treatment. There is no doubt that
every part of the body, all of the living substance, of these
fish was frozen, for specimens at this temperature could be
broken and pounded up into fine ice powder. But a tem-
perature of —20° C. killed the fish. Frogs lived after being
kept at a temperature of —28° C., centipeds at —50° C., and
certain snails endured a temperature of —120° C. without
dying. At the other extreme, instances are known of ani-
mals living in water (hot springs or water gradually heated
with the organisms in it) of a temperature as high as 50° C.
Experiments with Amebe show that these simplest animals
contract and cease active motion at 35° C., but are not killed
until a temperature of 40° to 45° C. is reached. The little
fish called blob or miller’s thumb (Cottus ictalops) has been
seen lying boiled in the bottom of the hot springs in the
Yellowstone Park; but it must have entered these springs
through streams of a temperature little below the boiling
point.

The pressure or weight of the atmosphere on the sur-
face of the earth is nearly fifteen pounds on each square
inch. This pressure is exerted equally in all directions, so
that an object on the earth’s surface sustains a pressure on
each square inch of its surface exposed to the air of fifteen
pounds. Thus all animals living on the earth’s surface or
near it live under this pressure, and know no other condi-
tion. For this reason they do not notice it. The animals
which live in water, however, sustain a much greater pres-
sure, this pressure increasing with the depth. Certain
ocean fishes live habitually at great depths, as two to five
miles, where the pressure is equivalent to that of many
hundred atmospheres. If these fishes are brought to the
surface their eyes bulge out fearfully, being pushed out
through reduced expansion; their scales fall off because of
the great expansion of the skin, and the stomach is pushed

110 ANIMAL LIFE

out from the mouth till it is wrong side out. Indeed, the
bodies sometimes burst. Their bodies are accustomed to
this great pressure, and when this outside pressure is sud-
denly removed the body may be bursted. Sometimes
such a fish is raised from its proper level by a struggle
with its prey, when both captor and victim may be de-
stroyed by the expansion of the body. Some fishes die on
being taken out of water through the swelling of the air
bladder and the bursting of its blood-vessels. If an animal
which lives normally on the surface of the earth is taken
up a very high mountain or is carried up in a balloon to a
great altitude where the pressure of the atmosphere is
much less than it is at the earth’s surface, serious conse-
quences may ensue, and if too high an altitude is reached
death occurs. This death may be in part due to the diffi-
culty in breathing in sufficient oxygen to maintain life, but
it is probably chiefly due to disturbances caused by the
removal of the pressure to which the body is accustomed
and is structurally adapted to withstand. A famous bal-
loon ascension was made in Paris in 1875 by three men.
After the balloon had reached a height of nearly 24,000
feet (almost five miles) the men began to lose conscious-
ness. On the sinking of the balloon to about 20,000 feet
the men regained consciousness again and threw out bal-
last so that the balloon rose to a height of over 25,000 feet.
This time all three became wholly unconscious, and on the
balloon sinking again only one regained consciousness.
The other two died in the foolhardy experiment. All liv-
ing animals are accustomed to live under a certain pres-
sure, and there are evidently limits of maximum or mini-
mum pressure beyond which no animal at present existing
can go and remain alive.

But in the case both of temperature and pressure con-
ditions it is easy to conceive that animals might exist which
could live under temperature and pressure conditions not
included between the minimum and maximum limits of each

THE PRIMARY CONDITIONS OF ANIMAL LIFE 111

as determined by animals so existing. But it is impossible
to conceive of animals which could live without oxygen or
without organic food. The necessities of oxygen and organic
food (and water) are the primary or essential conditions
for the existence of any animals.

Of course, we might include such conditions, among
the primary conditions, as the light and heat of the sun,
the action of gravitation, and other physical conditions
without which existence or life of any kind would be im-
possible on this earth. But we here consider by “ primary
conditions of animal life” rather those necessities of living
animals as opposed to the necessities of living plants.
Neither animals nor plants could exist without the sun,
whence they derive directly or indirectly all their energy.

66. Difference between animals and plants—It is easy to
distinguish between the animal and plant when a butterfly
is fluttering about a blossoming cherry tree or a cow feed-
ing in a field of clover. It is not so easy, if it is, indeed,
possible, to say which is plant and which is animal when
the simplest plants are compared with the simplest ani-
mals. It is almost impossible to so define animals as to
distinguish all of them from all plants, or so to define
plants as to distinguish all of them from all animals.
While most animals have the power of locomotion, some,
like the sponges and polyps and barnacles and numerous
parasites, are fixed. While most plants are fixed, some of
the low aquatic forms have the power of spontaneous loco-
motion, and all plants have some power of motion, as espe-
cially exemplified in the revolution of the apex of the
growing stem and root, and the spiral twisting of tendrils,
and in the sudden closing of the leaves of the sensitive
plant when touched. Among the green or chlorophyll-
bearing plants the food consists chiefly of inorganic sub-
stances, especially of carbon which is taken from the car-
bonic-acid gas in the atmosphere, and of water. But some
green-leayed plants feed also in part on organic food.

112 ANIMAL LIFE

Such are the pitcher-plants and sun-dews, and Venus-fly-
traps, which catch insects and use them for food nutrition.
But there are many plants, the fungi, which are not green
—that is, which do not possess chlorophyll, the substance
on which seems to depend the power to make organic
matter out of inorganic substances. These plants feed on
organic matter as animals do. The cells of plants (in their
young stages, at least) have a wall composed of a peculiar
carbohydrate substance called cellulose, and this cellulose
was for a long time believed not to occur in the body of
animals. But now it is known that certain sea-squirts
(Tunicata) possess cellulose. It is impossible to find any
set of characteristics, or even any one characteristic, which
is possessed only by plants or only by animals. But nearly
all of the many-celled plants and animals may be easily .
distinguished by their general characteristics. The power
of breaking up carbonic-acid gas into carbon and oxygen
and assimilating the carbon thus obtained, the presence of
chlorophyll, and the cell walls formed of cellulose, are char-
acteristics constant in all typical plants. In addition, the
fixed life of plants, and their general use of inorganic sub-
stances for food instead of organic, are characteristics
readily observed and practically characteristic of many-
celled plants. When the thousands of kinds of one-celled
organisms are compared, however, it is often a matter of
great difficulty or of real impossibility to say whether a
given organism should be assigned to the plant kingdom
or to the animal kingdom. In general the distinctive
characters of plants are grouped around the loss of the
power of locomotion and related to or dependent upon it.
67. Living organic matter and inorganic matter.—It would
seem to be an easy matter to distinguish an organism—that
is, a living animal or plant—from an inorganic substance. It
is easy to distinguish a dove or a sunflower from stone, and
practically there never is any difficulty in making such dis-
tinctions. But when we try to define living organic matter,

THE PRIMARY CONDITIONS OF ANIMAL LIFE 113

and to describe those characteristics which are peculiar to
it, which absolutely distinguish it from inorganic matter,
we meet with some difficulties. At least many of the char-
acteristics commonly ascribed to organisms, as peculiar to
them, are not so. The possession of organs, or the composi-
tion of the body of distinct parts, each with a distinct func-
tion, but all working together, and de pending on each other,
is as true of a steam-engine as of a horse. That the work
done by the steam-engine depends upon fuel is true; but
so it is that the work done by the horse depends upon fuel,
or food as we call it in the case of the animal. The oxida-
tion or burning of this fuel in the engine is wholly compar-
able with the oxidation of the food, or the muscle and fat
it is turned into, in the horse’s body. The composition of
the bodies of animals and plants of tiny structural units,
the cells, is in many ways comparable with the composition
of some rocks of tiny structural units, the crystals. But
not to carry such rather quibbling comparisons too far, it
may be said that organisms are distinguished from organic
substances by the following characteristics : Organization ;
the power to make over inorganic substances into organic
matter, or the changing of organic matter of one kind, as
plant matter, into another kind, as animal matter; motion,
the power of spontaneous movement in response to stimuli;
sensation, the power of being sensible of external stimuli;
reproduction, the power of producing new beings like them-
selves; and adaptation, the power of responding to external
conditions in a way useful to the organism. Through adap-
tation organisms continue to exist despite the changing of
conditions. If the conditions surrounding an inorganic
body change, even gradually, the inorganic body does not
change to adapt itself to these conditions, but resists them
until no longer able to do so, when it loses its identity or
integrity.

CHAPTER VII

THE CROWD OF ANIMALS AND THE STRUGGLE FOR
EXISTENCE

68. The crowd of animals,—All animals feed upon living
organisms, or on their dead bodies. Hence each animal
throughout its life is busy with the destruction of other
organisms, or with their removal after death. If those
creatures upon which others feed are to hold their own, there
must be enough born or developed to make good the drain
upon their numbers. If the plants did not fill up their
ranks and make good their losses, the animals that feed
on them would perish. If the plant-eating animals were
destroyed, the flesh-eating animals would in turn disappear.
But, fortunately, there is a vast excess in the process of
reproduction. More plants sprout than can find room to
grow. More animals are born than can possibly survive.
The process of increase among animals is correctly spoken
of as multiplication. Each species tends to increase in
geometric ratio, but as it multiplies its members it finds
the world already crowded with other species doing the
same thing. A single pair of any species whatsoever, if not
restrained by adverse conditions, would soon increase to
such an extent as to fill the whole world with its progeny.
An annual plant producing two seeds only would have
1,048,576 descendants at the end of twenty-one years, if
each seed sprouted and matured. The ratio of increase is
therefore a matter of minor importance. It is the ratio of
net increase above loss which determines the fate of a spe-
cies. Those species increase in numbers whose gain exceeds

114

THE STRUGGLE FOR EXISTENCE 115

the death rate, and those which “live beyond their means ”
must sooner or later disappear. One of the most abundant
of birds is the fulmar petrel, which lays but one egg yearly.
It has but few enemies, and this low rate of increase suf-
fices to cover the seas within its range with petrels.

It is difficult to realize the inordinate numbers in which
each species would exist were it not for the checks produced
by the presence of other animals. Certain Protozoa at their
normal rate of increase, if none were devoured or destroyed,
might fill the entire ocean in about a week. The conger-
ecl lays, it is said, 15,000,000 eggs. If each egg grew
up to maturity and reproduced itself in the same way in
less than ten years the sea would be solidly full of conger-
eels. If the eggs of a common house-fly should develop, and
each of its progeny should find the food and temperature it
needed, with no loss and no destruction, the people of a city in
which this might happen could not get away soon enough to
escape suffocation from a plague of flies. Whenever any in-
sect is able to develop a large percentage of the eggs laid, it
becomes at once a plague. Thus originate plagues of grass-
hoppers, locusts, and caterpillars. But the crowd of life is
such that no great danger exists. The scavenger destroys
the decaying flesh where the fly would lay its eggs. Minute
creatures, insects, bacteria, Protozoa are parasitic within
the larva and kill it. Millions of flies perish for want of
food. Millions more are destroyed by insectivorous birds,
and millions are slain by parasites. The final result is that
from year to year the number of flies does not increase.
Linneus once said that “three flies would devour a dead
horse as quickly as a lion.” Equally soon would it be de-
voured by three bacteria, for the decay of the horse is due
to the decomposition of its flesh by these microscopic plants
which feed upon it. “ Even slow-breeding man,” says Dar-
win, “has doubled in twenty-five years. At this rate in less
than a thousand years there would literally not be standing
room for his progeny. The elephant is reckoned the slow-

116 ANIMAL LIFE

est breeder of all known animals. It begins breeding when
thirty years old and goes on breeding until ninety years
old, bringing forth six young in the interval, and surviving
till a hundred years old. If this be so, after about eight
hundred years there would be 19,000,000 elephants alive,
descended from the first pair.” A few years more of the
‘unchecked multiplication of the elephant and every foot of
land on the earth would be covered by them.

Yet the number of elephants does not increase. In gen-
eral, the numbers of every species of animal in the state of
Nature remain about stationary. Under the influence of
man most of them slowly diminish. There are about as
many squirrels in the forest one year as another, about as
many butterflies in the field, about as many frogs in the
pond. Wolves, bears, deer, wild ducks, singing birds, fishes,
tend to grow fewer and fewer in inhabited regions, because
the losses from the hand of man are added to the losses in
the state of Nature.

It has been shown that at the normal rate in increase of
English sparrows, if none were to die save of old age, it
would take but twenty years to give one sparrow to every
square inch in the State of Indiana. Such an increase is
actually impossible, for more than a hundred other species
of similar birds are disputing the same territory with the
power of increase at a similar rate. There can not be food
and space for all. With such conditions a struggle is set
up between sparrow and sparrow, between sparrow and
other birds, and between sparrow and the conditions of life.
Such a conflict is known as the struggle for existence.

69. The struggle for existence.—The struggle for exist-
ence is threefold: (@) among individuals of one species,
as sparrow and sparrow; (4) between individuals of differ-
ent species, as sparrow with bluebird or robin; and (c) with
the conditions of life, as the effort of the sparrow to keep
warm in winter and to find water in summer. All three
forms of this struggle are constantly operative and with

r
re

THE STRUGGLE FOR EXISTENCE 117

every species. In some regions the one phase may be more
destructive, in others another. Where the conditions of
life are most easy, as in the tropics, the struggle of species
with species, of individual with individual, is the most
severe.

No living being can escape from any of these three
phases of the struggle for existence. For reasons which we
shall see later, it is not well that any should escape, for “ the
sheltered life,” the life withdrawn from the stress of effort,
brings the tendency to degeneration. 3

Because of the destruction resulting from the struggle
for existence, more of every species are born than can
possibly find space or food to mature. The majority fail
to reach their full growth because, for one reason or an-
other, they can not do so. All live who can. Each strives
to feed itself, to save its own life, to protect its young.
But with all their efforts only a portion of each species
succeed.

70. Selection by Nature——But the destruction in Nature
is not indiscriminate. In the long run those least fitted to
resist attack are the first to perish. It is the slowest ani-
mal which is soonest overtaken by those which feed upon
it. It is the weakest which is crowded away from the feed-
ing-place by its associates. It is the least adapted which is
first destroyed by extremes of heat and cold. Just as a
farmer improves his herd of cattle by destroying his weak-
est or roughest calves, reserving the strong and fit for par-
entage, so, on an inconceivably large scale, the forces of
Nature are at work purifying, strengthening, and fitting to
their surroundings the various species of animals. This
process has been called natural selection, or the survival of
the fittest. But by fittest in this sense we mean only best
adapted to the surroundings, for this process, like others in
Nature, has itself no necessarily moral element. The song-
bird becomes through this process more fit for the song-bird
life, the hawk becomes more capable of killing and tear-

118 ANIMAL LIFE

ing, and the woodpecker better fitted to extract grubs from
the tree.

In the struggle of species with species one may gain a
little one year and another the next, the numbers of each
species fluctuating a little with varying circumstances, but
after a time, unless disturbed by the hand of man, a point
will be reached when the loss will almost exactly balance
the increase. This produces a condition of apparent equi-
librium. The equilibrium is broken when any individual or
group of individuals becomes capable of doing something
more than hold its own in the struggle for existence.

When the conditions of life become adverse to the exist-
ence of a species it has three alternatives, or, better, one of
three things happens, namely, migration, adaptation, extinc-
tion. The migration of birds and some other animals is a
systematic changing of environment when conditions are
unfavorable to life. When the snow and ice come, the fur-
seal forsakes the islands on which it breeds, and which are
its real home, and spends the rest of the year in the open
sea, returning at the close of winter. Some other animals
migrate irregularly, removing from place to place as condi-
tions become severe or undesirable. The Rocky Mountain
locusts, which breed on the.great plateau along the eastern
base of the Rocky Mountains, sometimes increase so rapidly
in numbers that they can not find enough food in the scanty
vegetation of this region. Then great hosts of them fly
high into the air until they meet an air current moving
toward the southeast. The locusts are borne by this cur-
rent or wind hundreds of miles, until, when they come to
the great grain-growing Mississippi Valley, they descend
and feed to their hearts’ content, and to the dismay of the
Nebraska and Kansas farmer. These great forced migra-
tions used to occur only too often, but none has taken place
since 1878, and it is probable that none will ever occur
again. With the settlement of the Rocky Mountain plateau
by farmers, food is plenty at home. And the constant fight-

THE STRUGGLE FOR EXISTENCE 119

ing of the locusts by the farmers, by plowing up their eggs,
and crushing and burning the young hoppers, keeps down
their numbers.

Another animal of interesting migratory habits is the
lemming, a mouse-like animal nearly as large as a rat, which
lives in the arctic regions. At intervals varying from five
to twenty years the cultivated lands of Norway and Sweden,
where the lemming is ordinarily unknown, are overrun by
vast numbers of these little animals. They come as an
army, steadily and slowly advancing, always in the same
direction, and “regardless of all obstacles, swimming across
streams and even lakes of several miles in breadth, and
committing considerable devastation on their line of march
by the quantity of food they consume. In their turn they
are pursued and harassed by crowds of beasts and birds of
prey, as bears, wolves, foxes, dogs, wild cats, stoats, weasels,
eagles, hawks, and owls, and never spared by man; even
the domestic animals not usually predaceous, as cattle,
foals, and reindeer, are said to join in the destruction,
stamping them to the ground with their feet and even eat-
ing their bodies. Numbers also die from disease apparently
produced from overcrowding. None ever return by the
course by which they came, and the onward march of the
survivors never ceases until they reach the sea, into which
they plunge, and swimming onward in the same direction
as before perish in the waves.” One of these great migra-
tions lasts for from one to three years. But it always ends
in the total destruction of the migrating army. But the
migration may be of advantage to the lemmings which re-
main in the original breeding grounds, leaving them with
enough food, so that, on the whole, the migration results in
gain to the species.

But most animals can not migrate to their betterment.
In that case the only alternatives are adaptation or destruc-
tion. Some individuals by the possession of slight advan-
tageous variations of structure are able to meet the new

120 ANIMAL LIFE

demands and survive, the rest die. The survivors produce
young similarly advantageously different from the general
type, and the adaptation increases with successive genera-
tions.

71. Adjustment to surroundings a result of natural selec-
tion.— To such causes as these we must ascribe the nice
adjustment of each species to its surroundings. Ifa species
or a group of individuals can not adapt itself to its environ-
ment, it will be crowded out by others that can do so. The
former will disappear entirely from the earth, or else will be
limited to surroundings with which it comes into perfect
adjustment. A partial adjustment must with time become
a complete one, for the individuals not adapted will be
exterminated in the struggle for life. In this regard very
small variations may lead to great results. A side issue
apparently of little consequence may determine the fate of
a species. Any advantage, no matter how small, will turn
the scale of life in favor of its possessor and his progeny.
“ Battle within battle,” says a famous naturalist, “ must be
continually recurring, with varying success. Yet in the
long run the forces are soviicely balanced that the face of
Nature remains for a long time uniform, though assuredly
the merest trifle would give the victory to one organic being
over another.”

"2. Artificial selection.—It has been long known that the
nature of a herd or race of animals can be materially altered
by a conscious selection on the part of man of these indi-
viduals which are to become parents. To “weed out” a
herd artificially is to improve its blood. ‘To select for re-
production the swiftest horses, the best milk cows, the most
intelligent dogs, is to raise the standard of the herd or
race in each of these respects by the simple action of hered-
ity. Artificial selection has been called the “ magician’s
wand,” by which the breeder can summon up whatever
animal form he will. If the parentage is chosen to a defi-
nite end, the process of heredity will develop the form

THE STRUGGLE FOR EXISTENCE 121

desired by a force as unchanging as that by which a stream
turns a mill.

From the wild animals about him man has developed
the domestic animals which he finds useful. The dog
which man trains to care for his sheep is developed by
selection from the most tractable progeny of the wolf which
once devoured his flocks. By the process of artificial selec-
tion those individuals that are not useful to man or pleas-
ing to his fancy have been destroyed, and those which con-
tribute to his pleasure or welfare have been preserved and
allowed to reproduce their kind. The various fancy breeds
of pigeons—the carriers, pouters, tumblers, ruff-necks, and
fan-tails—are all the descendants of the wild dove of Eu-
rope (Columba livia). These breeds or races or varieties
have been produced by artificial selection. So it is with
the various breeds of cattle and of hogs and of horses
and dogs.

In this artificial selection new variations are more rap-
idly produced than in Nature by means of intercrossing
different races, and by a more rapid weeding out of un-
favorable—that is, of undesirable—variations. The rapid
production of variations and the careful preservation of
the desirable ones and rigid destruction of undesirable
ones are the means by which many races of domestic ani-
mals are produced. This is artificial selection.

73. Dependence of species on species—There was intro-
duced into California from Australia, on young orange trees,
a few years ago, an insect pest called the cottony cushion
scale (Icerya purchasi). This pest increased in numbers
with extraordinary rapidity, and in four or five years threat-
ened to destroy completely the great orange orchards of
California. Artificial remedies were of little avail. Finally,
an entomologist was sent to Australia to find out if this
scale insect had not some special natural enemy in its
native country. It was found that in Australia a certain
species of lady-bird beetle attacked and fed on the cottony

1292 ANIMAL LIFE

cushion scales and kept them in check. Some of these
lady-birds ( Vedalia cardinalis) were brought to California
and released in a scale-infested orchard. The lady-birds,
having plenty of food, thrived and produced many young.
Soon the lady-birds were in such numbers that numbers of
them could be distributed to other orchards. In two or
three years the Vedalias had become so numerous and
widely distributed that the cottony cushion scales began to
diminish perceptibly, and soon the pest was nearly wiped
out. But with the disappearance of the scales came also a
disappearance of the lady-birds, and it was then discovered
that the Vedalias fed only on cottony cushion scales and
could not live where the scales were not. So now, in order
to have a stock of Vedalias on hand in California it is neces-
sary to keep protected some colonies of the cottony cushion
scale to serve as food. Of course, with the disappearance
of the predaceous lady-birds the scale began to increase
again in various parts of the State, but with the sending of
Vedalias to these localities the scale was again crushed.
How close is the interdependence of these two species !

Similar relations can be traced in every group of ani-
mals. When the salmon cease to run in the Sacramento
River in California the otter which feeds on them takes, it
is said, to robbing the poultry-yards; and the bear, which
also feeds on fish, strikes out for other game, taking fruit
or chickens or bee-hives, whatever he may find.

CHAPTER VIII
ADAPTATIONS

74. Origin of adaptations—The strife for place in the
crowd of animals makes it necessary for each one to adjust
itself to the place it holds. As the individual becomes
fitted to its condition, so must the species as a whole. The
species is therefore made up of individuals that are fitted
or may become fitted for the conditions of life. As the
stress of existence becomes more severe, the individuals fit
to continue the species are chosen more closely. This
choice is the automatic work of the conditions of life, but
it is none the less effective in its operations, and in the
course of centuries it becomes unerring. When conditions
change, the perfection of adaptation in a species may be
the cause of its extinction. If the need of a special fitness
can not be met immediately, the species will disappear.
For example, the native sheep of England have developed
a long wool fitted to protect them in a cool, damp climate.
Such sheep transferred to Cuba died in a short time, leay-
ing no descendants. The warm fleece, so useful in Eng-
land, rendered them wholly unfit for survival in the tropics.
It is one advantage of man, as compared with other forms
of life, that so many of his adaptations are external to his
structure, and can be cast aside when necessity arises.

v5. Classification of adaptations.—The various forms of
adaptations may be roughly divided into five classes, as fol-
lows: (a) food securing, (4) self-protection, (c) rivalry, (d)
defense of young, (¢) surroundings.

The few examples which are given under each class,
128

Fig. 54.—The deep-sea angler (Corynolophus reinhardti), which has a dorsal spine
modified to be a luminous “fishing rod and lure,” attracting lantern-fishes
(Echiostoma and thophora). An extraordinary adaptation for securing food.
(The angler is drawn after a figure of LUTKEN’s.)

124 ANIMAL LIFE

some of them striking, some not especially so, are mostly
chosen from the vertebrates and from the insects, because
these two groups of animals are the groups with which be-
ginning students of zodlogy are likely to be familiar, and
the adaptations referred to are therefore most likely to be
best appreciated. Quite as good and obvious examples could
be selected from any other groups of animals. The student

She 5.

will find good practice in trying to discover examples shown
by the animals with which he may be familiar. That all
or any part of the body structure of any animal can be
called with truth an example of adaptation is plain from
what we know of how the various organs of the animal
body have come to exist. But by giving special attention
to such adaptations as are plainly obvious, beginning stu-

ADAPTATIONS 125

dents may be put in the
way of independent ob-
servation along an ex-
tremely interesting and
attractive line of zodlog-
ical study.

76. Adaptations for
securing food.— For the
purpose of capture of
their prey, some carniv-
orous animals are pro-
vided with strong claws,
sharp teeth, hooked

beaks, and other struc-
tures familiar to us in
the lion, tiger, dog, cat,
owl, and eagle. Insect-
eating mammals
contrivances

have
especially

Fie. 56.—Foot of the bald eagle, show-
ing claws for seizing its prey.

(CHAPMAN.)

Fig. 55.—The brown pelican, showing gular
sac, which it uses in catching and holding
fishes that form its food.

adapted for the catching of insects. The

ant-eater, for example, has a
curious, long sticky tongue
which it thrusts forth from
its cylindrical snout deep
into the recesses of the ant-
hill, bringing it out with its
sticky surface covered with
ants. Animals which feed on
nuts are fitted with strong
teeth or beaks for crack-
ing them. Similar teeth are
found in those fishes which
feed on crabs, snails, or sea-ur-
chins. Those mammals like
the horse and cow, that
feed on plants, have usually

Giraffes feeding.

57.—

‘

Fi

KAY \

ADAPTATIONS 127

broad chisel-like incisor teeth for cutting off the foliage,
and teeth of very similar form are developed in the dif-
ferent groups of plant-
eating fishes. Molar
teeth are found when it

Fie. 58.—Scorpion, showing the special devel-

opment of certain mouth parts (the maxil- | F1G. 59.—Head of mosquito (fe-
lary palpi) as pincer-like organs for grasp- male), showing the piercing
ing prey. At the posterior tip of the. body needle-like mouth parts which
is the poisonous sting. compose the ‘‘ bill.”’

is necessary that the food should be crushed or chewed,
and the sharp canine teeth go with a flesh diet. The
long neck of the giraffe
(Fig. 57) enables it to
browse on the foliage of
trees.

Insects like the leaf-
beetles and the grasshop-
pers, that feed on the
foliage of plants, have a Fig. 60.—The praying-horse (Mantis) with
pair of jaws, broad but fore legs developed as grasping organs.

128 ANIMAL. LIFE

sharply edged, for cutting off bits of leaves and stems.
Those which take only liquid food, as the butterflies and
sucking-bugs, have their mouth parts modified to form a
slender, hollow sucking beak or proboscis, which can be
thrust into a flower nectary,
or into the green tissue of
plants or the flesh of animals,
to suck up nectar or plant sap
or blood, depending on the
special food habits of the in-
sect. The honey-bee has a
very complicated equipment
of mouth parts fitted for tak-
ing either solid food like pol-
len, or liquid food like the
nectar of flowers. The mos-
quito has a “bill” (Fig. 59)
composed of six sharp, slender
needles for piercing and lac-
| erating the flesh, and a long
tubular under lip through
which the blood can flow into
the mouth. Some predaceous
insects, as the praying-horse
(Fig. 60), have their fore legs
developed into formidable
grasping organs for seizing and
holding their prey.
77. Adaptation for self-de-
pg Bee ; fense.—For self-protection, car-
1@. 61.—Acorns put into bark of tree 5 J
by the Californian woodpecker Divorous animals use the same
(AMelanerpes formicivorous bairdii). weapons to defend themselves
—From photograph, Stanford Uni- : .
versity, California. which serve to secure their
prey; but these as well as
other animals may protect themselves in other fashions.
Most of the hoofed animals are provided with horns, struc-

ADAPTATIONS 129

4. ae
a

Fie. 62.—Section of bark of live oak tree with acorns placed in it by the Californian

woodpecker (Melanerpes formicivorous bairdii).—From photograph, Stanford
University, California.

tures useless in procuring food but often of great effective-
ness as weapons of defense. T'o the category of structures
useful for self-defense belong the many peculiarities of col-

oration known as “recognition marks.” These are marks,
10

130 ANIMAL LIFE

not otherwise useful, which are supposed to enable mem-
bers of any one species to recognize their own kind among
the mass of animal life. To this category belongs the
black tip of the weasel’s tail, which re-
mains the same whatever the changes
in the outer fur. Another example is
seen in the white outer feathers of the
tail of the meadow-lark as well as in
certain sparrows and warblers. The
white on the skunk’s back and _ tail
serves the same purpose and also as a
warning. It is to the skunk’s advan-
tage not to be hidden, for to be seen in
the crowd of animals is to be avoided
by them. The songs of birds and the
calls of various creatures serve also as
recognition marks. Each species knows
and heeds its own characteristic song
or cry,,and it is a source of mutual
Fic. 63.—Centipea. The Protection. The fur-seal pup knows
foremost pair of legsis its mother’s call, even though ten thou-
aint kad eee sand other mothers are calling on the
gans. An adaptation rookery.
ee ari <a The ways in which animals make
themselves disagreeable or dangerous
to their captors are almost as varied as the animals them-
selves. Besides the teeth, claws, and horns of ordinary
attack and defense, we find among the mammals many
special structures or contrivances which serve for de-
fense through making their possession unpleasant. The
scent glands of the skunk and its relatives are noticed
above. ‘The porcupine has the bristles in its fur specialized
as quills, barbed and detachable. These quills fill the
mouth of an attacking fox or wolf, and serve well the pur-
pose of defense. The hedgehog of Europe, an animal of
different nature, being related rather to the mole than to

a a

ADAPTATIONS 131

the squirrel, has a similar armature of quills. The armadillo
of the tropics has movable shields, and when it withdraws its

7
ae

ad
“.

Fie. 64.—Flying fishes. (The upper one a species of Cypselurus, the lower of Haoce-
tus.) These fishes escape from their enemies by leaping into the air and sailing
or “flying” long distances.

head (which is also defended by a bony shield) it is as well
protected as a turtle.

4

As
Lf Pa Fa

e aes

Fig. 65.—The horned toad (Phrynosoma blainvillei). The spiny covering repels many
enemies.

_ Special organs for defense of this nature are rare among
birds, but numerous among reptiles. The turtles are all

132 ANIMAL LIFE

protected by bony shields, and some of them, the box-tur-
tles, may close their shields almost hermetically. The
snakes broaden their heads, swell their necks, or show their
forked tongues to frighten their enemies. Some of them

Fig. 66.—Nokee or poisonous scorpion-fish (Hmmydrichthys vulcanus) with poison-
ous pints from Tahiti.

are further armed with fangs connected with a venom gland,
so that to most animals their bite is deadly. Besides its
fangs the rattlesnake has a rattle on the tail made up of a

Fie. 67.—Mad tom (Schilbcodes furiosus) with poisoned pectoral spine.

succession of bony clappers, modified vertebra, and scales,
by which intruders are warned of their presence. This
sharp and insistent buzz is a warning to animals of other
species and a recognition signal to those of its own kind.

ADAPTATIONS 133

Even the fishes have many modes of self-defense through
giving pain or injury to those who would swallow them.
The cat-fishes or horned pouts when attacked set immoy-
ably the sharp spine of the
pectoral fin, inflicting a
jagged wound. Pelicans
who have swallowed a cat-
fish have been known to
die of the wounds inflicted
by the fish’s spine. In
the group of scorpion-
fishes and toad-fishes are
certain genera in which
these spines are provided
with poison glands. These
may inflict very severe
wounds to other fishes, or
even to birds or man. One of this group
of poison-fishes is the nokee (Hmmydrich-
thys, Fig. 66). A group of small fresh-
water cat-fishes, known as the mad toms
(Fig. 67), have also a poison gland attached
to the pectoral spine, and its sting is most
exasperating, like the sting of a wasp.
The sting-rays (Fig. 68) of many species Fie. 68—A sting-ray
have a strong, jagged spine on the tail, (/ephus gooaed,
covered with slime, and armed with broad
saw-like teeth. This inflicts a dangerous wound, not
through the presence of specific venom, but from the dan-
ger of blood poisoning arising from the slime, and the
ragged or unclean cut.

Many fishes are defended by.a coat of mail or a coat of
sharp thorns. The globe-fishes and porcupine-fishes (Fig.
69) are for the most part defended by spines, but their
instinct to swallow air gives them an additional safeguard.
When one of these fishes is disturbed it rises to the surface,

134 ANIMAL LIFE

gulps air until its capacious stomach is filled, and then
floats belly upward on the surface. It is thus protected
from other fishes, though easily taken by man. The torpe-
do, electric eel, electric cat-fish, and star-gazer, surprise and
stagger their captors by
eat ete means of electric shocks.

In the torpedo or electric
' ray (Fig. 70), found on
the sandy shores of all
warm seas, on either side
of the head is a large
honeycomb-like structure
which yields a_ strong
electric shock whenever
the live fish is touched.
This shock is felt severe-
ly if the fish be stabbed
with a knife or metallic
spear. The electric eel
of the rivers of Para-
guay and southern Bra-
zil is said to give severe
shocks to herds of wild
horses driven through
the streams, and similar
== accounts are given of the
=————— | electric cat-fish of the

= Nile.
Fig. 69.—Porcupine-fish (Diodon hystrix), the .
lower ones swimming normally, the upper Among the insects,

one floating belly upward, with inflated the possession of stings
parte a from specimens from the is not uncommon. The
wasps and bees are fa-

miliar examples of stinging insects, but many other kinds,
less familiar, are similarly protected. All insects have
their bodies covered with a coat of armor, composed of a
horny substance called chitin. In some cases this chitin-

ADAPTATIONS 135

ous coat is very thick and serves to protect them effectu-
ally. This is especially true of the beetles. Some insects
are inedible (as mentioned in Chapter XII), and are con-
spicuously colored so as to be readily recognized by in-
sectivorous birds. The birds, knowing by experience that
these insects are ill-tasting, avoid them. Others are ef-
fectively concealed from their enemies by their close
resemblance in color and marking to their surroundings.
These protective resem-
blances are discussed in
Chapter XII.

78. Adaptation for rivalry.
—In questions of attack and
defense, the need of meeting
animals of their own kind as
well as animals of other
races must be considered. In
struggles of species with
those of their own kind, the
term rivalry may be applied.
Actual warfare is confined
mainly to males in the breed-
ing season and to polyga-
mous animals. Among those
in which the male mates
with many females, he must
struggle with other males for
their possession. In all the
groups of vertebrates the
sexes are about equal in num- F's. 70.—Torpedo or electric ray (War-
bers: . Where mating exists, sco eine tpaaares showing electric
either for the season or for —
life, this condition does not involve serious struggle or
destructive rivalry.

Among monogamous birds, or those which pair, the
male courts the female of his choice by song and by display

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ADAPTATIONS 137

of his bright feathers. 'The female consents to be chosen
by the one which pleases her. It is believed that the hand-
somest, most vivacious, and most musical males are the
ones most successful in such courtship. With polygamous
animals there is intense rivalry among the males in the
mating season, which in almost all species is in the spring.
The strongest males survive and reproduce their strength.
The most notable adaptation is seen in the superior size
of teeth, horns, mane, or spurs. Among the polygamous
fur seals (Fig. 71) and sea lions the male is about four times

Fie. 72.—The Arctic black duck. Male, female, and precocial young.

the size of the female. In the polygamous family of deer,
buffalo, and the domestic cattle and sheep, the male is larger
and more powerfully armed than the female. In the polyg-
amous group to which the hen, turkey, and peacock belong
the males possess the display of plumage, and the structures
adapted for fighting, with the will to use them.

79. Adaptations for the defense of the young.—The pro-
tection of the young is the source of many adaptive struc-
tures as well as of the instincts by which such structures are

138 ANIMAL LIFE

utilized. In general, those animals are highest in develop-
ment, with best means of holding their own in the struggle

Pe ee : _ SCorler Bear
Fie. 73.—The altricial nestlings of the Canada jay (Perisoreus canadensis).

for life, that take best care of their young. The homes
of animals are elsewhere specially discussed (see Chapter

ADAPTATIONS 139

XV), but those instincts which lead to home-building
may all be regarded as useful adaptations in preserving the
young. Among the lower or more coarsely organized

gees
ee ‘|

Fie. 74.—Kangaroo (Macropus rufus) with young in pouch,

140 ‘* ANIMAL LIFE

birds, such as the chicken, the duck, and the auk, as with ~
the reptiles, the young animal is hatched with well-devel-
oped muscular system and sense
organs, and is capable of running
about, and, to some extent, of feed-
ing itself. Birds of this type are
\ known as precocial (Fig. 72), while
\ the name altricial (Fig. 73) is ap-
\\ plied to the more highly organized
forms, such as the thrushes, doves,
and song-birds generally. With
these the young are hatched in a-
wholly helpless condition, with in-
effective muscles, deficient senses,
and dependent wholly upon the
parent. The altricial condition de-
mands the building of a nest, the
2 ___ estabiishment of a home, and the
aan ee (Raja binoon continued care of one or both of
lata) cut open to show young the parents.

rally gt one end of the cae) Lhe very lowest mammals known,
the duck-bills (Monotremes) of

Australia, lay large eggs in a horny shell like those of a
turtle, and guard them with great jealousy. But with
almost all mammals the egg is very small and without
much food-yolk. The egg begins its development within
the body. It is nourished by the
blood of the mother, and after birth
the young is cherished by her, and
fed by milk secreted by specialized
glands of the skin. All these features
are adaptations tending toward the
preservation of the young. In the
division of mammals next lowest to the Monotremes—the
kangaroo, opossum, etc.—the young are born in a very im-
mature state and are at once seized by the mother and

Fie. 76.—Egg-case of the cock-
roach.

ADAPTATIONS. 141.
thrust into a pouch or fold of skin along the abdomen,
where they are kept until they are able to take care of
themselves (Fig. 74). This is an interesting and ingenious
adaptation, but less specialized and
less perfect an adaptation than the
conditions found in ordinary mam-
mals.

Among the insects, the special
provisions for the protection and
care of the eggs and the young are
wide-spread and various. Some of
those adaptations which take the
special form of nests or “homes”
will be described in a later chapter ,
(see Chapter XV). The eggs of Fic. 77.—Giant water-bug (Ser-
the common cockroach are laid in 72%), Male carrying cess
small packets inclosed in a firm wall
(Fig. 76). The eggs of the great water-bugs are carried on
the back of the male (Fig. 77); and the spiders lay their
eggs in a silken sac or cocoon, and some of the ground or

Fie. 78.—Cocoon inclosing the pupa of the great Cecropia moth. Spun of silk by the
larva before pupation.

running spiders (Lycoside) drag this egg-sac, attached to
the tip of the abdomen, about with them. The young
spiders when hatched live for some days inside this sac,
feeding on each other! Many insects have long, sharp,

142 - ANIMAL LIFE

piercing ovipositors, by means of which the eggs are de-
posited in the ground or in the leaves or stems of green
plants, or even in the hard wood of tree-trunks. Some of

Fie. 79.—The cottony cushion scale
insect (lcerya purchasi), from
California. The male is winged,
the female wingless and with a
large waxen egg-sac (e.s.) attached
to her body. (The lines at the left
of each figure indicate the size of
the insects.)

the scale insects se-
crete wax from their
bodies and form a
large, often beautiful
egg-case, attached to
and nearly covering the body in
which eggs are deposited (Fig.
79). The various gall insects lay
their eggs in the soft tissue of
plants, and on the hatching of
the larve an abnormal growth
of the plant occurs about the
young insect, forming an in-
closing gall that serves not only
to protect the insect within,
but to furnish it with an abun-
dance of plant-sap, its food. The
young insect remains in the gall
until it completes its develop-
ment and growth, when it

gnaws its way out. Such insect galls are especially abun-

dant on oak trees (Fig. 80).

The care of the eggs and the

young of the social insects, as the bees and ants, are de-

scribed in Chapter IX.

ADAPTATIONS 143

80. Adaptations concerned with surroundings in life—A
large part of the life of the animal is a struggle with the
environment itself; in this struggle only those that are
adapted live and leave descendants fitted like themselves.
The fur of mammals fits them to their surroundings. As
the fur differs, so may the habits change. Some animals
are active in winter; others, as the bear, hibernate, sleep-
ing in caves or hollow trees or in burrows until conditions
are favorable for their activity. Most snakes and lizards
hibernate in cold weather. In the swamps of Louisiana,

Fie. 80.—The giant gall of the white oak (California), made by the gall insect Andri-
cus californicus. The gall at the right cut open to show tunnels made by the
insects in escaping from the gall.—From photograph.

in winter, the bottom may often be seen covered with water
snakes lying as inert as dead twigs. Usually, however,
hibernation is accompanied by concealment. Some animals
in hibernation may be frozen alive without apparent injury.
The blackfish of the Alaska swamps, fed to dogs when
frozen solid, has been known to revive in the heat of the
dog’s stomach and to wriggle out and escape. As animals
resist heat and cold by adaptations of structure or habits,
so may they resist dryness, Certain fishes hold reservoirs

144 ANIMAL LIFE

of water above their gills, by means of which they can
breathe during short excursions from the water. Still
others (mud-fishes) retain the primitive lung-like structure
of the swim-bladder, and are able to breathe air when, in the
dry season, the water of the pools is reduced to mud.

Another series of adaptations is concerned with the
places chosen by animals for their homes. The fishes that
live in water have special organs for
breathing under water (Fig. 82).
Many of the South American mon-
keys have the tip of the tail adapted
for clinging to limbs of trees or to
the bodies of other monkeys of its
own kind. The hooked claws of the
bat hold on to rocks, the bricks of
chimneys, or to the surface of hollow
trees where the bat sleeps through
the day. The tree-frogs (Fig. 83) or
tree-toads have the tips of the toes
swollen, forming little pads by which
they cling to the bark of trees.

Among other adaptations relat-
ing to special surroundings or con-
ditions of life are the great cheek
pouches of the pocket gophers,
which carry off the soil dug up by
the large shovel-like feet when the
gopher excavates its burrow.

Those insects which live under-
ground, making burrows or tunnels”
in the soil, have their legs or other parts adapted for dig-
ging and burrowing. The mole cricket (Fig. 84) has its
legs stout and short, with broad, shovel-like feet. Some
water-beetles (Fig. 85) and water-bugs have one or more of
the pairs of legs flattened and broad to serve as oars or pad.
dles for swimming. The grasshoppers or locusts, who leap,

Fig. 81.—Insect galls on leaf.

ADAPTATIONS 145

_ have their hind legs greatly enlarged and elon-
gated, and provided with strong muscles, so as
to make of them “leaping legs.” The grubs

Fig. 82.—Head of rainbow trout (Salmo irideus) with gill cover bent back to show
gills, the breathing organs.

or ibe of beetles which live as “borers” in tree-trunks

have mere rudiments of legs, or none at all (Fig. 86).

They have great, strong, biting jaws for cutting away

the hard wood. They move simply by wriggling along

in their burrows or tunnels.
Insects that live

in water either come

up to the surface to

breathe or take down

air underneath their

wings, or in some

other way, or have

gills for breathing the

air which is mixed

with the water. These Fig. 83.—Tree-toad (Hyla regilla).

gills are special adap- :

tive structures which present a great variety of form and

appearance. In the young of the May-flies they are deli-

cate plate-like flaps projecting from the sides of the body.

They are kept in constant motion, gently waving back and
11

146 ANIMAL LIFE

forth in the water so as to maintain currents to bring fresh
water in contact with them. Young mosquitoes (Fig. 87)
do not have gills, but come
up to the surface to breathe.
The larve, or wrigglers,
breathe through a special

Fig. 84.—The mole cricket (Gryllotalpa), Fie. 85.—A water-beetle (Hydroph-
with fore feet modified for digging. ilus).

tube at the posterior tip of the body, while the pupx have
a pair of horn-like tubes on the back of the head end of
the body.

81. Degree of structural change in adaptations.— While
among the higher or vertebrate animals, especially the
fishes and reptiles, most remarkable cases of adaptations
occur, yet the structural changes are for the most part ex-
ternal, never seriously affecting the development of the
internal organs other
than the skeleton. The
organization of these
higher animals is much >)
less plastic than SEROUS iia: 86.—W 0od-boring beetle larva (Prionus).
the invertebrates. In
general, the higher the type the more persistent and un-
changeable are those structures not immediately exposed

ADAPTATIONS 147

to the influence of the struggle for existence. It is thus
the outside of an animal that tells where its ancestors
have lived. The inside, suffering little change, whatever
the surroundings, tells the real nature of the animal.

82. Vestigial organs.—In general, all the peculiarities of
animal structure find their explanation in some need of
adaptation. When this need ceases, the structure itself
tends to disappear or else to serve some other need. In
the bodies of most animals there are certain incomplete
or rudimentary organs
or structures which
serve no distinct use-
ful purpose. They are
structures which, in the
ancestors of the ani-
mals now possessing
them, were fully devel-
oped functional organs,
but which, because of a
change in habits or con-
ditions of living, are of
no further need, and
are gradually dying out.
Such organs are called
vestigial organs. Ex-
amples are the disused
ear muscles of man, the
vermiform appendix in
man, which is the reduced and now useless anterior end
of the large intestine. In the lower animals, the thumb or
degenerate first finger of the bird with its two or three little
quills servesasanexample. So also the reduced and elevated
hind toe of certain birds, the splint bones or rudimentary
side toes of the horse, the rudimentary eyes of blind fishes,
the minute barbel or beard of the horned dace or chub, and
the rudimentary teeth of the right whales and sword-fish.

Fie. 87.—Young stages of the mosquito.
a, larva (wriggler) ; 6, pupa.

148 ANIMAL LIFE

Each of these vestigial organs tells a story of some past
adaptation to conditions, one that is no longer needed in
the life of the species. They have the same place in the
study of animals that silent letters have in the study of
words. For example, in our word knight the & and gh are
no longer sounded; but our ancestors used them both, as
the Germans do to-day in their cognate word Knecht. So
with the French word ¢emps, which means time, in which
both p and s are silent. The Romans, from whom the
French took this word, needed all its letters, for they spelled
and pronounced it ¢empus. In general, every silent letter
in every word was once sounded. In like manner, every
vestigial structure was once in use and helpful or necessary
to the life of the animal which possessed it.

— Saaae xo es mT
Bee
.

ian eS a

Horns of two male elk interlocked while fighting. Permission of G. O. SH1EeLDs,
publisher of Recreation,

CHAPTER IX
ANIMAL COMMUNITIES AND SOCIAL LIFE

83. Man not the only social animal Man is commonly
called the social animal, but he is not the only one to
which this term may be applied. There are many others
which possess a social or communal life. A moment’s
thought brings to mind the familiar facts of the communal
life of the honey-bee and of the ants. And there are many
other kinds of animals, not so well known to us, that live
in communities or colonies, and live a life which in greater
or less degree is communal or social. In this connection
we may use the term communal for the life of those ani-
mals in which the division of labor is such that the indi-
vidual is dependent for its continual existence on the com-
munity as a whole. The term social life would refer to a
lower degree of mutual aid and mutual dependence.

84. The honey-bee.—Honey-bees live together, as we
know, in large communities. We are accustomed to think
of honey-bees as the inhabitants of bee-hives, but there
were bees before there were hives. The “bee-tree” is
familiar to many of us. The bees, in Nature, make their
home in the hollow of some dead or decaying tree-trunk,
and carry on there all the industries which characterize
the busy communities in the hives. A honey-bee com-
munity comprises three kinds of individuals (Fig. 88)—
namely, a fertile female or queen, numerous males or
drones, and many infertile females or workers. These
three kinds of individuals differ in external appearance
sufficiently to be readily recognizable. The workers are

149

150 ANIMAL LIFE

smaller than the queens and drones, and the last two differ
in the shape of the abdomen, or hind body, the abdomen of
the queen being longer and more slender than that of the

Fie. 88.—Honey-bee. a, drone or male; 0, worker or infertile female; c, queen or
fertile female.

male or drone. In a single community there is one queen,
a few hundred drones, and.ten to thirty thousand workers.
The number of drones aud workers varies at different
times of the year, being smallest in winter. Each kind of
individual has certain work or business to do for the whole
community. The queen lays all the eggs from which new
bees are born; that is, she is the mother of the entire
community. The drones or males have simply to act as
royal consorts; upon them depends the fertilization of the
eggs. The workers undertake all the food-getting, the
care of the young bees, the comb-building, the honey-mak-
ing—all the industries with which we are more or less
familiar that are carried on in the hive. And all the
work done by the workers is strictly work for the whole
community; in no case does the worker bee work for itself
alone; it works for itself only in so far as it is a member
of the community.

How varied and elaborately perfected these industries
are may be perceived from a brief account of the life his-
tory of a bee community. The interior of the hollow in
the bee-tree or of the hive is filled with “ comb ”—that is,
with wax molded into hexagonal cells and supports for
these cells. The molding of these thousands of symmet-

ANIMAL COMMUNITIES AND SOCIAL LIFE 151

rical cells is accomplished by the workers by means of their
specially modified trowel-like mandibles or jaws. The wax
itself, of which the cells are made, comes from the bodies

of the workers in the form of small
liquid drops which exude from the skin
on the under side of the abdomen or
hinder body rings. These droplets
run together, harden and become flat-
tened, and are removed from the wax
plates, as the peculiarly modified parts
of the skin which produce the wax
are called, by means of the hind legs,
which are furnished with scissor-like
contrivances for cutting off the wax
(Fig. 89). In certain of the cells are
stored the pollen and honey, which
serve as food for the community. The
pollen is gathered by the workers from
certain favorite flowers and is carried
by them from the flowers to the hive
in the “pollen baskets,” the slightly
concave outer surfaces of one of the
segments of the broadened and flattened
hind legs. This concave surface is lined
on each margin with a row of incurved
stiff hairs which hold the pollen mass
securely in place (Fig. 89). The “ honey ”
is the nectar of flowers which has been
sucked up by the workers by means of
their elaborate lapping and sucking
mouth parts and swallowed into a sort
of honey-sac or stomach, then brought
to the hive and regurgitated into the

Fia. 89.—Posterior leg of
worker honey-bee. The
concave surface of the
upper large joint with
the marginal hairs is
the pollen basket ; the
wax shears are the cut-
ting surfaces of the
angle between the two
large segments of the
leg.

cells. This nectar is at first too watery to be good
honey, so the bees have to evaporate some of this water.
Many of the workers gather above the cells containing

152 ANIMAL LIFE

nectar, and buzz—that is, vibrate their wings violently.
This creates currents of air which pass over the exposed
nectar and increase the evaporation of the water. The
violent buzzing raises the temperature of the bees’ bodies,
and this warmth given off to the air also helps make evap-
oration more rapid. In addition to bringing in food the
workers also bring in, when necessary, “ propolis,” or the
resinous gum of certain trees, which they use in repairing
the hive, as closing up cracks and crevices in it.

In many of the cells there will be found, not pollen or
honey, but the eggs or the young bees in larval or pupal
condition (Fig. 90).
The queen moves
about through the
hive, laying eggs.
She deposits only one
egg in a cell. In
three days the egg
hatches, and the
young bee appears
as a helpless, soft,
white, footless grub
Fig. 90.—Cells containing eggs, larve, and pupe of or larva. It is cared

te nner ee A ous ines freee lt tor ‘by certain of the

workers, that may be
called nurses. These nurses do not differ structurally from
the other workers, but they have the special duty of caring
for the helpless young bees. They do not go out for pollen
or honey, but stay in the hive. They are usually the new
bees—i. e., the youngest or most recently added workers.
After they act as nurses for a week or so they take their
places with the food-gathering workers, and other new
bees act as nurses. The nurses feed the young or larval
bees at first with a highly nutritious food called bee-jelly,
which the nurses make in their stomach, and regurgitate
for the larve.- After the larve are two or three days old

ANIMAL COMMUNITIES AND SOCIAL LIFE 153

they are fed with pollen and honey. Finally, a small mass
of food is put into the cell, and the cell is “capped ” or
covered with wax. The larva, after eating all the food, in
two or three days more changes into a pupa, which lies
quiescent without eating for thirteen days, when it changes
into a full-grown bee. The new bee breaks open the cap
of the cell with its jaws, and comes out into the hive, ready
to take up its share of the work for the community. Ina
few cases, however, the life history is different. The nurses
will tear down several cells around some single one, and
enlarge this inner one into a great irregular vase-shaped
cell. When the egg hatches, the grub or larva is fed bee-
jelly as long as it remains a larva, never being given ordi-
nary pollen and honey at all. This larva finally pupates,
and there issues from the pupa not a worker or drone bee,
but a new queen. The egg from which the queen is pro-
duced is the same as the other eggs, but the worker nurses
by feeding the larva only the highly nutritious bee-jelly
make it certain that the new bee shall become a queen
instead of a worker. It is also to be noted that the male
bees or drones are hatched from eggs that are not ferti-
lized, the queen having it in her power to lay either ferti-
lized or unfertilized eggs. From the fertilized eggs hatch
larve which develop into queens or workers, depending on
the manner of their nourishment; from the unfertilized
eggs hatch the males.

When several queens appear there is much excitement
in the community. Each community has normally a single
one, so that when additional queens appear some rearrange-
ment is necessary. This rearrangement comes about first
by fighting among the queens until only one of the new
queens is left alive. Then the old or mother queen issues
from the hive or tree followed by many of the workers.
She and her followers fly away together, finally alighting
on some tree branch and massing there in a dense swarm.
This is the familiar phenomenon of “swarming.” The

154 ANIMAL LIFE

swarm finally finds a new hollow tree, or in the case of the
hive-bee (Fig. 91) the swarm is put into a new hive, where
the bees build cells, gather food, produce young, and thus

nth, ne)

Fie. 91.—Hiving a swarm of honey-bees. Photograph by 8. J. HUNTER.

found a new community. This swarming is simply an emi-
gration, which results in the wider distribution and in the
increase of the number of the species. It is a peculiar but
effective mode of distributing and perpetuating the species.

There are many other interesting and suggestive things
which might be told of the life in a bee community: how
the community protects itself from the dangers of starva-
tion when food is scarce or winter comes on by killing the
useless drones and the immature bees in egg and larval
stage; how the instinct of home-finding has been so highly
developed that the worker bees go miles away for honey
and nectar, flying with unerring accuracy back to the hive ;
of the extraordinarily nice structural modifications which
adapt the bee so perfectly for its complex and varied busi-
nesses ; and of the tireless persistence of the workers until

ANIMAL COMMUNITIES AND SOCIAL LIFE 155

they fall exhausted and dying in the performance of their
duties. The community, it is important to note, is a per-
sistent or continuous one. The workers do not live long,
the spring broods usually not over two or three months,
and the fall broods not more than six or eight months;
but new ones are hatching while the old ones are dying,
and the community as a whole always persists. The queen
may live several years, perhaps as many as five.* She lays
about one million eggs a year.

85. The ants—There are many species of ants, two
thousand or more, and all of them live in communities and
show a truly communal life. There is much variety of
habit in the lives of different kinds of ants, and the degree
in which the communal or social life is specialized or elab-
orated varies much. But certain general conditions pre-
vail in the life of all the different kinds of individuals—

Fig. 92.—Female (a), male (0), and worker (c) of an ant (Camponotus sp.).

sexually developed males and females that possess wings,
and sexually undeveloped workers that are wingless (Fig.
92). In some kinds the workers show structural differ-

* A queen bee has been kept alive in captivity for fifteen years,

156 ANIMAL LIFE

ences among themselves, being divided into small workers,
large workers, and soldiers. The workers are not, as with
the bees, all infertile females, but they are both male and
female, both being infertile. Although the life of the ant
communities is much less familiar and fully known than
that of the bees, it is even more remarkable in its speciali-
zations and elaborateness. ‘The ant home, or nest, or formi-
cary, is, with most species, a very elaborate underground,
many-storied labyrinth of galleries and chambers. Certain
rooms are used for the storage of food; certain others as
“nurseries ” for the reception and care of the young; and
others as stables for the ants’ cattle, certain plant-lice or
scale-insects which are sometimes collected and cared for by
the ants. The food of ants comprises many kinds of vege-
table and animal substances, but the favorite food, or “ na-
tional dish,” as it has been called, is a sweet fluid which is
produced by certain small insects, the plant-lice (Aphide)
and scale-insects (Coccide). These insects live on the sap
of plants ; rose-bushes are especially favored with their pres-
ence. The worker ants (and we rarely see any ants but
the wingless workers, the winged males and females appear-
ing out of the nest only at mating time) find these honey-
secreting insects, and gently touch or stroke them with their
feelers (antennz), when the plant-lice allows tiny drops of
the honey to issue from the body, which are eagerly drunk
by the ants. It is manifestly to the advantage of the ants
that the plant-lice should thrive; but they are soft-bodied,
defenseless insects, and readily fall a prey to the wander-
ing predaceous insects like the lady-birds and aphis lions.
So the ants often guard small groups of plant-lice, attack-
ing, and driving away the would-be ravagers. When the
branch on which the plant-lice are gets withered and dry,
the ants have been observed to carry the plant-lice care-
fully to a fresh, green branch. In the Mississippi Valley a
certain kind of plant-louse lives on the roots of corn. Its
eggs are deposited in the ground in the autumn and hatch

ANIMAL COMMUNITIES AND SOCIAL LIFE 157

the following spring before the corn is planted. Now, the
common little brown ant lives abundantly in the corn-
fields, and is specially fond of the honey secreted by the
corn-root plant-louse. So, when the plant-lice hatch in the
spring before there are corn roots for them to feed on, the
little brown ants with great solicitude carefully place the
plant-lice on the roots of a certain kind of knotweed which
grows in the field, and protect them until the corn ger-
minates. Then the ants remove the plant-lice to the roots
of the corn, their favorite food plant. In the arid lands of
New Mexico and Arizona the ants rear their scale-insects
on the roots of cactus. Other kinds of ants carry plant-
lice into their nests and provide them with food there.
Because the ants obtain food from the plant-lice and take
care of them, the plant-lice are not inaptly called the ants’
cattle.

Like the honey-bees, the young ants are helpless little
grubs or larve, and are cared for and fed by nurses. The
so-called ants’ eggs, little white, oval masses, which we
often see being carried in the mouths of ants in and out of
an ants’ nest, are not eggs, but are the pupx which are
being brought out to enjoy the warmth and light of the
sun or being taken back into the nest afterward.

In addition to the workers that build the nest and col-
lect food and care for the plant-lice, there is in many
species of ants a kind of individuals called soldiers. These
are wingless, like the workers, and are also, like the work-
ers, not capable of laying or of fertilizing eggs. It is the
business of the soldiers, as their name suggests, to fight.
They protect the community by attacking and driving
away predaceous insects, especially other ants. The ants
are among the most warlike of insects. The soldiers of a
community of one species of ant often sally forth and
attack a community of some other species. If successful
in battle the workers of the victorious community take
possession of the food stores of the conquered and carry

158 ANIMAL LIFE

them to their own nest. Indeed, they go even further ; they
may make slaves of the conquered ants. There are numer-
ous species of the so-called slave-making ants. The slave-
makers carry into their own nest the eggs and larve and
pup of the conquered community, and when these come
to maturity they act as slaves of the victors—that is, they
collect food, build additions to the nests, and care for the
young of the slave-makers. This specialization goes so far
in the case of some kinds of ants, like the robber-ant of
South America (Zciton), that all of the Hciton workers have
become soldiers, which no longer do any work for them-
selves. The whole community lives, therefore, wholly by
pillage or by making slaves of other kinds of ants. There
are four kinds of individuals in a robber-ant community—
winged males, winged females, and small and large wing-
less soldiers. There are many more of the small soldiers
than of the large, and some naturalists believe that the few
latter, which are distinguished by heads and jaws of great
size, act as officers. On the march the small soldiers are
arranged in a long, narrow column, while the large soldiers
are scattered along on either side of the column and appear
to act as sentinels and directors of the army. The obser-
vations made by the famous Swiss students of ants, Huber
and Forel, and by other naturalists, read like fairy tales,
and yet are the well-attested and often reobserved actual
phenomena of the extremely specialized communal and
social life of these animals.

86. Other communal insects—The termites or white ants
(not true ants) are communal insects. Some species of
termites in Africa live in great mounds of earth, often
fifteen feet high. The community comprises hundreds of
thousands of individuals, which are of eight kinds (Fig 93),
viz., sexually active winged males, sexually active winged
females, other fertile males and females which are wingless,
wingless workers of both sexes not capable of reproduc-
tion, and wingless soldiers of both sexes also incapable of

ANIMAL COMMUNITIES AND SOCIAL LIFE 159

reproduction. The production of new individuals is the
sole business of the fertile males and females ; the workers
build the nest and collect food, and the soldiers protect the
community from the attacks of marauding insects. The
egg-laying queen grows to monstrous size, being sometimes

Fic. 93.—Termites. a, queen; 0, male; ¢, worker; d, soldier.

five or six inches long, while the other individuals of the
community are not more than half or three quarters of
an inch long. The great size of the queen is due to the
enormous number of eggs in her body.

The bumble-bees live in communities, but their social
arrangements are very simple ones compared with those of
the honey-bee. There is, in fact, among the bees a series
of gradations from solitary to communal life. The inter-
esting little green carpenter-bees live a truly solitary life.
Each female bores out the pith from five or six inches of
an elder branch or raspberry cane, and divides this space
into a few cells by means of transverse partitions (Fig. 94).
In each cell she lays an egg, and puts with it enough food
—flower pollen—to last the grub or larva through its life.

160 ANIMAL LIFE

She then waits in an upper cell of the nest until the young
bees issue from their cells, when she leads them off, and
each begins active life on its own account. The mining-

C wy Wd A
LY’
Fie. 94.—Nest of carpenter-bee. Fia. 95.—Nest of Andrena, the mining-bee.

bees (Andrena), which make little burrows (Fig. 95) in a
clay bank, live in large colonies—that is, they make their
nest burrows close together in the same clay bank, but each
female makes her own burrow, lays her own eggs in it, fur-
nishes it with food—a kind of paste of nectar and pollen—
and takes no further care of her young. Nor has she at
any time any special interest in her neighbors. But with
the smaller mining-bees, belonging to the genus Halictus,
several females unite In making a common burrow, after
which each female makes side passages of her own, extend-

ANIMAL COMMUNITIES AND SOCIAL LIFE 161

ing from the main or public entrance burrow. As a well-
known entomologist has said, Andrena builds villages com-
posed of individual homes, while Halictus makes cities
composed of apartment houses. The bumble-bee (Fig. 96),
however, establishes a real community with a truly com-
munal life, although a very simple one. The few bumble-
bees which we see in winter time are queens; all other
bumble-bees die in the autumn. In the spring a queen
selects some deserted nest of a field-mouse, or a hole in
the ground, gathers pollen which she molds into a rather
large irregular mass and puts into
the hole, and lays a few eggs on the
pollen mass. The young grubs or
larvee which soon hatch feed on the
pollen, grow, pupate, and issue as
workers—winged bees a little small-
er than the queen. These workers
bring more pollen, enlarge the nest,
and make irregular cells in the pol-
len mass, in each of which the queen
lays an egg. She gathers no more
pollen, does no more work except
that of egg-laying. From these new
eggs are produced more workers, and
so on until the community may come
to be pretty large. Later in the sum-
mer males and females are produced
and mate. With the approach of
winter all the workers and males die,
leaving only the fertilized females,
the queens, to live through the win- Fie. 96.—Bumble-bees. a,
ter and found new communities in weap g peer
the spring.

The social wasps show a communal life like that of the
bumble-bees. The only yellow-jackets and hornets that

live through the winter are fertilized females or queens.
12

162 ANIMAL LIFE

When spring comes each queen builds a small nest sus-
pended from a tree branch, and consisting of a small comb
inclosed in a covering or envelope open at the lower end.
The nest is composed of “wasp paper,” made by chewing
bits of weather-beaten wood taken from old fences or out-
buildings. In each of the cells the tjueen lays an egg.
_ She deposits in the cell a small mass of food, consisting of
some chewed insects or spiders. From these eggs hatch
grubs which eat the food prepared for them, grow, pupate,
and issue as worker bees, winged and slightly smaller
than the queen (Fig. 97). The workers enlarge the nest,
adding more combs and making many cells, in each of
which the queen lays an egg. The workers provision the
cell with chewed insects, and other broods of workers are
rapidly hatched. The
community grows in
numbers and the nest
grows in size until it
comes to be the great
ball-like oval mass
which we know so well
as a hornets’ nest (Figs.
98 and 99), a thing to be
left untouched. Some-
times the nest is built
underground. When
disturbed, they swarm
out of the hole and
fiercely attack any in-

vading foe in sight.

Fie. 97.—The yellow-jacket (Vespa), a social
wasp. a, worker; 0, queen. After a number of

broods of workers has
been produced, broods of males and females appear and
mating takes place. In the late fall the males and all of
the many workers die, leaving only the new queens to live
through the winter.

ANIMAL COMMUNITIES AND SOCIAL LIFE 163

The bumble-bees and social wasps show an intermediate
condition between the simply gregarious or neighborly

Fie. 98.—Nest of Vespa, a social Fic. 99.—Nest of Vespa opened to show
wasp. From photograph. combs within.

mining-bees and the highly developed, permanent honey-
bee community. Naturalists believe that the highly or-
ganized communal life of the honey-bees and the ants is
a development from some simple condition like that of the
bumble-bees and social wasps, which in its turn has grown
out of a still simpler, mere gregarious assembly of the
individuals of one species. It is not difficult to see how
such a development could in the course of a long time take
place.

87. Gregariousness and mutual aid—The simplest form
of social life is shown among those kinds of animals in
which many individuals of one species keep together, form-
ing a great band or herd. In this case there is not much
division of labor, and the safety of the individual is not
wholly bound up in the fate of the herd. Such animals are

164 ANIMAL LIFE

said to be gregarious in habit. The habit undoubtedly is
advantageous in the mutual protection and aid afforded
the individuals of the band. This mutual help in the case
of many gregarious animals is of a very positive and obvious
character. In other cases this gregariousness is reduced to
a matter of slight or temporary convenience, possessing but
little of the element of mutual aid. The great herds of
reindeer in the north, and of the bison or buffalo which
once ranged over the Western American plains, are examples
of a gregariousness in which mutual protection from ene-
mies, like wolves, seems to be the principal advantage gained.
The bands of wolves which hunted the buffalo show the
advantage of mutual help in aggression as well as in pro-
tection. In this banding together of wolves there is active
co-operation among individuals to obtain a common food
supply. What one wolf can not do—that is, tear down a
buffalo from the edge of the herd—a dozen can do, and all
are gainers by the operation. On the other hand, the vast
assembling of sea-birds (Fig. 100) on certain ocean islands
and rocks is a condition probably brought about rather by
the special suitableness of a few places for safe breeding
than from any special mutual aid afforded; still, these sea-
birds undoubtedly combine to drive off attacking eagles
and hawks. Eagles are usually considered to be strictly
solitary in habit (the unit of solitariness being a pair, not
an individual); but the description, by a Russian naturalist,
of the hunting habits of the great white-tailed eagle (Hali-
etos albicilla) on the Russian steppes shows that this kind
of eagle at least has adopted a gregarious habit, in which
mutual help is plainly obvious. This naturalist once saw an
eagle high in the air, circling slowly and widely in perfect
silence. Suddenly the eagle screamed loudly. “Its cry
was soon answered by another eagle, which approached it,
and was followed by a third, a fourth, and so on, till nine
or ten eagles came together and soon disappeared.” The
naturalist, following them, soon discovered them gathered

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kq ydeaSoioyg ‘vog Sutog ut dnois joliqud oy} Jo uo “puvxys] snayeM UO po[quiossv (DLLD DIQULO) DILQ) SIIINUI §,SBI[TRI—‘OOT ‘PTA

166 ANIMAL LIFE

about the dead body of a horse. The food found by the
first was being shared by all. The well-known association
of pelicans in fishing is a good example of the advantage of
a gregarious and mutually helpful habit. The pelicans go
fishing in great bands, and, after having chosen an appro-
priate place near the shore, they form a wide half-circle
facing the shore, and narrow it by paddling toward the
land, catching the fish which they inclose in the ever-nar-
rowing circle.

The wary. Rocky Mountain sheep (Fig. 101) live to-
gether in small bands, posting sentinels whenever they
are feeding or resting, who watch for and give warning
of the approach of enemies. The beavers furnish a well-
known and very interesting example of mutual help, and
they exhibit a truly communal life, although a simple
one. They live in “villages” or communities, all helping
to build the dam across the stream, which is necessary to
form the broad marsh or pool in which the nests or houses
are built. Prairie-dogs live in great villages or communi-
ties which spread over many acres. They tell each other by
shrill cries of the approach of enemies, and they seem to
visit each other and to enjoy each other’s society a great
deal, although that they afford each other much actual
active help is not apparent. Birds in migration are grega-
rious, although at other times they may live comparatively
alone. In their long flights they keep together, often with
definite leaders who seem to discover and decide on the
course of flight for the whole great flock. The wedge-
shaped flocks of wild geese flying high and uttering their
sharp, metallic call in their southward migrations are well
known in many parts of the United States. Indeed, the
more one studies the habits of animals the more examples
of social life and mutual help will be found. Probably most
animals are in some degree gregarious in habit, and in all
cases of gregariousness there is probably some degree of
mutual aid.

Fie. 101.—Rocky Mountain or bighorn sheep. By permission of the
publishers of Outing.

168 ANIMAL LIFE

88. Division of labor and basis of communal life——We have
learned in Chapters II and IV that the complexity of the
bodies of the higher animals depends on a specialization or
differentiation of parts, due to the assumption of different
functions or duties by different parts of the body; that the
degree of structural differentiation depends on the degree
or extent of division of labor shown in the economy of the
animal. It is obvious that the same principle of division of
labor with accompanying modification of structure is the
basis of colonial and communal life. It is simply a mani-
festation of the principle among individuals instead of
among organs. The division of the necessary labors of life
among the different zooids of the colonial jelly-fish is plain-
ly the reason for the profound and striking, but always
reasonable and explicable modifications of the typical polyp
or medusa body, which is shown by the swimming zooids,
the feeding zooids, the sense zooids, and the others of the
colony. And similarly in the case of the termite commu-
nity, the soldier individuals are different structurally from
the worker individuals because of the different work they
have to do. And the queen differs from all the others, be-
cause of the extraordinary prolificacy demanded of her to
maintain the great community.

It is important to note, however, that among those ani-
mals that show the most highly organized or specialized
communal or social life, the structural differences among
the individuals are the least marked, or at least are not the
most profound. The three kinds of honey-bee individuals
differ but little; indeed, as two of the kinds, male and
female, are to be found in the case of almost all kinds of
animals, whether communal in habit or not, the only unu-
sual structural specialization in the case of the honey-bee, is
the presence of the worker individual, which differs from
the usual individuals in but little more than the rudimen-
tary condition of the reproductive glands. Finally, in the
case of man, with whom the communal or social habit is so

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OY} OB S[vos OL, ‘“BVyeVyoUVyYy Fo ‘spuv[sy sopuvuu1oy oy Jo suo ‘tupeyW uo vivyedez yw souloy Jo sdnois IO soWoyoor [vos-INJ—ZOL “HT

aan ~ er Ser | v2 ay ry ory a sy 7 Tre ad o

170 ANIMAL LIFE

all-important as to gain for him the name of “ the social
animal,” there is no differentiation of individuals adapted
only for certain kinds of work. Among these highest
examples of social animals, the presence of an advanced
mental endowment, the specialization of the mental power,
the power of reason, have taken the place of and made
unnecessary the structural differentiation of individuals.
The honey-bee workers do different kinds of work: some
gather food, some care for the young, and some make wax
and build cells, but the individuals are interchangeable ;
each one knows enough to do these various things. There
is a structural differentiation in the matter of only one
special work or function, that of reproduction.

With the ants there is, in some cases, a considerable
structural divergence among individuals, as in the genus
Atta of South America with six kinds of individuals—
namely, winged males, winged females, wingless soldiers,
and wingless workers of three distinct sizes. In the case
of other kinds with quite as highly organized a communal
life there are but three kinds of individuals, the winged
males and females and the wingless workers. The workers
gather food, build the nest, guard the “ cattle” (aphids),
make war, and care for the young. Each one knows enough
to do all these various distinct things. Its body is not so
modified that it can do but one kind of thing, which thing
it must always do.

The increase of intelligence, the development of the
power of reasoning, is the most potent factor in the devel-
opment of a highly specialized social life. Man is the
example of the highest development of this sort in the ani-
mal kingdom, but the highest form of social development
is not by any means the most perfectly communal.

89. Advantages of communal life—The advantages of
communal or social life, of co-operation and mutual aid, are
real. The animals that have adopted such a life are among
the most successful of all animals in the struggle for exist-

ANIMAL COMMUNITIES AND SOCIAL LIFE 171

ence. The termite individual is one of the most defense-
less, and, for those animals that prey on insects, one of
the most toothsome luxuries to be found in the insect
world. But the termite is one of the most abundant and
widespread and successfully living insect kinds in all the
tropics. Where ants are not, few insects are. The honey-
bee is a popular type of a successful life. The artificial
protection afforded the honey-bee by man may aid in its
struggle for existence, but it gains this protection because
of certain features of its communal life, and in Nature the
honey-bee takes care of itself well. The Little Bee People
of Kipling’s Jungle Book, who live in great communities in
the rocks of Indian hills, can put to rout the largest and
fiercest of the jungle animals. Co-operation and mutual
aid are among the most important factors which help in
the struggle for existence. Its great advantages are, how-
ever, in some degree balanced by the fact that mutual help
brings mutual dependence. The community or society can
accomplish greater things than the solitary individuals, but
co-operation limits freedom, and often sacrifices the indi-
vidual to the whole.

CHAPTER X
COMMENSALISM AND SYMBIOSIS

90. Association between animals of different species—The
living together and mutual help discussed in the last chap-
ter concerned in each instance a single species of animal.
All the various members of a pack of wolves or of a com-
munity of ants are individuals of the same species. But
there are many instances of an association of individuals
of different kinds of animals. The number of individuals
concerned, however, is usually but two—that is, one of
each of the two kinds of animals. In many cases of an
association of individuals of different species one kind
derives great benefit and the other suffers more or less
injury from the association. One kind lives at the expense
of the other. This association is called parasitism, and is
discussed in the next chapter. In some cases, however,
neither kind of animal suffers from the presence of the
other. The two live together in harmony and presumably
to their mutual advantage. In some cases this mutual
advantage is obvious. This kind of association is called
commensalism or symbiosis. The term commensalism may
be used to denote a condition where the two animals are
not so intimately associated nor derive such obvious mu-
tual advantage from the association, as in that condition
of very intimate and permanent association with obvious
co-operative and marked advantage that may be called
symbiosis. A few examples of each of these interesting
conditions of association between which it is impossible to

make any sharp distinction, will be given.
172

COMMENSALISM AND SYMBIOSIS 173

91. Commensalism.—A curious example of commensalism
is afforded by the different species of Remoras (Hchenidide)
which attach themselves to sharks, barracudas, and other
large fishes by means of a sucking disk on the top of the
head (Fig. 103). This disk is made by a modification of

Fie. 103.—Remora, with dorsal fin modified to be a sucking plate by which the
fish attaches itself to a shark.

the dorsal fin. The Remora thus attached to a shark may
be carried about for weeks, leaving its host only to secure
food. This is done by a sudden dash through the water.
The Remora injures the shark in no way save, perhaps, by
the slight check its presence gives to the shark’s speed in
swimming.

Whales, similarly, often carry barnacles about with
them. ‘These barnacles are permanently attached to the
skin of the whale just as they would be to a stone or
wooden pile. Many small crustaceans, annelids, mollusks,
and other invertebrates burrow into the substance of living
sponges, not for the purpose of feeding on them, but for
shelter. On the other hand, the little boring sponge
(Cliona) burrows in the shells of oysters and other bivalves
for protection. These are hardly true cases of even that
lesser degree of mutually advantageous association which
we are calling commensalism. But some species of sponge
“are never found growing except on the backs or legs of
certain crabs.” In these cases the sponge, with its many
plant-like branches, protects the crab by concealing it from
its enemies, while the sponge is benefited by being carried
about by the crab to new food supplies. Certain sponges

174 ANIMAL LIFE

and polyps are always found growing in close association,
though what the mutual advantage of this association is
has not yet been found out.

Among the coral reefs near Thursday Island (between
New Guinea and Australia) there lives an enormous kind
of sea-anemone or polyp. Individuals of this great polyp
measure two feet across the disk when fully expanded.
In the interior, the stomach cavity, which communicates
freely with the outside by means of the large mouth open-
ing at the free end of the polyp, there may often be found
a small fish (Amphiprion percula). That this fish is pur-
posely in the gastral cavity of the polyp is proved by the
fact that when it is dislodged it invariably returns to its
singular lodging-place. ‘The fish is brightly colored, being
of a brilliant vermilion hue with three broad white cross
bands. The discoverer of this peculiar habit suggests that
there are mutual benefits to fish and polyp from this habit.
“The fish being conspicuous, is liable to attacks, which it
escapes by a rapid retreat into the sea-anemone ; its enemies
in hot pursuit blunder against the outspread tentacles of
the anemone and are at once narcotized by the ‘thread
cells’ shot out in innumerable showers from the tentacles,
and afterward drawn into the stomach of the anemone and
digested.”

Small fish of the genus Nomeus may often be found
accompanying the beautiful Portuguese man-of-war (Phy-
salia) as it sails slowly about on the ocean’s surface (Fig.
104). These little fish lurk underneath the float and
among the various hanging thread-like parts of the Phy-
salia, which are provided with stinging cells. The fish are
protected from their enemies by their proximity to these
stinging threads, but of what advantage to the man-of-
war their presence is is not understood. Similarly, several
kinds of medusz are known to harbor or to be accompanied
by young or small adult fishes.

In the nests of the various species of ants and termites

COMMENSALISM AND SYMBIOSIS 175

many different kinds of other insects have been found.
Some of these are harmful to their hosts, in that they feed
on the food stores gathered by the industrious and provi-
dent ant, but others appear

to feed only on refuse or use-
less substances in the nest.
Some may even be of help to
their hosts. Over one thou-
sand species of these myrme-
cophilous (ant-loving) and
termitophilous (termite - lov-
ing) insects have been re-
corded by collectors as living
habitually in the nests of ants
and termites. The owls and
rattlesnakes which live with
the prairie-dogs in their vil-
lages afford a familiar exam-
ple of commensalism.

92. Symbiosis. —Of a more
intimate character, and of
more obvious and certain mu-
tual advantage, is the well-
known case of the symbiotic
association of some of the
numerous species of hermit-
crabs and certain species of
sea-anemones. The hermit-

Fie. 104.—A Portuguese man-of-war

crab always takes for his (Physalia), with man-of-war fishes

habitation the shell of an- Bee ee ne
; shelter of the stinging feelers.

other animal, often that of Specimens from off Tampa, Fla.

the common whelk. All of

the hind part of the crab lies inside the shell, while its
head with its great claws project from the opening of the
shell. On the surface of the shell near the opening there
is usually to be found a sea-anemone, or sea-rose (Fig. 105).

176 ANIMAL LIFE

This sea-anemone is fastened securely to the shell, and has
its mouth opening and tentacles near the head of the crab.
The sea-anemone is carried from place to place by the her-
mit-crab, and in this way is much aided in obtaining food.
On the other hand, the crab is protected from its enemies
by the well-armed and dangerous tentacles of the sea-anem-

Fie. 105.—Hermit-crab (Pagurus) in shell, with a sea-anemone (Adamsia palliata)
attached to the shell._After HeRTw1e.

one. In the tentacles there are many thousand long,
slender stinging threads, and the fish that would obtain
the hermit-crab for food must first deal with the stinging
anemone. There is no doubt here of the mutual advan-
tage gained by these two widely different but intimately
associated companions. If the sea-anemone be torn away
from the shell inhabited by one of these crabs, the crab
will wander about, carefully seeking for another anemone.
When he finds it he struggles to loosen it from its rock
or from whatever it may be growing on, and does not rest
until he has torn it loose and placed it on his shell.

There are numerous small crabs called pea-crabs (Pin-
notheres) which live habitually inside the shells of living

COMMENSALISM AND SYMBIOSIS 177

mussels. The mussels and the crabs live together in per-
fect harmony and to their mutual benefit.

There are a few extremely interesting cases of symbiosis
in which not different kinds of animals are concerned, but
animals and plants. It has long been known that some
sea-anemones pos-
sess certain body
cells which con-
tain chlorophyll,
that green sub-
stance character-
istic of the green
plants, and only
in few cases pos-
sessed by animals.
When these chlo-
rophyll-bearing
sea-anemones were
first found, it was
believed that the
chlorophyll cells

Fie. 106.—The crab Zpizoanthus paguriphilus, with
really belonged to the sea-anemone Parapagurus pilosiramus on its

the animal’s body, shell.

and that this con-

dition broke down one of the chiefest and most readily
apparent distinctions between animals and plants. But
it is now known that these chlorophyll-bearing cells are
microscopic, one-celled plants, green alge, which live ha-
bitually in the bodies of the sea-anemone. It is a case
of true symbiosis. The alge, or plants, use as food the
carbonic-acid gas which is given off in the respiratory
processes of the sea-anemone, and the sea-anemone breathes
in the oxygen given off by the alge in the process of ex-
tracting the carbon for food from the carbonic-acid gas.
These alge, or one-celled plants, lie regularly only in the
innermost of the three cell layers which compose the wall

18

178 ANIMAL LIFE

or body of the sea-anemone (Fig. 107). They penetrate
into and lie in the interior of the cells of this layer whose
special function is that of digestion. They give this inner-

Fie. 107.—Diagrammatic section of sea-anemone. 4,
the inner cell layer containing alga cells, the two
isolated cells at right being cells of this layer with
contained alge; 6, middle body wall layer; c, outer
body wall layer.—After HERtTwie.

most layer of cells
a distinct green
color.

There are other
examples known of
the symbiotic asso-
ciation of plants
and animals; and
if we were to fol-
low the study of
symbiosis into the
plant kingdom we
should find that in
one of the large
groups of plants,
the familiar lichens
which grow on

rocks and tree trunks and old fences, every member lives
symbiotically. A lichen is not a single plant, but is always
composed of two plants, an alga (chlorophyll-bearing) and
a fungus (without chlorophyll) living together in a most
intimate, mutually advantageous association.

CHAPTER XI
PARASITISM AND DEGENERATION

93. Relation of parasite and host.—In addition to the vari-
ous ways of living together of animals already described,
namely, the social life of individuals of a single species and
the commensal and symbiotic life of individuals of differ-
ent species, there is another kind of association among ani-
mals that is very common. In cases of symbiosis the two
animals living together are of mutual advantage to each
other; both profit by the association. But tnere are many
instances in the animal kingdom of an association between
two animals by which one gains advantages great or small,
sometimes even obtaining all the necessities of life, while
the other gains nothing, but suffers corresponding disad-
vantage, often even the loss of life itself. This is the asso-
ciation of parasite and host; the relation between two ani-
mals whereby one, the parasite, lives on or in the other, the
host, and at the expense of the host. Parasitism is a com-
mon phenomenon in all groups of animals, although the
parasites themselves are for the most part confined to the
classes of invertebrates. Among the simplest animals or
Protozoa there are parasites, as Gregarina, which lives in
the bodies of insects and crustaceans; there are parasitic
worms, and parasitic crustaceans and mollusks and insects,
and a few vertebrates. When an animal can get along
more safely or more easily by living at the expense of some
other animal and takes up such a life, it becomes a parasite.
Parasitism is naturally, therefore, not confined to any one

group or class of animals.
179

180 ANIMAL LIFE

94. Kinds of parasitism.—The bird-lice (Mallophaga),
which infest the bodies of all kinds of birds and are found
especially abundant on domestic fowls, live upon the out-
side of the bodies of their hosts, feeding upon the feathers
and dermal scales. They are examples of external parasites.
Other examples are fleas and ticks, and the crustaceans called
fish-lice and whale-lice, which are attached to marine ani-
mals. On the other hand, almost all animals are infested by
certain parasitic worms which live in the alimentary canal,
like the tape-worm, or imbedded in the muscles, like the
trichina. These are examples of internal parasites. Such
parasites belong mostly to the class of worms, and some of
them are very injurious, sucking the blood from the tissues
of the host, while others feed solely on the partly digested
food. There are also parasites that live partly within and
partly on the outside of the body, like the Sacculina, which
lives on various kinds of crabs. The body of the Sacculina
consists of a soft sac which lies on the outside of the crab’s
body, and of a number of long, slender root-like processes
which penetrate deeply into the crab’s body, and take up
nourishment from within. The Sacculina is itself a crus-
tacean or crab-like creature. The classification of para-
sites as external and internal is purely arbitrary, but it is
often a matter of convenience.

Some parasites live for their whole lifetime on or in the
body of the host, as is the case with the bird-lice. Their
eggs are laid on the feathers of the bird host; the young
when hatched remain on the bird during growth and deyel-
opment, and the adults only rarely leave the body, usually
never. These may be called permanent parasites. On the
other hand, fleas leap off or on a dog as caprice dictates ;
or, as in other cases, the parasite may pass some definite
part of its life as a free, non-parasitic organism, attaching
itself, after development, to some animal, and remaining
there for the rest of its life. These parasites may be called
temporary parasites. But this grouping or classification,

PARASITISM AND DEGENERATION 181

like that of the external and internal parasites, is simply a
matter of convenience, and does not indicate at all any
blood relationship among the members of any one group.
95. The simple structure of parasites—In all cases the
body of a parasite is simpler in structure than the body of
other animals which are closely related to the parasite—
that is, animals that live parasitically have simpler bodies
than animals that live free active lives, competing for
food with the other animals about them. This simplicity
is not primitive, but results from the loss or atrophy of the
structures which the mode of life renders useless. Many
parasites are attached firmly to their host, and do not move
about. They have no need of the power of locomotion.
They are carried by their host. Such parasites are usually
without wings, legs, or other locomotory organs. Because
they have given up locomotion they have no need of or-
gans of orientation, those special sense organs like eyes
and ears and feelers which serve to guide and direct the
moving animal; and most non-locomotory parasites will
be found to have no eyes, nor any of the organs of special
sense which are accessory to locomotion and which serve
for the detection of food or of enemies. Because these im-
portant organs, which depend for their successful activity
on a highly organized nervous system, are lacking, the
nervous system of parasites is usually very simple and un-
developed. Again, because the parasite usually has for
its sustenance the already digested highly nutritious food
elaborated by its host, most parasites have a very simple
alimentary canal, or even no alimentary canal at all.
Finally, as the fixed parasite leads a wholly sedentary and
inactive life, the breaking down and rebuilding of tissue in
its body go on very slowly and in minimum degree, and
there is no need of highly developed respiratory and circu-
latory organs; so that most fixed parasites have these sys-
tems of organs in simple condition. Altogether the body
of a fixed, permanent parasite is so simplified and so want-

189 ANIMAL LIFE

ing in all those special structures which characterize the
higher, active, complex animals, that it often presents a
very different appearance from those: animals with which
we know it to be nearly related.

The simplicity of parasites does not indicate that they |
all belong to the groups of primitive simple animals.
Parasitism is found in the whole range of animal life,
from primitive to highest. Their simplicity is something
that has resulted from their mode of life. It is the result
of a change in the body-structure which we can often
trace in the development of the individual parasite. Many
parasites in their young stages are free, active animals
with a better or more complex body than they possess in
their fully developed or adult stage. The simplicity of
parasites is the result of degeneration—a degeneration
that has been brought about by their adoption of a seden-
tary, non-competitive parasitic life. And this simplicity of
degeneration, and the simplicity of primitiveness should be
sharply distinguished. Animals that are primitively simple
have had only simple ancestors; animals that are simple
by degeneration often have had highly organized, complex
ancestors. And while in the life history or development of
a primitively simple animal all the young stages are simpler
than the adult, in a degenerate animal the young stages
may be, and usually are, more complex and more highly
organized than the adult stage.

In the examples of parasitism that are described in
the following pages all these general statements are illus-
trated.

96. Gregarina.—In the intestines of cray-fishes, centi-
peds, and several kinds of insects may often be found
certain one-celled animals (Protozoa) which are living as
parasites. Their food, which they take into their minute
body by absorption, is the intestinal fluids in which they
lie. These parasitic Protozoa belong to the genus Grega-
rina (Fig. 9) (see Chapter I). Because the body of any

PARASITISM AND DEGENERATION 183

protozoan is as simple as an animal’s body can be, being
composed of but a single cell, degeneration can not occur
in the cases of these parasites. There are, besides (rega-
rina, numerous other parasitic one-celled animals, several
kinds living inside the cells of their host’s body. One
kind lives in the blood-corpuscles of the frog, and another
in the cells of the liver of the rabbit.

97. The tape-worm and other flat-worms.—In the great
group of flat-worms (Platyhelminthes), that group of ani-
mals which of all the principal animal groups is widest
in its distribution, perhaps a major-
ity of the species are parasites. In-
stead of being the exception, the
parasitic life is the rule among these
worms. Of the three classes into
which the flat-worms are divided
almost all of the members of two of
the classes are parasites. The com-
mon tape-worm (Tenia) (Fig. 108),
which lives parasitically in the intes-
tine of man, is a good example of
one of these classes. “It has the
form of a narrow ribbon, which may
attain the length of several yards,
attached at one end to the wall of fre. 108—rTape-worm (Tenia
the intestine, the remainder hanging — um). _In upper left-

ps i P h.nd corner of figure the
freely in the interior.” Its body is jeaa much magnified. —
composed of segments or serially After Leuckarr.
arranged parts, of which there are
about eight hundred and fifty altogether. It has no mouth
nor alimentary canal. It feeds simply by absorbing into
its body, through the surface, the nutritious, already di-
gested liquid food in the intestine. There are no eyes
nor other special sense organs, nor any organs of locomo-
tion. The body is very degenerate. The life history of
the tape-worm is interesting, because of the necessity of

184 ANIMAL LIFE

two hosts for its completion. The eggs of the tape-worm
pass from the intestine with the excreta, and must be
taken into the body of some other animal in order to de-
velop. In the case of one of the several species of tape-
worms that infest man this other host must be the pig.
In the alimentary canal of the pig the young tape-worm
develops, and later bores its way through the walls of the
canal and becomes imbedded in the muscles. There it lies,
until it finds its way into the alimentary canal of man by
his eating the flesh of the pig. In the intestine of man
the tape-worm continues to develop
until it becomes full grown.

In a lake in Yellowstone Park
the suckers are infested by one of.
the flat-worms (Ligula) that at-
tains a size of nearly one fourth
the size of the fish in whose in-
testines it lives. If the tape-worm
of man attained such a compara-
tive size, a man of two hundred
pounds’ weight would be infested by
a parasite of fifty pounds’ weight.

98. Trichina and other round-
worms.— Another group of animals,
many of whose numbers are para-
sites, are the round-worms or thread-
worms (Nemathelminthes). The
free-living round-worms are active,

Fig. 109. — Trichina spiralis
(after Cravs). a, male; 2, well-organized animals, but the

encysted form in muscle ; ¢,

ree parasitic kinds all show a greater

or less degree of degeneration. One
of the most terrible parasites of man is a round-worm called
Trichina spiralis (Fig. 109). It is a minute worm, from
one to three millimetres long, which in its adult condition
lives in the intestine of man or of the pig or other mam-
mals. The young are born alive and bore through the walls

PARASITISM AND DEGENERATION 185

of the intestine. They migrate to the voluntary muscles
of the hosts, especially those of the limbs and _ back, and
here each worm coils itself up in a muscle fiber and be-
comes inclosed in a spindle-shaped cyst or cell (Fig. 109, 0).
A single muscle may be infested by hundreds of thousands of
these minute worms. It has been estimated that fully one
hundred million encysted worms have existed in the mus-
cles of a “trichinized” human body. The muscles undergo
more or less degeneration, and the death of the host may
occur. It is necessary, for the further development of the
worms, that the flesh of the host be eaten by another mam-
mal, as the flesh of the pig by man, or the flesh of man by
a pig or rat. The 7Zrichine in the alimentary canal of
the new host develop into active adult worms and produce
new young.

In the Yellowstone Lake the trout are infested by the
larve or young of a round-worm (Bothriocephalus cordiceps)
which reaches a length of twenty inches, and which is
often found stitched, as it were, through the viscera and
the muscles of the fish. The infested trout become feeble
and die, or are eaten by the pelicans which fish in this
lake. In the alimentary canal of the pelican the worms
become adult, and parts of the worms containing eggs
escape from the alimentary canal with the excreta. These
portions of worms are eaten by the trout, and the eggs give
birth to new worms which develop in the bodies of the
fish with disastrous effects. It is estimated that for each
pelican in Yellowstone Lake over five million eggs of the
parasitic worms are discharged into the lake.

The young of various carnivorous animals are often
infested by one of the species of round-worms called “ pup-
worms” (Uncinaria). Recent investigations show that
thousands of the young or pup fur-seals are destroyed each
year by these parasites. The eggs of the worm lie through
the winter in the sands of the breeding grounds of the fur-
seal. The young receive them from the fur of the mother

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‘puvsy] [nv_ 49 ‘Atoyxoor 1098]O], JO spuvs oy) uo (M2WYUZUQ) WIOM oIISRIEd B 4q poy ‘sdnd [vos-ngq— ort ‘pit

PARASITISM AND DEGENERATION 187

and the worm develops in the upper intestine. It feeds on
the blood of the young seal, which finally dies from anemia.
On the beaches of the seal islands in Bering Sea there are
sometimes hundreds of dead seal pups which have been
killed by this parasite (Fig. 110).

99. Sacculina—Among the more highly organized ani-
mals the results of a parasitic life, in degree of structural
degeneration, can be more readily seen. A well-known para-
site, belonging to the crustacea—the class of shrimps, crabs,
lobsters, and cray-fishes—is Sacculina. The young Sac-
culina is an active, free-swimming larva much like a young
prawn or young crab. But the adult bears absolutely no
resemblance to such a typical crustacean as a cray-fish or
crab. The Sacculina after a short period of independent
existence at-
taches itself to

the abdomen of Vou)
a crab, and Wy
there completes KW AW if

its develop-
ment while liy-
ing as a para-
site In its
adult condition
(Fig. 111) it is
simply a great

Fie. 111.—Sacculina, a crustacean parasite of crabs. 4, at-

tumor-like sac, tached to a crab, with root-like processes penetrating the
bearing many crab’s body ; 0, removed from the crab.

delicate root-
like suckers which penetrate the body of the crab host and
absorb nutriment. The Sacculina has no eyes, no mouth
parts, no legs, or other appendages, and hardly any of the
usual organs except reproductive organs. Degeneration
here is carried very far.

Other parasitic crustacea, as the numerous kinds of
fish-lice (Fig. 112) which live attached to the gills or to

WVZ
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Sy
VR

188 ANIMAL LIFE

other parts of fish, and derive all their nutriment from the
body of the fish, show various degrees of degeneration. With
some of these fish-lice the female,
which looks like a puffed-out worm,
is attached to the fish or other aquatic
animal, while the male, which is per-
haps only a tenth of the size of the
female, is permanently attached to
the female, living parasitically on her.
100. Parasitic insects. — Among
the insects there are many kinds
that live parasitically for part of
their life, and not a few that live as

gantih pate i parasites for their whole life. The
co" true sucking lice (Fig. 113) and the
bird-lice (Fig. 114) live for their whole lives as external
parasites on the bodies of their host, but they are not
fixed —that is, they retain
their legs and power of loco-
motion, although they have
lost their wings through de-
generation. The eggs of the
lice are deposited. on the hair
of the mammal or bird that

J

\
He
I
1
j
‘
i

Fie. 113.—Sucking louse (Pediculus) of
human body. Fig. 114.—Bird louse (Lipeurus densus).

serves as host; the young hatch and immediately begin to
live as parasites, either sucking the blood or feeding on the

PARASITISM AND DEGENERATION 189

hair or feathers of the host. In the order Hymenoptera
there are several families, all of whose members live during
their larval stage as parasites. We may call all these hy-
menopterous parasites ichneumon flies. ‘The ichneumon
flies are parasites of other insects, especially of the larve of
beetles and moths and butterflies. In fact, the ichneumon
flies do more to keep in check the increase of injurious and
destructive caterpillars than do all our artificial remedies
for these insect pests. The adult ichneumon fly is four-
winged and lives an active, independent life. It lays its
eggs either in or on or near some caterpillar or beetle grub,
and the young ichneumon, when hatched, burrows about in
the body of its host, feeding on its tissues, but not attacking
such organs as the heart or nervous ganglia, whose injury
would mean immediate death to the host. The caterpillar
lives with the ichneumon grub within it, usually until nearly

erlu COA

e.
x

Ui AON

ae

A catia ililaine

Fie. 115.—Parasitized caterpillar from which the ichneumon fly parasites have
issued, showing the circular holes of exit in the skin.

time for its pupation. In many instances, indeed, it pu-
pates, with the parasite still feeding within its body, but it
never comes to maturity. The larval ichneumon fly pupates
either within the body of its host (Fig. 115) or in a tiny
silken cocoon outside of its body (Fig. 116). From the
cocoons the adult winged ichneumon flies emerge, and
after mating find another host on whose body to lay their
eggs.

One of the most interesting ichneumon flies is T'halessa
(Fig. 119), which has a remarkably long, slender, flexible
ovipositor, or egg-laying organ. An insect known as the

190 ANIMAL LIFE

pigeon horn-tail (Zremex columba) (Fig. 117) deposits its
eggs, by means of a strong, piercing ovipositor, half an inch
deep in the trunk wood of growing trees. The young or

Fie. 116.—Caterpillar with cocoons of the pup of ichneumon fly parasites, and
(above) one of the adult ichneumon flies. The lines indicate natural dimensions,

larval Tremex is a soft-bodied white grub, which bores
deeply into the trunk of the tree, filling up the burrow be-
hind it with small chips. The Vhalessa is a parasite of the
Tremex, and “ when a female 7halessa finds a tree infested
by Tremex, she selects a place which she judges is opposite

PARASITISM AND DEGENERATION 191

a Tremex burrow, and, elevating her long ovipositor in a
loop over her back, with its tip on the bark of the tree (Fig.

Fie. 117.—The pigeon horn-tail (7remex
columba), with strong boring ovipositor.

Fie. 119.—The large ichneumon fly
Fig. 118.—Thalessa lunator boring.—After Thalessa, with long flexible oviposi-
Comstock. tor. The various parts of this ovi-
positor are spread apart in the fig-
: ure ; naturally they lie together to
118), she makes a derrick out piel ‘anes claweine iia
of her body and proceeds with

great skill and precision to drill a hole into the tree. When
the Tremez burrow is reached she deposits an egg in it.

192 ANIMAL LIFE

The larva that hatches from this egg creeps along this
burrow until it reaches its victim, and then fastens itself to
the horn-tail larva, which it destroys by sucking its blood.

Fie. 120.—Wasp (Polistes), with female Stylops para-
site (@) in body.

The larva of Thales-
sa, When full grown,
changes to a pupa
within the burrow
of its host, and the
adult gnaws a hole
out through the bark
if it does not find the
hole already made by
the Tremez.”

The beetles of
the family Stylopide
present an interest-
ing case of parasit-

ism. The adult males are winged, but the adult females

are wingless and grub-like.

itself to a wasp or bee, and bores into its abdomen.

pupates within the abdomen of the
wasp or bee, and lies there with its
head projecting slightly from a su-
ture between two of the body rings
of its host (Fig. 120). The adult
finally issues and leaves the host’s
body.

Almost all of the mites and ticks,
which are more nearly allied to the
spiders than to the true insects, live
parasitically. Most of them live as
external parasites, sucking the blood
of their host, but some live under-
neath the skin like the itch-mites

The larval stylopid attaches

It

Fie. 121.—The itch-mite
(Sarcoptes scabei).

(Fig. 121), which cause, in man, the disease known as

the itch.

PARASITISM AND DEGENERATION 193

101. Parasitic vertebrates—Among the vertebrate ani-
mals there are not many examples of true parasitism. The
hag-fishes or borers (Myzxine, Heptatrenia, Polistotrema) are
long and cylindrical, eel-like creatures, very slimy and very
low in structure. The mouth is without jaws, but forms a
sucking disk, by which the hag-fish attaches itself to the
body of some other fish. By means of the rasping teeth on
its tongue, it makes a round hole through the skin, usually
at the throat. It then devours all the muscular substance
of the fish, leaving the viscera untouched. When the fish
finally dies it is a mere hulk of skin, scales, bones, and
viscera, nearly all the muscle being gone. Then the hag-
fish slips out and attacks another individual.

The lamprey, another low fish, in similar fashion feeds
leech-like on the flesh of other fishes, which it scrapes out
with its rasp-like teeth, remaining attached by the round
sucking disk of its mouth.

Certain birds, as the cow-bird and the European cuckoo,
have a parasitic habit, laying their eggs in the nests of
other birds, leaving their young to be hatched and reared
by their unwilling hosts. This is, however, not bodily para-
sitism, such as is seen among lower forms.

102. Degeneration through quiescence.— While parasitism
is the principal cause of degeneration among animals, yet
it is not the sole cause. It is evident that if for any other
reason animals should become fixed, and live inactive or
sedentary lives, they would degenerate. And there are not
a few instances of degeneration due simply to a quiescent
life, unaccompanied by parasitism. The Tunicata, or sea-
squirts (Fig. 122), are animals which have become simple
through degeneration, due to the adoption of a sedentary
life, the withdrawal from the crowd of animals and from
the struggle which it necessitates. The young tunicate is
a free-swimming, active, tadpole-like or fish-like creature,
which possesses organs very like those of the adult of the

simplest fishes or fish-like forms. That is, the sea-squirt
14

194 ANIMAL LIFE

begins life as a primitively simple vertebrate. It possesses
in its larval stage a notochord, the delicate structure which
precedes the formation of a backbone, extending along the
upper part of the body,
below the spinal cord. It
is found in all young ver-
tebrates, and is charac-
teristic of the class. The
other organs of the young
tunicate are all of verte-
bral type. But the young
sea-squirt passes a period
of active and free life as
a little fish, after which
it settles down and at-
taches itself to a stone or
shell or wooden pier by
means of suckers, and re-
mains for the rest of its
life fixed. Instead of go-
ing on and developing
into a fish-like creature, it

Fig. 122.—A sea-squirt, or tunicate. loses its notochord, its

special sense organs, and
other organs; it loses its complexity and high organiza-
tion, and becomes a “ mere rooted bag with a double neck,”
a thoroughly degenerate animal.

A barnacle is another example of degeneration through
quiescence. The barnacles are crustaceans related most
nearly to the crabs and shrimps. The young barnacle just
from the egg (Fig. 123, f) is a six-legged, free-swimming
nauplius, very like a young prawn or crab, with single eye.
In its next larval stage it has six pairs of swimming feet,
two compound eyes, and two large antenne or feelers, and
still lives an independent, free-swimming life. When it -
makes its final change to the adult condition, it attaches

PARASITISM AND DEGENERATION 195

itself to some stone or shell, or pile or ship’s bottom, loses
its compound eyes and feelers, develops a protecting shell,
and gives up all power of locomotion. Its swimming feet
become changed into grasping organs, and it loses most of
its outward resemblances to the other members of its class
(Fig. 123, e).

Fig. 123.—Three adult crustaceans and their larve. a, prawn (Peneus), active and
free-living ; }, larva of prawn; c, Sacculina, parasite; d, larva of Sacculina;
é, barnacle (Zepas), with fixed quiescent life; jf, larva of barnacle.—After
HAECKEL. ;

Certain insects live sedentary or fixed lives. All the
members of the family of scale insects (Coccide), in one
sex at least, show degeneration, that has been caused by
quiescence. One of these coccids, called the red orange
scale (Fig. 124), is very abundant in Florida and California
and in other orange-growing regions. The male is a beau-
tiful, tiny, two-winged midge, but the female is a wingless,

196 ANIMAL LIFE

footless little sac without eyes or other organs of special
sense, which lies motionless under a flat, thin, circular, red-
dish scale composed of wax and two or three cast skins of
the insect itself. The insect has a long, slender, flexible,
sucking beak, which is thrust into the leaf or stem or fruit
of the orange on which the “scale bug” lives and through
which the insect sucks the orange sap, which is its only

Fig. 124.—The red orange scale of California. a, bit of leaf with scales; 0, adult
female; ¢, wax scale under which adult female lives; d, larva; e, adult male.

food. It lays eggs under its body, and thus also under the
protecting wax scale, and dies. From the eggs hatch active
little larval scale-bugs with eyes and feelers and six legs.
They crawl from under the wax scale and roam about over
the orange tree. Finally, they settle down, thrusting their
sucking beak into the plant tissues, and cast their skin.
The females lose at this molt their legs and eyes and

PARASITISM AND DEGENERATION 197

feelers. Each becomes a mere motionless sac capable only
of sucking up sap and of laying eggs. The young males,
however, lose their sucking beak and can no longer take
food, but they gain a pair of wings and an additional pair
of eyes. They fly about and fertilize the sac-like females,
which then molt again and secrete the thin wax scale over
them.

Throughout the animal kingdom loss of the need of
movement is followed by the loss of the power to move, and
of all structures related to it.

103. Degeneration through other causes,—Loss of certain
organs may occur through other causes than parasitism and
a fixed life. Many insects live but a short time in their
adult stage. May-flies live for but a few hours or, at most,
a few days. They do not need to take food to sustain life
for so short a time, and so their mouth parts have become
rudimentary and functionless or are entirely lost. This is
true of some moths and numerous other specially short-
lived insects. Among the social insects the workers of the
termites and of the true ants are wingless, although they
are born of winged parents, and are descendants of winged
ancestors. The modification of structure dependent upon
the division of labor among the individuals of the com-
munity has taken the form, in the case of the workers, of a
degeneration in the loss of the wings. Insects that live
in caves are mostly blind; they have lost the eyes, whose
function could not be exercised in the darkness of the cave.
Certain island-inhabiting insects have lost their wings,
flight being attended with too much danger. The strong
sea-breezes may at any time carry a flying insect off the
small island to sea. Only those which do not fly much sur-
vive, and by natural selection wingless breeds or species are
produced. Finally, we may mention the great modifications
of structure, often resulting in the loss of certain organs,
which take place to produce protective resemblances (see
Chapter XII). In such cases the body may be modified in

198 ANIMAL LIFE

color and shape so as to resemble some part of the envi-
ronment, and thus the animal may be unperceived by its
enemies. Many insects have lost their wings through this
cause.
104. Immediate causes of degeneration—When we say
that a parasitic or quiescent mode of life leads to or causes
degeneration, we have explained the stimulus or the ulti-
mate cause of degenerative changes, but we have not
shown just how parasitism or quiescence actually produces
these changes. Degeneration or the atrophy and disap-
pearance of organs or parts of a body is often said to be
due to disuse. That is, the disuse of a part is believed by
many naturalists to be the sufficient cause for its gradual
dwindling and final loss. That disuse can so affect parts
of a body during the lifetime of an individual is true. A
muscle unused becomes soft and flabby and small. Whether
the effects of such disuse can be inherited, however, is open
to serious doubt. Such inheritance must be assumed if
disuse is to account for the gradual growing less and final
disappearance of an organ in the course of many genera-
tions. Some naturalists believe that the results of such
disuse can be inherited, but as yet such belief rests on no
certain knowledge. If characters assumed during the life-
time of the individual are subject to inheritance, disuse
alone may explain degeneration. If not, some other imme-
diate cause, or some other cause along with disuse, must
be found. Such a cause must be sought for in the action of
natural selection, preserving the advantages of simplicity of
’ structure where action is not required.

105. Advantages and disadvantages of parasitism and de-
generation.— We are accustomed, perhaps, to think of degen-
eration as necessarily implying a disadvantage in life. A
degenerate animal is considered to be not the equal of a non-
degenerate animal, and this would be true if both kinds of
animals had to face the same conditions of life. The blind,
footless, simple, degenerate animal could not cope with the

PARASITISM AND DEGENERATION 199

active, keen-sighted, highly organized non-degenerate in
free competition. But free competition is exactly what
the degenerate animal has nothing to do with. Certainly
the Sacculina lives successfully ; it is well adapted for its
own peculiar kind of life. For the life of a scale insect,
no better type of structure could be devised. A parasite
enjoys certain obvious advantages in life, and even extreme
degeneration is no drawback, but rather favors it in the
advantageousness of its sheltered and easy life. As long
as the host is successful in eluding its enemies and avoid-
ing accident and injury, the parasite is safe. It needs to
exercise no activity or vigilance of its own; its life is easy
as long as its host lives. But the disadvantages of para-
sitism and degeneration are apparent also. The fate of the
parasite is usually bound up with the fate of the host.
When the enemy of the host crab prevails, the Sacculina
goes down without a chance to struggle in its own defense.
But far more important than the disadvantage in such par-
ticular or individual cases is the disadvantage of the fact
that the parasite can not adapt itself in any considerable
degree to new conditions. It has become so specialized,
so greatly modified and changed to adapt itself to the one
set of conditions under which it now lives; it has gone so
far in its giving up of organs and body parts, that if pres-
ent conditions should change and new ones come to exist,
the parasite could not adapt itself tothem. The independ-
ent, active animal with all its organs and all its functions
intact, holds itself, one may say, ready and able to adapt
itself to any new conditions of life which may gradually
come into existence. The parasite has risked everything
for the sake of a sure and easy life under the presently
existing conditions. Change of conditions means its ex-
tinction.

106. Human degeneration.—It is not proposed in these
pages to discuss the application of the laws of animal life
to man. But each and every one extends upward, and can

200° >. ANIMAL LIFE

be traced in the relation of men and society. Thus, among
men as among animals, self-dependence favors complexity
of power. Dependence, parasitism, quiescence favor de-
. generation. . Degeneration means loss of complexity, the
narrowing of the range of powers and capabilities. It is
not necessarily a phase of disease or the precursor of death.
But as intellectual and moral excellence are matters associ-
ated with high development in man, dependence is unfa-
vorable to them.

Degeneration has been called animal pauperism. Pau-
perism in all its forms, whether due to idleness, pampering,
or misery, is human degeneration. It has been shown that
a large part of the criminality and pauperism among men
is hereditary, due to the survival of the tendency toward
living at the expense of others. The tendency to live with-
out self-activity passes from generation to generation.
Beggary is more profitable than unskilled and inefficient
labor, and our ways of careless charity tend to propagate
the beggar. That form of charity which does not render
its recipient self-helpful is an incentive toward degenera-
tion. Withdrawal from the competition of life, withdrawal
from self-helpful activity, aided by the voluntary or invol-
untary assistance of others—these factors bring about de-
generation. The same results follow in all ages and with
all races, with the lower animals as with men.

——— a

foe wt eee

pt: ve

CHAPTER XII

PROTECTIVE RESEMBLANCES, AND MIMICRY

107. Protective resemblance defined.—If a grasshopper
be startled from the ground, you may watch it and deter-
mine exactly where it alights after its leap or flight, and
yet, on going to the spot, be wholly unable to find it. The
colors and marking of the insect so harmonize with its sur-
roundings of soil and vegetation that it is nearly indistin-
guishable as long as it remains at rest. And if you were
intent on capturing grasshoppers for fish-bait, this resem-
blance in appearance to their surroundings would be very
annoying to you, while it would be a great advantage to
the grasshoppers, protecting some of them from capture and
death. This is protective resemblance. Mere casual obser-
vation reveals to us that such instances of protective resem-
blance are very common among animals. A rabbit or grouse
crouching close to the ground and remaining motionless
is almost indistinguishable. Green caterpillars lying out-
stretched along green grass-blades or on green leaves may
be touched before being recognized by sight. In arctic
regions of perpetual snow the polar bears, the snowy arctic
foxes, and the hares are all pure white instead of brown
and red and gray like their cousins of temperate and warm
regions. Animals of the desert are almost without excep-
tion obscurely mottled with gray and sand color, so as to
harmonize with their surroundings.

In the struggle for existence anything that may give
an animal an advantage, however slight, may be sufficient
to turn the scale in favor of the organism possessing the

201

202 ANIMAL LIFE

advantage. Such an advantage may be swiftness of move-
ment, or unusual strength or capacity to withstand unfa-
vorable meteorological conditions, or the possession of such
color and markings or peculiar shape as tend to conceal the
animal from its enemies or from its prey. Resemblances
may serve the purpose of aggression as well as protection.
In the case of the polar bears and other predaceous ani-
mals that show color likenesses to their surroundings, the
resemblance can better be called aggressive than protective.
The concealment afforded by the resemblance allows them
to steal unperceived on their prey. This, of course, is an
advantage to them as truly as escape from enemies would be.

We have already seen that by the action of natural
selection and heredity those variations or conditions that
give animals advantages in the struggle for life are pre-
served and emphasized. And so it has come about that
advantageous protective resemblances are very widespread
among animals, and assume in many cases extraordinarily
striking and interesting forms. In fact, the explanation
of much of the coloring and patterning of animals depends
on this principle of protective resemblance.

Before considering further the general conditions of
protective resemblances, it will be advisable to refer to
specific examples classified roughly into groups or special
kinds of advantageous colorings and markings.

108. General protective or aggressive resemblance,—As
examples of general protective resemblance—that is, a gen-
eral color effect harmonizing with the usual surroundings
and tending to hide or render indistinguishable the animal
—may be mentioned the hue of the green parrots of the
evergreen tropical forests; of the green tree-frogs and tree-
snakes which live habitually in the green foliage; of the
mottled gray and tawny lizards, birds, and small mam-
mals of the deserts; and of the white hares and foxes
and snowy owls and ptarmigans of the snow-covered arc-
tic regions. Of the same nature is the slaty blue of the

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204 ANIMAL LIFE

gulls and terns, colored like the sea. In the brooks most
fishes are dark olive or greenish above and white below.
To the birds and other enemies which look down on them
from above they are colored like the bottom. To their fish
enemies which look up from below, their color is like the
white light above them, and their forms are not clearly
seen. The fishes of the deep sea in perpetual darkness are

Fia. 126.—Alligator lizard (Gerrhonotus scincicauda) on granite rock. Photograph
by J. O. SNYDER, Stanford University, California.

inky violet in color below as well as above. Those that
live among sea-weeds are red, grass-green, or olive, like
the plants they frequent. General protective resemblance
is very widespread among animals, and is not easily appre-
ciated when the animal is seen in museums or zodlogical
gardens—that is, away from its natural or normal environ-
ment. A modification of general color resemblance found
in many animals may be called variable protective resem-
blance. Certain hares and other animals that live in
northern latitudes are wholly white during the winter when
the snow covers everything, but in summer, when much of
the snow melts, revealing the brown and gray rocks and

PROTECTIVE RESEMBLANCES, AND MIMICRY 905

withered leaves, these creatures change color, putting on
a grayish and brownish coat of hair. The ptarmigan of
the Rocky Mountains (one of the grouse), which lives on
the snow and rocks of the high peaks, is almost wholly
white in winter, but in summer when most of the snow is
melted its plumage is chiefly brown. On the campus at
Stanford University there is a little pond whose shores are
covered in some places with bits of bluish rock, in other
places with bits of reddish rock, and in still other places
with sand. A small insect called the toad-bug (Galgulus
oculatus) lives abundantly on the banks of this pond.
Specimens collected from the blue rocks are bluish in
color, those from the red rocks are reddish, and those from
the sand are sand-colored. Such changes of color to suit
the changing surroundings can be quickly made in the case
of some animals. The chameleons of the tropics, whose
skin changes color momentarily from green to brown,
blackish or golden, is an excellent example of this highly
specialized condition. The same change is shown by a
small lizard of our Southern States (Anolius), which from its
habit is called the Florida
chameleon. There is a lit-
tle fish (Oligocotius snyderi)
which is common in the tide
pools of the bay of Monterey,
in California, whose color
changes quickly to harmo-
nize with the different colors
of the rocks it happens to
rest above. Some of the tree-
frogs show this variable col-
oring. A very striking in- Fie. 127.—Chrysalid of swallow-tail but-
P - terfly (Papilio), harmonizing with the
stance of variable protective bavic oy which ié rests,
resemblance is shown by the
chrysalids of certain butterflies. An eminent English nat-
uralist collected many caterpillars of a certain species of

206 ANIMAL LIFE

butterfly, and put them, just as they were about to change
into pupe or chrysalids, into various boxes, lined with paper
of different colors. The color of the chrysalid was found

Fie. 128.—Chrysalid of butterfly (lower left-hand projection from stem), showing pro-
tective resemblance. Photograph from Nature. °

to harmonize very plainly with the color of the lining of
the box in which the chrysalid hung. It is a familiar fact
to entomologists that most butterfly chrysalids resemble in

PROTECTIVE RESEMBLANCES, AND MIMICRY 907

color and general external appearance the surface of the
object on which they rest (Figs. 127 and 128).

109. Special protective resemblance.—Far more striking
are those cases of protective resemblance in which the ani-
mal resembles in color and shape, sometimes in extraor-
dinary detail, some particular object or part of its usual
environment. Certain parts of the Atlantic Ocean are
covered with great patches of sea-weed called the gulf-weed
(Sargassum), and many kinds of animals—fishes and other
creatures—live upon and among the alge. No one can
fail to note the extraordinary color resemblances which exist
between those animals and the weed itself. The gulf-weed
is of an olive-yellow color, and the crabs and shrimps, a cer-
tain flat-worm, a certain mollusk, and a little fish, all of
which live among the Sargassum, are exactly of the same
shade of yellow as the weed, and have small white markings
on their bodies which are characteristic also of the Sargas-
sum. The mouse-fish or Sargassum fish and the little sea-
horses, often attached to the gulf- weed, show the same traits
of coloration (Fig. 129).. In the black rocks about Tahiti
is found the black nokee or lava-fish (HZmmydrichthys vul-
canus) (Fig. 66), which corresponds perfectly in color and
form to a piece of lava. This fish is also noteworthy for
having envenomed spines in the fin on its back. The
slender grass-green caterpillars of many moths and butter-
flies resemble very closely the thin grass-blades among
which they live. The larve of the geometrid moths, called
inch-worms or span-worms, are twig-like in appearance,
and have the habit, when disturbed, of standing out stiffly
from the twig or branch upon which they rest, so as to re-
semble in position as well as in color and markings a short
or a broken twig. One of the most striking resemblances
of this sort is shown by the large geometrid larva illus-
trated in Fig. 130, which was found near Ithaca, New York,
The body of this caterpillar has a few small, irregular spots
or humps, resembling very exactly the scars left by fallen

Fig. 129.—The mouse-fish (Pterophryne histrio) in the Sargassum or gulf-weed. The
fishes are marked and colored so as to be nearly indistinguishable from the masses
of the gulf-weed. In the lower right-hand corner of figure are two sea-horses, also
shaped and marked so as to be concealed.

PROTECTIVE RESEMBLANCES, AND MIMICRY 9209

buds or twigs. These caterpillars have a special muscular
development to enable them to hold themselves rigidly for

Wa

=

Fig. 130.—A geometrid larva ona branch. (The Fie. 131.—A walking-stick insect
larva is the upper right-hand projection from (Diapheromera’ femorata) on
the stem.) twig.

long times in this trying attitude. They also lack the
middle prop-legs of the body, common to other lepidopter-
19

210 : ANIMAL LIFE

ous larve, which would tend to destroy the illusion so
successfully carried out by them. The common walking-
stick (Diapheromera) (Fig. 131), with its wingless, greatly
elongate, dull-colored body, is an excellent example of spe-
cial protective resemblance. It is quite indistinguishable,
when at rest, from the twigs to which it is clinging. An-
other member of the family of insects to which the walk-
ing-stick belongs is the famous green-leaf insect (Phylliwm)
(Fig. 132). It is found in
South America and is of a
bright green color, with broad
leaf-like wings and body, with
markings which imitate the
leaf veins, and small irregu-
lar yellowish spots which
mimic decaying or stained
or fungus-covered spots in
the leaf.

There are many butter-
flies that resemble dead
leaves. All our common
meadow browns ((rapta),
brown and reddish butter-
flies with ragged-edged wings,
that appear in the autumn
F Me and flutter aimlessly about ex-

Fig. 132.—The green-leaf insect actly like the falling leaves,

(Phyllium). show this resemblance. But

most remarkable of all is a

large butterfly (Aallima) (Fig. 133) of the East Indian
region. The upper sides of the wings are dark, with
purplish and orange markings, not at all resembling a
dead leaf. But the butterflies when at rest hold their
wings together over the back, so that only the under sides
of the wings are exposed. The under sides of Kallima’s
wings are exactly the color of a dead and dried leaf, and

PROTECTIVE RESEMBLANCES, AND MIMICRY 911

the wings are so held that all combine to mimic with ex-
traordinary fidelity a dead leaf still attached to the twig by
a short pedicle or leaf-stalk imitated by a short tail on the

Fig. 133.—Kallima, the ‘‘ dead-leaf butterfly.”

hind wings, and showing midrib, oblique veins, and, most
remarkable of all, two apparent holes, like those made in
leaves by insects, but in the butterfly imitated by two small
circular spots free from scales and hence clear and trans-

212 ANIMAL LIFE

parent. With the héad and feelers concealed beneath the
wings, it makes the resemblance wonderfully exact.

There are numerous instances of special protective
resemblance among spiders. Many spiders (Fig. 134) that

Fie. 134.—Spiders showing unusual shapes and patterns, for purposes of
aggressive resemblance.

live habitually on tree trunks resemble bits of bark or small,
irregular masses of lichen. A whole family of spiders,
which live in flower-cups lying in wait for insects, are white
and pink and party-colored, resembling the markings of the
special flowers frequented by them. This is, of course, a

Fie. 135.—A pipe-fish (Phyllopteryx) resembling sea-weed, in which it lives.

special resemblance not so much for protection as for ag-
gression ; the insects coming to visit the flowers are unable
to distinguish the spiders and fall an easy prey to them.
110. Warning colors and terrifying appearances.—In the
cases of advantageous coloring and patterning so far dis-

PROTECTIVE RESEMBLANCES, AND MIMICRY . 913

cussed the advantage to the animal lies in the resemblance
between the animals and their surroundings, in the incon-
spicuousness and concealment afforded by the coloration.
But there is another interesting phase of advantageous
coloration in which the advantage derived is in render-
ing the animals as conspicuous and as readily recogniz-
able as possible. While many animals are very inconspicu-
ously colored, or are manifestly colored so as to resemble
their surroundings, generally or specifically, many other
animals are very brightly and conspicuously colored and
patterned. If we are struck by the numerous cases of imi-
tative coloring among insects, we must be no less impressed
by the many cases of bizarre and conspicuous coloration
among them.

Many animals, as we well know, possess special and
effective weapons of defense, as the poison-fangs of the
venomous snakes and the stings of bees and wasps. Other
animals, and with these cases most of us are not so well
acquainted, possess a means of defense, or rather safety, in
being inedible—that is, in possessing some acrid or ill-
tasting substance in the body which renders them unpala-
table to predaceous animals. Many caterpillars have been
found, by observation in Nature and by experiment, to be
distasteful to insectivorous birds. Now, it is obvious that
it would be a great advantage to these caterpillars if they
could be readily recognized by birds, for a severe stroke by
a bird’s bill is about as fatal to a caterpillar as being wholly
eaten. Its soft, distended body suffers mortal hurt if cut
or bitten by the bird’s beak. This advantage of being
readily recognizable is possessed by many if not all ill-
tasting caterpillars by being brilliantly and conspicuously
colored and marked. Such colors and markings are called
warning colors. They are intended to inform birds of the
fact that the caterpillar displaying them is an ill-tasting
insect, a caterpillar to be let alone. The conspicuously
black-and-yellow banded larva (Fig. 43, 0) of the common

914 ANIMAL LIFE

Monarch butterfly is a good example of the possession of
warning colors by distasteful caterpillars.

These warning colors are possessed not only by the ill-
tasting caterpillars, but by many animals which have spe-
cial means of defense. The wasps and bees, provided with
stings—dangerous animals to trouble—are almost all con-
spicuously marked with yellow and black. The lady-bird
beetles (Fig. 136), composing a whole family of small beetles

Fie. 136.—Two lady-bird beetles, conspicuously colored and marked.

which are all ill-tasting, are brightly and conspicuously col-
ored and spotted. The Gila monster (Heloderma), the only
poisonous lizard, differs from most other lizards in being
strikingly patterned with black and brown. Some of the
venomous snakes are conspicuously colored, as the coral
snakes (laps) or coralillos of the tropics. The naturalist
Belt, whose observations in Nicaragua have added much to
our knowledge of tropical animals, describes as follows an
interesting example of warning colors in a species of frog:
**In the woods around Santo Domingo (Nicaragua) there
are many frogs. Some are green or brown and imitate
green or dead leaves, and live among foliage. Others are
dull earth-colored, and hide in holes or under logs. All
these come out only at night to feed, and they are all
preyed upon by snakes and birds. In contrast with these
obscurely colored species, another little frog hops about in

PROTECTIVE RESEMBLANCES, AND MIMICRY 915

the daytime, dressed in a bright livery of red and blue.
He can not be mistaken for any other, and his flaming
breast and blue stockings show that he does not court con-
cealment. He is very abundant in the damp woods, and I
was convinced he was uneatable so soon as I made his
acquaintance and saw the happy sense of security with
which he hopped about. I took a few specimens home
with me, and tried my fowls and ducks with them, but
none would touch them. At last, by throwing down pieces
of meat, for which there was a great competition among
them, I managed to entice a young duck into snatching up
one of the little frogs. Instead of swallowing it, however,
it instantly threw it out of its mouth, and went about jerk-
ing its head, as if trying to throw off some unpleasant
taste.”

Certain animals which are without special means of
defense and are not at all formidable or dangerous are yet
so marked or shaped and so behave as to present a threat-
ening or terrifying appearance. The large green caterpil-
lars (Fig. 137) of the Sphinx moths—the tomato-worm is a
familiar one of these larvee—have a formidable-looking,

Fig. 137.—A ‘‘tomato-worm” larva of ‘the Sphinx moth, Phlegethontius carolina,
showing terrifying appearance.

sharp horn on the back of the next to last body ring.
When disturbed they lift the hinder part of the body, bear-
ing the horn, and move it about threateningly. As a mat-
ter of fact, the horn is not at all a weapon of defense, but is
quite harmless. Numerous insects when disturbed lift
the hind part of the body, and by making threatening mo-

Btn
piers

16 ANIMAL LIFE

tions lead enemies to believe that they possess a sting.
The striking eye-spots of many insects are believed by some
entomologists to be of the nature of terrifying appearances.
The larva (Fig. 138) of the Puss moth (Cerura) has been
often referred to as a striking example of terrifying appear-
ances. When one of these larve is disturbed, “it retracts
its head into the
first body ring in-
flating the mar-
gin, which is of a
bright red color.
There are two in-
tensely black spots
on this margin in the
appropriate position for
eyes, and the whole ap-
pearance is that of a large
flat face extending to the
outer edge of the red mar-
gin. The effect is an in-
=< tensely exaggerated cari-
Fie. 138.—Larva of the Puss moth (Cerura). cature of a vertebrate
Upper figure shows the larva as it appears face, which is probably
when undisturbed ; lower figure, when dis- :
turbed.—After PouLTon. alarming to the verte-
brate enemies of the cat-
erpillar. .. . The effect is also greatly strengthened by two
pink whips which are swiftly protruded from the prongs
of the fork in which the body terminates. . .. The end
of the body is at the same time curved forward over the
back, so that the pink filaments are brandished above the
head.”

111. Alluring coloration—A few animals show what are
called alluring colors—that is, they display a color pattern
so arranged as to resemble or mimic a flower or other lure,
and thus to entice to them other animals, their natural prey. -
This is a special kind of aggressive resemblance. A species

PROTECTIVE RESEMBLANCES, AND MIMICRY 917

of predatory insect called a “ praying-horse” (allied to the
genus Mantis), found in India, has the shape and color of
an orchid. Small insects are attracted and fall a prey to it.
Certain Brazilian fly-catching birds have a brilliantly colored
crest which can be displayed in the shape of a flower-cup.
The insects attracted by the apparent flower furnish the fly-
catcher with food. An Asiatic lizard is wholly colored like
the sand upon which it lives except for a peculiar red fold
of skin at each angle of the mouth. This fold is arranged
in flower-like shape, ‘“‘ exactly resembling a little red flower
which grows in the sand.” Insects attracted by these
flowers find out their mistake too late. In the tribe of
fishes called the “ anglers” or fishing frogs the front rays
of the dorsal fin are prolonged in shape of long, slender fila-
ments, the foremost and longest of which has a flattened
and divided extremity like the bait on a hook. The fish
conceals itself in the mud or in the cavities of a coral reef
and waves the filaments back and forth. Small fish are at--
tracted by the lure, mistaking it for worms writhing about
in the water or among the weeds. As they approach they
are ingulfed in the mouth of the angler, which in some of
the species is of enormous size. One of these species is
known to fishermen as the “all-mouth.” These fishes
(Lophius piscatorius), which live in the mud, are colored
like mud or clay. Other forms of anglers, living among
coral reefs, are brown and red (Antennarius), their colora-
tion imitating in minutest detail the markings and out-
growths on the reef itself, the lure itself imitating a worm
of the reef. In a certain group of deep-sea anglers, the sea-
devils (Ceratiide), certain species show a still further spe-
cialization of the curious fishing-rod. In one species (Co-
rynolophus reinhardti) (Fig. 54), living off the coast of
Greenland at a depth of upward of a mile, the fishing-rod
or first dorsal spine has a luminous bulb at its tip around
which are fleshy, worm-like streamers. At the abyssal
depths of a mile, more or less, frequented by these sea-

218 ANIMAL LIFE

devils there is no light, the inky darkness being absolute.
This shining lure is therefore a most effective means of
securing food.

112. Mimicry.—Although the word mimicry could often
have been used aptly in the foregoing account of protective
resemblances, it has been reserved for use in connection,
with a certain specific group of cases. It has been reserved
to be applied exclusively to those rather numerous instances
where an otherwise defenseless animal, one without poison-
fangs or sting, and without an ill-tasting substance in its
body, mimics some other specially defended or inedible ani-
mal sufficiently to be mistaken for it and so to escape
attack. Such cases of protective resemblance are called
true mimicry, and they are especially to be observed among
insects.

In Fig. 139 are pictured three familiar American butter-
flies. One of these, the Monarch butterfly (Anosia plexip-
pus), is perhaps the most abundant and widespread butter-
fly of our country. It is a fact well known to entomologists
that the Monarch is distasteful to birds and is let alone by
them. It is a conspicuous butterfly, being large and chiefly
of a red-brown color. The Viceroy butterfly (Basilarchia
archippus), also red-brown and much like the Monarch, is
not, as its appearance would seem to indicate, a very near
relative of the Monarch, belonging to the same genus, but
on the contrary it belongs to the same genus with the third
butterfly figured, the black and white Basilarchia. All the
butterflies of the genus Basilarchia are black and white
except this species, the Viceroy, and one other. The Vice-
roy is not distasteful to birds; it is edible, but it mimics the
inedible Monarch so closely that the deception is not de-
tected by the birds, and so it is not molested.

In the tropics there have been discovered numerous
similar instances of mimicry by edible butterflies of inedi-
ble kinds. The members of two great families of butterflies
(Danaide and Heliconide) are distasteful to birds, and are

Fie. 139.—The mimicking of the inedible Monarch butterfly by the edible Viceroy.
Upper figure is the Monarch (Anosia plexippus); middle figure is the Viceroy
(Basilarchia archippus); lowest figure is another member of the same genus
(Basilarchia), to show the usual color pattern of the species of the genus.

220 ANIMAL LIFE

mimicked by members of the other butterfly families (espe-
cially the Pieride), to which family our common white
cabbage-butterfly belongs, and by the swallow-tails (Papi-
lionide).

The bees and wasps are protected by their stings. They
are usually conspicuous, being banded with yellow and black.
They are mimicked by numerous other insects, especially
moths and flies, two defenseless kinds of insects. This
mimicking of bees and wasps by flies is very common, and
can be observed readily at any flowering shrub. The flower-
flies (Syrphidz), which, with the bees, visit flowers, can be
distinguished from the bees only by sharp observing. When
these bees and flies can be caught and examined in hand, it
will be found that the flies have but two wings while the
bees have four.

A remarkable and interesting case of mimicry among
insects of different orders is that of certain South Ameri-
can tree-hoppers (of the family Membracide, of the order
Hemiptera), which mimic the famous leaf-cutting ant
(Sauba) of the Amazons (Fig. 140). These ants have the
curious habit of cutting off, with their sharp jaws, bits of
green leaves and carry-
ing them to their nests.
In carrying the bits of
leaves the ants hold them
vertically above their
heads. The leaf-hoppers
= 24S — mimic the ants and their
Fig. 140.—Tyee-hopper (Membracid), which burdens with remarka-

mimics the ee ant (Sauba) of Bra- ble exactitude b having

ce right-hand insect is the tree- aS ae Be Pe : ody aia:

vated in the form of a

thin, jagged-edged ridge no thicker than a leaf. This part

of the body is green like the leaves, while the under part
of the body and the legs are brown like the ants.

Some examples of mimicry among other animals than

PROTECTIVE RESEMBLANCES, AND MIMICRY 991

insects are known, but not many. The conspicuously
marked venomous coral-snake or coralillos (laps) is mim-
icked by certain non-venomous snakes called king-snakes ©
(Lampropeltis, Osceola). The pattern of red and black
bands surrounding the cylindrical body is perfectly imi-
tated. But whether this is true mimicry brought about
for purposes of protection may be doubted. Instances
among birds have been described, and a single case has
been recorded in the class of mammals. But it is among
the insects that the best attested instances occur. The
simple fact of the close resemblance of two widely related
animals can not be taken to prove the existence of mimicry.
Two animals may both come to resemble some particular
part in their common environment and thus to resemble
closely each other. Here we have simply two instances
of special protective resemblance, and not an instance of
mimicry. The student of zodlogy will do well to watch
sharply for examples of protective resemblance or mimicry,
for but few of the instances that undoubtedly exist are as
yet known.

113. Protective resemblances and mimicry most common
among insects—The large majority of the preceding exam-
ples have been taken from among the insects. This is
explained by the fact that the phenomena of protective
resemblances and mimicry have been studied especially
among insects; the theory of mimicry was worked out
chiefly from the observation and study of the colors and
markings of insects and of the economy of insect life.
Why protective resemblances and mimicry among insects
have been chiefly studied is because these conditions are
specially common among insects. The great class Insecta
includes more than two thirds of all the known living
species of animals. The struggle for existence among the
insects is especially severe and bitter. All kinds of “shifts
for a living” are pushed to extremes; and as insect colors
and patterns are especially varied and conspicuous, it is

229 ANIMAL LIFE

only to be expected that this useful modification of colors
and patterns, that results in the striking phenomena of
special protective resemblances and mimicry, should be
specially widespread and pronounced among insects. More-
over, they are mostly deficient in other means of defense,
and seem to be the favorite food for many different kinds
of animals. Protective resemblance is their best and most
widely adopted means of preserving life.

114. No volition in mimicry.—The use of the word mim-
icry has been criticised because it suggests the exercise of
volition or intent on the part of the mimicking animal.
The student should not entertain this conception of mim-
icry. In the use of “mimicry” in connection with the
phenomena just described, the biologist ascribes to it a
technical meaning, which excludes any suggestion of voli-
tion or intent on the part of the mimic. Just how such
extraordinary and perfect cases of mimicry as shown by
Phyllium and Kallima have come to exist is a problem
whose solution is not agreed on by naturalists, but none of
them makes volition—the will or intent of the animal—any
part of his proposed solution. Each case of mimicry is the
result of a slow and gradual change, through a long series
of ancestors. The mimicry may indeed include the adop-
tion of certain habits of action which strengthen and make
more pronounced the deception of shape and color. But
these habits, too, are the result of a long development, and
are instinctive or reflex—that is, performed without the
exercise of volition or reason.

115. Color; its utility and beauty.—The causes of color,
and the uses of color in animals and in plants are subjects
to which naturalists have paid and are paying much atten-
tion. The subject of “protective resemblances and mim-
icry” is only one, though one of the most interesting,
branches or subordinate subjects of the general theory of
the uses of color. Other uses are obvious. Bright colors
and markings may serve for the attraction of mates; thus

PROTECTIVE RESEMBLANCES, AND MIMICRY 993

are explained by some naturalists the brilliant plumage of
the male birds, as in the case of the bird-of-paradise and
the pheasants. Or they may serve for recognition charac-
ters, enabling the individuals of a band of animals readily
to recognize their companions; the conspicuous whiteness
of the short tail of the antelopes and cotton-tail rabbits,
the black tail of the black-tail deer, and the white tail-
feathers of the meadow-lark, are explained by many natu-
ralists on this ground. Recognition marks of this type
are especially numerous among the birds, hardly a species
being without one or more of them, if their meaning is cor-
rectly interpreted. The white color of arctic animals may
be useful not alone in rendering them inconspicuous, but
may serve also a direct physiological function in preventing
the loss of heat from the body by radiation. And the dark
colors of animals may be of value to them in absorbing heat
rays and thus helping them to keep warm. But “by far
the most widespread use of color is to assist an animal in
escaping from its enemies or in capturing its prey.”

The colors of an animal may indeed not be useful to
it at all. Many color patterns exist on present-day birds
simply because, preserved by heredity, they are handed
down by their ancestors, to whom, under different condi-
tions of life, they may have been of direct use. For the
most part, however, we can look on the varied colors and
the striking patterns exhibited by animals as being in some
way or another of real use and value. We can enjoy the
exquisite coloration of the wings of a butterfly none the
less, however, because we know that these beautiful colors
and their arrangement tend to preserve the life of the
dainty creature, and have been produced by the operation
of fixed laws of Nature working through the ages.

CHAPTER XIII

THE SPECIAL SENSES

116. Importance of the special senses—The means by
which animals become acquainted with the outer world
are the special senses, such as feeling, tasting, smelling,
hearing, and seeing. The behavior of animals with regard
to their surroundings, with regard to all the world outside
of their own body, depends upon what they learn of this
outer world through the exercise of these special senses.
Habits are formed on the basis of experience or knowledge
of the outer world gained by the special senses, and the
development of the power to reason or to have sense de-
pends on their pre-existence.

117. Difficulty of the study of the special senses—We are
accustomed to think of the organs of the special senses as
extremely complex parts of the body, and this is certainly
true in the case of the higher animals. In our own body
the ears and eyes are organs of most specialized and highly
developed condition. But we must not overlook the fact
that the animal kingdom is composed of creatures of widely
varying degrees of organization, and that in any considera-
tion of matters common to all animals those animals of
simplest and most lowly organization must be studied as
well as those of high development. The study of the spe-
cial senses presents two phases, namely, the study of the
structure of the organs of special sense, and the study of
the physiology of special sense—that is, the functions of
these organs. It will be recognized that in the study of

how other animals feel and taste and smell and hear and
224

.

THE SPECIAL SENSES 225

see, we shall have to base all our study on our own experi-
ence. We know of hearing and seeing only by what we
know of our own hearing and seeing; but by examination
of the structure of the hearing and seeing organs of cer-
tain other animals, and by observation and experiments,
zoologists are convinced that some animals hear sounds
that we can not hear, and some see colors that we can
not see.

While that phase of the study of the special senses
which concerns their structure may be quite successfully
undertaken, the physiological phase of the study of the
actual tasting and seeing and hearing of the lower animals
is a matter of much difficulty. The condition and char-
acter of the special senses vary notably among different
animals. There may even exist other special senses than
the ones we possess. Some zodlogists believe that certain
marine animals possess a “density or pressure sense ”—
that is, a sense which enables them to tell approximately
how deep in the water they may be at any time. To
certain animals is ascribed a “temperature sense,” and
some zodlogists believe that what we call the homing in-
stinct of animals as shown by the homing pigeons and
honey-bees and other animals, depends on their possession
of a special sense which man does not possess. Recent
experiments, however, seem to show that the homing of
pigeons depends on their keen sight. In numerous animals
there exist, besides the organs of the five special senses
which we possess, organs whose structure compels us to be-
lieve them to be organs of special sense, but whose func-
tion is wholly unknown to us. Thus in the study of the
special senses we are made to see plainly that we can not
rely simply on our knowledge of our own body structure
for an understanding of the structure and functions of
other animals.

118. Special senses of the simplest animals.—In the Amba
(see Chapter I), that type of the simplest animals, with

16

296 ANIMAL LIFE

one-celled body, without organs, and yet with its capacity
for performing the necessary life processes, there are no
special senses except one (perhaps two). The Ame@da can
feel. It possesses the tactile sense. And there are no
special sense organs except one, which is the whole of the
outer surface of the body. If the Ameda be touched with
a fine point it feels the touch, for the soft viscous proto-
plasm of its body flows slowly away from the foreign ob-
ject. The sense of feeling or touch, the tactile sense, is
the simplest or most primitive of the special senses, and
the simplest, most primitive organ of special sense is the
outer surface or skin of the body. Among those simple
animals that possess the simplest organs of hearing and
perceiving light, we shall find these organs to be simply
specialized parts of the skin or outer cell layer of the
body, and it is a fact that all the special sense organs of
all animals are derived or developed from the outer cell
layer, ectoblast, of the embryo. This is true also of the
whole nervous system, the brain and spinal cord of the
vertebrates, and the ganglia and nerve commissures of
the invertebrates. And while in the higher animals the
nervous system lies underneath the surface of the body,
in many of the lower, many-celled animals all the ganglia
and nerves, all of the nervous system, lie on the outer
surface of the body, being simply a specialized part of
the skin.

119. The sense of touch—In some of the lower, many-
celled animals, as among the polyps, there are on the skin
certain sense cells, either isolated or in small groups, which .
seem to be stimulated not alone by the touching of foreign
substances, but also by warmth and light. They are not
limited to a single special sense. They are the primitive
or generalized organs of special sense, and can develop into
specialized organs for any one of the special senses.

The simplest and most widespread of these special
senses with, as a whole, the simplest organs, is the tactile

THE SPECIAL SENSES 227

sense, or the sense of touch. The special organs of this
sense are usually simple hairs or papilla connecting with a
nerve. These tactile hairs or papille may be distributed
pretty evenly over most of the body, or may be mainly con-
centrated upon certain parts in crowded groups. Many of
the lower animals have projecting parts, like the feeling
tentacles of many marine invertebrates, or the antennz
(feelers) of crabs and insects, which are the special seat
of the tactile organs. Among the vertebrates the tactile
organs are either like those of the invertebrates, or are
little sac-like bodies of connective tissue in which the
end of a nerve is curiously folded and convoluted (Fig.
141). These little touch corpuscles simply lie in the cell
layer of the skin, covered over thinly by the cuticle. Some-
times they are simply free, branched
nerve-endings in the skin. These
tactile corpuscles or free nerve-end-
ings are especially abundant in those
parts of the body which can be best
used for feeling. In man the fin-
ger-tips are thus especially supplied ;
in certain tailed monkeys the tip of
the tail, and in hogs the end of the
snout. The difference in abundance
of these tactile corpuscles of the skin
can be readily shown by experiment.

With a pair of compasses, whose "4, ‘4)— Tactile papilla of
points have been slightly blunted, — after KozzurKen.
touch the skin of the forearm of a

person who has his eyes shut, with the points about tind
inches apart and in the direction of the length of the arm.
The person touched will feel the points as two. Repeat
the touching several times, gradually lessening the dis-
tance between the points. When the points are not more
than an inch to an inch and a half apart, the person
touched will feel but a single touch—that is, the touching

228 ANIMAL LIFE

of both points will give the sensation of but a single con-
tact. Repeat the experiment on the tip of the forefinger,
and both points will be felt until the points are only about
one tenth of an inch apart.

120. The sense of taste—The sense of taste enables us to
test in some degree the chemical constitution of substances
which are taken into the mouth as food. We discriminate
by the taste organs between good food and bad, well-tasting
and ill-tasting. These organs are, with us and the other air-
breathing animals, located in the mouth or on the mouth
parts. They must be located so as to come into contact
with the food, and it is also necessary that the food sub-
stance to be tasted be made liquid. This is accomplished
by the fluids poured into the mouth from the salivary
glands. With the lower aquatic animals it is not improb-
able that taste organs are situated on other parts of the
body besides the mouth, and that taste is used not only to
test food substances, but also to test the chemical char-
acter of the fluid medium in which they live.

The taste organs are much like the tactile organs, except
that the ending of the nerve is exposed, so that small par-
ticles of the substance to be tasted can come into actual
contact with it.. The nerve-ending is usually in a small
raised papilla or depressed pit. In the simplest animals
there is no special organ of taste, and yet Ameba and
other Protozoa show that they appreciate the chemical con-
stitution of the liquid in which they lie. They taste—that
is, test the chemical constitution of the substances—by
means of their undifferentiated body surface. The taste
organs are not always to be told from the organs of smell.
Where an animal has a certain special seat of smell, like
the nose of the higher animals, then the special sense
organs of the mouth can be fairly assumed to be taste
organs; but where the seat of both smell and taste is in
the mouth or mouth parts, it is often impossible to distin-
guish between the two kinds of organs.

THE SPECIAL SENSES 229

In mammals taste organs are situated on certain parts of
the tongue, and have the form of rather large, low, broad
papille, each bearing many small taste-buds (Fig. 142).
In fishes similar papillee and buds have been found in vari-
ous places on the sur-
face of the body, from
which it is believed that
the sense of taste in
fishes is not limited to
the mouth. In insects
the taste-papille and
taste - pits are grouped
in certain places on the Fie. 142.—Vertical section of large papilla on
mouth parts, bein g es- gets of a calf; ¢.0., taste-buds. — After
pecially abundant on
the tips of small, segmented, feeler-like processes called
palpi, which project from the under lip and from the so-
called maxille.

121. The sense of smell.— Smelling and tasting are closely
allied, the one testing substances dissolved, the other test-
ing substances vaporized. The organs of the sense of
smell are, like those of taste, simple nerve-endings in papil-
le or pits. The substance to be smelled must, however,
be in a very finely divided form; it must come to the or-
gans of smell as a gas or vapor, and not, as to the organs of
taste, in liquid condition. The organs of smell are situated
usually on the head, but as the sense of smell is used not
alone for the testing of food, but for many other purposes,
the organs of smell are not, like those of taste, situated
principally in or near the mouth. Smell is a special sense
of much wider range of use than taste. By smell animals
can discover food, avoid enemies, and find their mates.
They can test the air they breathe as well as the food they
eat. In the matter of the testing of food the senses of
both taste and smell are constantly used, and are indeed
intimately associated.

230 ANIMAL LIFE

The sense of smell varies a great deal in its degree of
development in various animals. With the strictly aquatic
animals—and these include most of the lower invertebrates,
as the polyps, the star-fishes, sea-urchins, and most of the
worms and mollusks—the sense of smell is probably but
little developed. There is little opportunity for a gas or
vapor to come to these animals, and only as a gas or vapor
can a substance be smelled. With these animals the sense
of taste must take the place of the olfactory sense. But
among the insects, mostly terrestrial animals, there is an
extraordinary development of the sense of smell. It is in-
deed probably their principal special sense. Insects must
depend on smell far more than on sight or hearing for
the discovery of food, for becoming
aware of the presence of their enemies
and of the proximity of their mates
and companions. The organs of
smell of insects are situated princi-
pally on the antenne or feelers, a
single pair of which is borne on the
head of every insect (Fig. 143). That —
many insects have an extraordinarily
keen sense of smell has been shown
by numerous experiments, and is con-
stantly proved by well-known habits.
If a small bit of decaying flesh be in-
closed in a box so that it is wholly
Fie. 143.—Antenna of aleat. Concealed, it will nevertheless soon

eating beetle, showing be found by the flies and carrion
ocdetin didi beetles that either feed on carrion
or must always lay their eggs in de-

caying matter so that their carrion-eating larvee may be
provided with food. It is believed that ants find their
way back to their nests by the sense of smell, and that
they can recognize by scent among hundreds of individ-
uals taken from various communities the members of their

THE SPECIAL SENSES 231

own community. In the insectary at Cornell University,
a few years ago, a few females of the beautiful promethea
moth (Callosamia promethea) were inclosed in a box,
which was kept inside the insectary building. No males
had been seen about the insectary nor in its immediate
vicinity, although they had been sought for by collectors.
A few hours after the beginning of the captivity of the
female moths there were forty male prometheas fluttering
about over the glass roof of the insectary. They could not

Fig. 144.—Promethea moth, male, showing specialized antenne.

see the females, and yet had discovered their presence in
the building. The discovery was undoubtedly made by the
sense of smell. These moths have very elaborately devel-
oped antenne (Fig. 144), finely branched or feathered,
affording opportunity for the existence of very many smell-
ing-pits.

The keenness of scent of hounds and bird dogs is famil-
iar to all, although ever a fresh source of astonishment as
we watch these animals when hunting. We recently
watched a retriever dog select unerringly, by the sense of
smell, any particular duck out of a pile of a hundred. In

239 ANIMAL LIFE

the case of man the sense of smell is not nearly so well
developed as among many of the other vertebrates. This
inferiority is largely due to degeneration through lessened
need; for in Indians and primitive races the sense of
smell is keener and better developed than in civilized
races. Where man has to make his living by hunting, and
has to avoid his enemies of jungle and plain, his special
senses are better developed than where the necessity of
protection and advantage by means of such keenness of
scent and hearing is done away with by the arts of civi-
lization.

122. The sense of hearing.—Hearing is the perception
of certain vibrations of bodies. These vibrations give rise
to waves—sound waves as they are called—which proceed
from the vibrating body in all directions, and which, com-
ing to an animal, stimulate the special auditory or hearing
organs, that transmit this stimulation along the auditory
nerve to the brain, where it is translated as sound. These
sound waves come to animals usually through the air, or,
in the case of aquatic animals, through water, or through
both air and water.

The organs of hearing are of very complex structure
in the case of man and the higher vertebrates. Our ears,
which are adapted for perceiving or being stimulated by
vibrations ranging from 16 to 40,000 a second—that is, for
hearing all those sounds produced by vibrations of a rapid-
ity not less than 16 to a second nor greater than 40,000 to
a second—are of such complexity of structure that many
pages would be required for their description. But among
the lower or less highly organized animals the ears, or au-
ditory organs, are much simpler.

In most animals the auditory organs shoe the common
characteristic of being wholly composed of, or having as
an essential part, a small sac filled with liquid in which
one or more tiny spherical hard bodies called ofoliths are
held. This auditory sac is formed of or lined internally by

THE SPECIAL SENSES 233

auditory cells, specialized nerve cells, which often bear
delicate vibratile hairs (Fig. 145). Auditory organs of this
general character are known among the polyps, the worms,
- the crustaceans, and the mollusks. In the common cray-
fish the “ears” are situated in the basal segment of the
inner antenne or feelers (Fig. 146). They consist each of
a small sac filled with liquid in which
are suspended several grains of sand
or other hard bodies. The inner

Fie. 145.—Auditory organ of a mollusk. @, audi- Fie. 146.— Antenna of

tory nerve; 6, outer wall of connective tissue ; cray - fish, with audi-
ec, cells with auditory hairs; d, otolith.—After tory sac at base.—
LEyDpie. After HUXLEY.

surface of the sac is lined with fine auditory hairs. The
sound waves coming through the air or water outside strike
against this sac, which lies in a hollow on the upper or
outer side of the antennsz. The sound waves are taken up
by the contents of the sac and stimulate the fine hairs,
which in turn give this stimulus to the nerves which run
from them to the principal auditory nerve and thus to the
brain of the cray-fish. Among the insects other kinds of
auditory organs exist. The common locust or grasshopper

234 ANIMAL LIFE

has on the upper surface of the first abdominal segment
a pair of tympana or ear-drums (Fig. 147), composed sim-
ply of the thinned, tightly stretched chitinous
cuticle of the body. On the inner surface of this

Fie. 147.—Grasshopper, showing auditory organ (a. 0.) in first segment of abdomen.
(Wings of one side removed.)

ear-drum there are a tiny auditory sac, a fine nerve lead-

ing from it to a small auditory ganglion lying near the

tympanum, and a large nerve leading from this ganglion

to one of the larger ganglia situated on the floor of the

Fie. 148.—A cricket, showing auditory organ (a. 0.) in fore-leg.

thorax. In the crickets and katydids, insects related to
the locusts, the auditory organs or ears are situated in the
fore-legs (Fig. 148).

Certain other insects, as the mosquitoes and other midges

THE SPECIAL SENSES 235

or gnats, undoubtedly hear by means of numerous delicate
hairs borne on the antenne. The male mosquitoes (Fig.
149) have many hundreds of these long, fine antennal hairs,
and on the sounding of a tuning-fork these hairs have been
observed to vibrate strongly. In the base of each antenna
there is a most elaborate organ,
composed of fine chitinous
rods, and accompanying nerves
and nerve cells whose function
it is to take up and transmit
through the auditory nerve to
the brain the stimuli received
from the external auditory
hairs.

123. Sound -making. — The
sense of hearing enables ani-
mals not only to hear the
warning natural sounds of
storms and falling trees and
plunging avalanches, but the
sounds made by each other.
Sound-making among animals
serves to aid in frightening
away enemies or in warning atts i ren re tyes ae
companions of their approach, __ tenne.
for recognition among mates
and members of a band or species, for the attracting and
wooing of mates, and for the interchange of information.
With the cries and roars of mammals, the songs of birds,
and the shrilling and calling of insects all of us are familiar.
These are all sounds that can be heard by the human ear.
But that there are many sounds made by animals that
we can not hear—that is, that are of too high a pitch for
our hearing organs to be stimulated by—is believed by nat-
uralists. Especially is this almost certainly true in the case
of the insects. The peculiar sound-producing organs of

236 ANIMAL LIFE

many sound-making insects are known; but certain other
insects, which make no sound that we can hear, neverthe-
less possess similar sound-making organs.

Sound is produced by mammals and birds by the strik-
ing of the air which goes to and comes from the lungs
against certain vibratory cords or flaps in the air-tubes.
Sounds made by this vibration are re-enforced and made
louder by arrangements of the air-tubes and mouth for
resonance, and the character or quality of the sound is
modified at will to a greater or less degree by the lips and
teeth and other mouth structures. Sounds so made are
said to be produced by a voice, or animals making sounds
in this way are said to possess a voice. Animals possessing
a voice have far more range and variety in their sound-
making than most of the animals which produce sounds in
other ways. The marvelous variety and the great strength
of the singing of birds and of the cries and roars of mam-
mals are unequaled by the sounds of any other animals.

But many animals without a voice—that is, which do not
make sounds from the air-tubes—make sounds, and some
of them, as certain insects, show much variety and range
in their singing. The sounds of insects are made by the
rapid vibrations of the wings, as the humming or buzzing
of bees and flies, by the passage of air out or into the body
through the many breathing pores or spiracles (a kind
of voice), by the vibration of a stretched membrane or
tympanum, as the loud shrilling of the cicada, and most
commonly by stridulation—that is, by rubbing together
two roughened parts of the body. The male crickets and
the male katydids rub together the bases of their wing
covers to produce their shrill singing. The locusts or
grasshoppers make sounds when at rest by rubbing the
roughened inside of their great leaping legs against the
upper surface of their wing covers, and when in flight by
striking the two wings of each side together. Numerous
other insects make sounds by stridulation, but many of

THE SPECIAL SENSES 237

these sounds are so feeble or so high in pitch that they are
rarely heard by us. Certain butterflies make an odd click-
ing sound, as do some of the water-beetles. In Japan,
where small things which are beautiful are prized not less
than large ones, singing insects are kept in cages and
highly valued, so that their capture becomes a lucrative
industry, just as it is with song birds in Europe and Amer-
ica. Among the many species of Japanese singing insects
is a night cricket, known as the bridle-bit insect, because
its note resembles the jingling of a bridle-bit.

124. The sense of sight.—Not all animals have eyes.
The moles which live underground, insects, and other ani-
mals that live in caves, and the deep-sea fishes which live
in waters so deep that the light of the sun never comes
to them, have no eyes at all, or have eyes of so rudimentary
a character that they can no longer be used for seeing.
But all these eyeless animals have no eyes because they
live under conditions where eyes are useless. They have
lost their eyes by degeneration. There are, however, many
animals that have no eyes, nor have they or their ancestors
ever had eyes. These are the simplest, most lowly organ-
ized animals. Many, perhaps all eyeless animals are, how-
ever, capable of distinguishing light from darkness. They
are sensitive to light. An investigator placed several indi-
viduals of the common, tiny fresh-water polyp (Hydra) in a
glass cylinder the walls of which were painted black. He
left a small part of the cylinder unpainted, and in this part
- of the cylinder where the light penetrated the Hydras all
gathered. The eyeless maggots or larve of flies, when
placed in the light will wriggle and squirm away into dark
crevices. They are conscious of light when exposed to it,
and endeavor to shun it. Most plants turn their leaves
toward the light; the sunflowers turn on their stems to
face the sun. Light seems to stimulate organisms whether
they have eyes or not, and the organisms either try to get
into the light or to avoid it. But this is not seeing.

238 ANIMAL LIFE

The simplest eyes, if we may call them eyes, are not
capable of forming an image or picture of external objects.
They only make the animal better capable of distinguish-
ing between light and darkness or shadow. Many lowly
organized animals, as some polyps, and worms, have certain
cells of the skin specially provided with pigment. These
cells grouped together form what is called a pigment fleck,
which can, because of the presence of the pigment, absorb
more light than the skin cells, and are more sensitive to
the light. By such pigment-flecks, or eye-spots, the animal
can detect, by their shadows, the passing near them of moy-
ing bodies, and thus be in some measure informed of the
approach of enemies or of prey. Some of these eye-flecks
are provided, not simply with pigment, but with a simple
sort of lens that serves to concentrate rays of light and
make this simplest
sort of eye even
more sensitive to
changes in the in-
tensity of light
(Fig. 150).

Most of the
many -celled ani-
mals possess eyes
by means of which
a picture of exter-
nal objects more or less nearly complete and perfect can
be formed. There is great variety in the finer structure .
of these picture-forming eyes, but each consists essentially
of an inner delicate or sensitive nervous surface called the
retina, which is stimulated by light, and is connected with
the brain by a large optic nerve, and of a transparent light-
refracting lens lying outside of the retina and exposed to
the light. These are the constant essential parts of an
image-forming and image-perceiving eye. In most eyes
there are other accessory parts which may make the whole

Fig. 150.—The simple eye of a jelly-fish (Lizzia
koellikeri).—After O. and R. HERTWIe.

THE SPECIAL SENSES 239

eye an organ of excessively complicated structure and of
remarkably perfect seeing capacity. Our own eyes are
organs of extreme structural complexity and of high de-
velopment, although some of the other vertebrates have
undoubtedly a keener and more nearly perfected sight.
The crustaceans and insects have eyes of a peculiar
character called compound eyes. In addition most insects
have smaller simple eyes. Each of the compound eyes is
composed of many (from a few, as in certain ants, to as
many as twenty-five thousand, as in certain beetles) eye ele-
ments, each eye element seeing independently of the other
eye elements and seeing only a very small part of any ob-
ject in front of the whole eye. All these small parts of
the external object seen by the many distinct eye elements
are combined so as to form an image in mosaic—that is,
made up of separate small parts—of the external object.
If the head of a dragon-fly be exam-
ined, it will be seen that
two thirds or more of the

F Fig. 152.—Some of the facets
Fie. 151.—A dragon-fly, showing the large com- of the compound eye of a
pound eyes on the head. dragon-fly.

whole head is made up of the two large compound eyes
(Fig. 151), and with a lens it may be seen that the outer
surface of each of these eyes is composed of many small
spaces or facets (Fig. 152) which are the outer lenses of
the many eye elements composing the whole eye.

CHAPTER XIV

INSTINCT AND REASON

125. Irritability—All animals of whatever degree of
organization show in life the quality of irritability or re-
sponse to external stimulus. Contact with external things
produces some effect on each of them, and this effect is
something more than the mere mechanical effect on the
matter of which the animal is composed. In the one-
celled animals the functions of response to external stimu-
lus are not localized. They are the property of any part of
the protoplasm of the body. Just as breathing or digestion
is a function of the whole cell, so are sensation and response
in action. In the higher or many-celled animals each of
these functions is specialized and localized. A certain set
of cells is set apart for each function, and each organ or
series of cells is released from all functions save its own.

126. Nerve cells and fibers—In the development of the
individual animal certain cells from the primitive external
layer or ectoblast of the embryo are set apart to preside
over the relations of the creature to its environment.
These cells are highly specialized, and while some of them
are highly sensitive, others are adapted for carrying or
transmitting the stimuli received by the sensitive cells, and
still others have the function of receiving sense-impressions
and of translating them into impulses of motion. The
nerve cells are receivers of impressions. These are gathered
together in nerve masses or ganglia, the largest of these
being known as the brain, the ganglia in general being
known as nerve centers. The nerves are of two classes.

240

INSTINCT AND REASON 241

The one class, called sensory nerves, extends from the skin
or other organ of sensation to the nerve center. The nerves
of the other class, motor nerves, carry impulses to motion.

127. The brain or sensorium.—The brain or other nerve
center sits in darkness surrounded by a bony protecting
box. To this main nerve center, or sensorium, come the
nerves from all parts of the body that have sensation,
the external skin as well as the special organs of sight,
hearing, taste, smell. With these come nerves bearing sen-
sations of pain, temperature, muscular effort—all kinds of
sensation which the brain can receive. These nerves are
the sole sources of knowledge to any animal organism.
Whatever idea its brain may contain must be built up
through these nerve impressions. The aggregate of these
impressions constitute the world as the organism knows it.
All sensation is related to action. If an organism is not
to act, it can not feel, and the intensity of its feeling is
related to its power to act.

128. Reflex action—These impressions brought to the
brain by the sensory nerves represent in some degree the
facts in the animal’s environment. They teach something
as to its food or its safety. The power of locomotion is
characteristic of animals. If they move, their actions must
depend on the indications carried to the nerve center from
the outside; if they feed on living organisms, they must
seek their food; if, as in many cases, other living organ-
isms prey on them, they must bestir themselves to escape.
The impulse of hunger on the one hand and of fear on the
other are elemental. The sensorium receives an impression
that food exists in a certain direction. At once an impulse
to motion is sent out from it to the muscles necessary to
move the body in that direction. In the higher animals
these movements are more rapid and more exact. This is
because organs of sense, muscles, nerve fibers, and nerve
cells are all alike highly specialized. In the star-fish the
sensation is slow, the muscular response sluggish, but the

17

949 ANIMAL LIFE

method remains the same. This is simple reflex action, an
impulse from the environment carried to the brain and
then unconsciously reflected back as motion. The impulse
of fear is of the same nature. Strike at a dog with a whip,
and he will instinctively shrink away, perhaps with a cry.
Perhaps he will leap at you, and you unconsciously will try
to escape from him. Reflex action is in general uncon-
scious, but with animals as with man it shades by degrees
into conscious action, and into volition or action “ done on
purpose.”

129. Instinet.—Different one-celled animals show differ-
ences in method or degree of response to external influences.
The feelers of the Ameba will avoid contact with the feel-
ers or pseudopodia of another Ameba, while it does not
shrink from contact with itself or with an organism of un-
like kind on which it may feed. Most Protozoa will discard
grains of sand, crystals of acid, or other indigestible object.
Such peculiarities of different forms of life constitute the
basis of instinct.

Instinct is automatic obedience to the demands of ex-
ternal conditions. As these conditions vary with each kind
of animal, so must the demands vary, and from this arises
the great variety actually seen in the instincts of different
animals. As the demands of life become complex, so may
the instincts become so. The greater the stress of envi-
ronment, the more perfect the automatism, for impulses to
safe action are necessarily adequate to the duty they have
to perform. If the instinct were inadequate, the species
would have become extinct. The fact that its individuals
persist shows that they are provided with the instincts
necessary to that end. Instinct differs from other allied
forms of response to external condition in being hereditary,
continuous from generation to generation. This suffi-
ciently distinguishes it from reason, but the line between
instinct and reason and other forms of reflex action can
not be sharply drawn.

INSTINCT AND REASON 243

It is not necessary to consider here the question of the
origin of instincts. Some writers regard them as “ inherited
habits,” while others, with apparent justice, doubt if mere
habits or voluntary actions repeated till they become a
“second nature” ever leave a trace upon heredity. Such
investigators regard instinct as the natural survival of those
methods of automatic response which were most useful to
the life of the animal, the individuals having less effective
methods of reflex action having perished, leaving no pos-
terity.

An example in point would be the homing instinct of
the fur-seal. When the arctic winter descends on its home
in the Pribilof Islands in Befting Sea, these animals take
to the open ocean, many of them swimming southward as
far as the Santa Barbara Islands in California, more than
three thousand miles from home. While on the long swim
they never go on shore, but in the spring they return to
the northward, finding the little islands hidden in the arc-
tic fogs, often landing on the very spot from which they
were driven by the ice six months before, and their arrival
timed from year to year almost to the same day. The per-
fection of this homing instinct is vital to their life. If
defective in any individual, he would be lost to the herd
and would leave no descendants. Those who return be-
come the parents of the herd. As to the others the rough
sea tells no tales. We know that, of those that set forth, a
large percentage never comes back. To those that return
the homing instinct has proved adequate. This must be so
so long as the race exists. The failure of instinct would
mean the extinction of the species.

130. Classification of instincts—The instincts of animals
may be roughly classified as to their relation to the indi-
vidual into egoistic and altruistic instincts.

Eqgoistic instincts are those which concern chiefly the
individual animal itself. To this class belong the instincts
of feeding, those of self-defense and of strife, the instincts

244 ANIMAL LIFE

of play, the climatic instincts, and environmental instincts,
those which direct the animal’s mode of life.

Altruistic instincts are those which relate to parent-
hood and those which are concerned with the mass of indi-
viduals of the same species. The latter may be called the
social instincts. In the former class, the instincts of par-
enthood, may be included the instincts of courtship, re-
production, home-making, nest-building, and care for the
young.

131. Feeding.—The instincts of feeding are primitively
simple, growing complex through complex conditions.
The protozoan absorbs smaller creatures which contain
nutriment. The sea-anemone closes its tentacles over its
prey. The barnacle waves its feelers to bring edible crea-
tures within its mouth. The fish seizes its prey by direct
motion. The higher vertebrates in general do the same,
but the conditions of life modify this simple action to a
very great degree.

In general, animals decide by reflex actions what is
suitable food, and by the same processes they reject poisons
or unsuitable substances. The dog rejects an apple, while
the horse rejects a piece of meat. Either will turn away
from an offered stone. Almost all animals reject poisons
instantly. Those who fail in this regard in a state of
nature die and leave no descendants. The wild vetches or
“ loco-weeds ” of the arid regions affect the nerve centers of
animals and cause dizziness or death. The native ponies
reject these instinctively. This may be because all ponies
which have not this reflex dislike have been destroyed.
The imported horse has no such instinct and is poisoned.
Very few animals will eat any poisonous object with which
their instincts are familiar, unless it be concealed from smell
and taste.

In some cases, very elaborate instincts arise in connec-
tion with feeding habits. With the California woodpeckers
(Melanerpes formicivorus bairdit) a large number of them

INSTINCT AND REASON 245

together select a live-oak tree for their operations. They
first bore its bark full of holes, each large enough to hold
an acorn. Then into each hole an acorn is thrust (Figs.
61 and 62). Only one tree in several square miles may be
selected, and when their work is finished all those inter-
ested go about their business elsewhere. At irregular in-
tervals a dozen or so come back with much clamorous dis-
cussion to look at the tree. When the right time comes,
they all return, open the acorns one by one, devouring
apparently the substance of the nut, and probably also the
grubs of beetles which have developed within. When the
nuts are ripe, again they return to the same tree and the
same process is repeated. In the tree figured this has been
noticed each year since 1891.

132. Self-defense—The instinct of self-defense is even
more varied in its manifestations. It may show itself
either in the impulse to make war on an intruder or in the
desire to flee from its enemies. Among the flesh-eating
mammals and birds fierceness of demeanor serves both for
the securing of food and for protection against enemies.
The stealthy movements of the lion, the skulking habits of
the wolf, the sly selfishness of the fox, the blundering good-
natured power of the bear, the greediness of the hyena, are
all proverbial, and similar traits in the eagle, owl, hawk,
and vulture are scarcely less matters of common observa-
tion.

: Herbivorous animals, as a rule, make little direct resist-

ance to their enemies, depending rather on swiftness of
foot, or in some cases on simple insignificance. To the lat-
ter cause the abundance of mice and mouse-like rodents
may be attributed, for all are the prey of carnivorous beasts
and birds, and even snakes.

Even young animals of any species show great fear of
their hereditary enemies. The nestlings in a nest of the
American bittern when one week old showed no fear of
man, but when two weeks old this fear was very manifest

9246 ANIMAL LIFE

(Figs. 153 and 154). Young mocking-birds will go into
spasms at the sight of an owl or a cat, while they pay little
attention to a dog or a hen. Monkeys that have never
seen a snake show almost hysterical fear at first sight of
one, and the same kind of feeling is common to most
men. A monkey was allowed to open a paper bag which

Y, \\

Fie. 158.—Nestlings of the American bittern. Two of a brood of four birds one week
old, at which age they showed no fear of man. Photograph by E. H. Tapor,
Meridian, N. Y., May 31, 1898. (Permission of Macmillan Company, publishers of
Bird-Lore.)

contained a live snake. He was staggered by the sight,
but after a while went back and looked in again, to repeat
the experience. Each wild animal has its special instinct
of resistance or method of keeping off its enemies. The
stamping of a sheep, the kicking of a horse, the running
in a circle of a hare, and the skulking in a circle of some
foxes, are examples of this sort of instinct.

INSTINCT AND REASON 247

133. Play.—The play instinct is developed in numerous
animals. To this class belong the wrestlings and mimic
fights of young dogs, bear cubs, seal pups, and young
beasts generally. Cats and kittens play with mice. Squir-

Fie. 154.—Nestlings of the American bittern. The four members of the brood of
which two are shown in Fig. 153, two weeks old, when they showed marked fear
of man. Photograph by F. M. Cuapman, Meridian, N. Y., June 8, 1898. (Per-
mission of Macmillan Company, publishers of Bird-Lore.)

rels play in the trees. Perhaps it is the play impulse which
leads the shrike or butcher-bird to impale small birds and
beetles on the thorns about its nest, a ghastly kind of orna-
ment that seems to confer satisfaction on the bird itself.
The talking of parrots and their imitations of the sounds
they hear seem to be of the nature of play. The greater

948 ANIMAL LIFE

their superfluous energy the more they will talk. Much of
the singing of birds, and the crying, calling, and howling of
other animals, are mere play, although singing primarily be-
longs to the period of reproduction, and other calls and
cries result from social instincts or from the instinct to
care for the young.

134. Climate.—Climatic instincts are those which arise
from the change of seasons. When the winter comes the
fur-seal takes its long swim to the southward; the wild
geese range themselves in wedge-shaped flocks and fly high
and far, calling loudly as they go; the bobolinks straggle
away one at a time, flying mostly in the night, and most of
the smaller birds in cold countries move away toward the
tropics. All these movements spring from the migratory
instinct. Another climatic instinct leads the bear to hide
in a cave or hollow tree, where he sleeps or hibernates till
spring. In some cases the climatic instinct merges in the
homing instinct and the instinct of reproduction. When
the birds move north in the spring they sing, mate, and
build their nests. The fur-seal goes home to rear its young.
The bear exchanges its bed for its lair, and its first business
after waking is to make ready to rear its young.

135. Environment.—Environmental instincts concern
the creature’s mode of life. Such are the burrowing instincts
of certain rodents, the woodchucks, gophers, and the like.
To enumerate the chief phases of such instincts would be
difficult, for as all animals are related to their environ-
ment, this relation must show itself in characteristic in-
stincts.

136. Courtship.—The instincts of courtship relate chiefly
to the male, the female being more or less passive. Among
many fishes the male struts before the female, spreading
his fins, intensifying his pigmented colors through muscu-
lar tension, and in such fashion as he can makes himself the
preferred of the female. In the little brooks in spring
male minnows can be found with warts on the nose or head,

INSTINCT AND REASON 949

with crimson pigment on the fins, or blue pigment on the
back, or jet-black pigment all over the head, or with varied
combinations of all these. Their instinct is to display all
these to the best advantage, even though the conspicuous
hues lead to their own destruction. Against this contin-
gency Nature provides a superfluity of males.

Among the birds the male in spring is in very many
species provided with an ornamental plumage which he
sheds when the breeding season is over. The scarlet, crim-
son, orange, blue, black, and lustrous colors of birds are
commonly seen only on the males in the breeding season,
the young males and all males in the fall having the plain
brown gray or streaky colors of the female. Among the
singing birds it is chiefly the male that sings, and his voice
and the instinct to use it are commonly lost when the young
are hatched in the nest.

Among polygamous mammals the male is usually much
larger than the female, and his courtship is often a
struggle with other males for the possession of the female.
Among the deer the male, armed with great horns, fight
to the death for the possession of the female or for the
mastery of the herd. The fur-seal has on an average a
family of about thirty-two females (Fig. 71), and for the
control of his harem others are ready at all times to dispute
the possession. But with monogamous animals like the
true or hair seal or the fox, where a male mates with a
single female, there is no such discrepancy in size and
strength, and the warlike force of the male is spent on out-
side enemies, not on his own species.

137. Reproduction.—The movements of many migra-
tory animals are mainly controlled by the impulse to repro-
duce. Some pelagic fishes, especially flying-fishes and fishes
allied to the mackerel, swim long distances to a region
favorable for a deposition of spawn. Some species are
known only in the waters they make their breeding homes,
the individuals being scattered through the wide seas at

250 ANIMAL LIFE

other times. Many fresh-water fishes, as trout, suckers,
etc., forsake the large streams in the spring, ascending the
small brooks where they can rear their young in greater
safety. Still others, known as anadromous fishes, feed
and mature in the sea, but ascend the rivers as the im-
pulse of reproduction grows strong. Among such species
are the salmon, shad, alewife, sturgeon, and striped bass in
American waters. The most noteworthy case of the ana-
dromous instinct is found in the king salmon or quinnat
of the Pacific coast. This great fish spawns in November.
In the Columbia River it begins running in March and
April, spending the whole summer in the ascent of the
river without feeding. By autumn the individuals are
greatly changed in appearance, discolored, worn, and distort-
ed. On reaching the spawning beds, some of them a thou-
sand miles from the sea, the female deposits her eggs in
the gravel of some shallow brook. After they are fertilized
both male and female drift tail foremost and helpless down
the stream, none of them ever surviving to reach the sea.
The same habits are found in other species of salmon of
the Pacific, but in most cases the individuals of other spe-
cies do not start so early or run so far. <A few species of
fishes, as the eel, reverse this order, feeding in the rivers
and brackish creeks, dropping down to the sea to spawn.
The migration of birds has relation to reproduction as
well as to changes of weather. As soon as they reach their
summer homes, courtship, mating, nest-building, and the
care of the young occupy the attention of every species.
138. Care of the young.—In the animal kingdom one of
the great factors in development has been the care of the
young. This feature is a prominent one in the specializa-
tion of birds and mammals. When the young are cared for
the percentage of loss in the struggle for life is greatly re-
duced, the number of births necessary to the maintenance
of the species is much less, and the opportunities for spe-
cialization in other relations of life are much greater.

INSTINCT AND REASON 251

In these regards, the nest-building and home-making
animals have the advantage over those that have not these
instincts. The animals that mate for life have the advan-
tage over polygamous animals, and those whose social or
mating habits give rise to a division of labor over those
with instincts less highly specialized.

The interesting instincts and habits connected with nest
or home building and the care of the young are discussed
in the next chapter.

139. Variability of instincts —When we study instincts
of animals with care and in detail, we find that their regu-
larity is much less than has been supposed. There is as
much yariation in regard to instinct among individuals as
there is with regard to other characters of the species.
Some power of choice is found in almost every operation of
instinct. Even the most machine-like instinct shows some
degree of adaptability to new conditions. On the other
hand, in no animal does reason show entire freedom from
automatism or reflex action. ‘“ The fundamental identity
of instinct with intelligence,” says an able investigator, “is
shown in their dependence upon the same structural mech-
anism (the brain and nerves) and in their responsive adap-
tability.”

140. Reason.—Reason or intellect, as distinguished from
instinct, is the choice, more or less conscious, among re-
sponses to external impressions. Its basis, like that of in-
stinct, is in reflex action. Its operations, often repeated,
become similarly reflex by repetition, and are known as
habit. A habit is a voluntary action repeated until it be-
comes reflex. It is essentially like instinct in all its mani-
festations. The only evident difference is in its origin.
Instinct is inherited. Habit is the reaction produced with-
in the individual by its own repeated actions. In the
varied relations of life the pure reflex action becomes inade-
quate. The sensorium is offered a choice of responses. To
choose one and to reject the others is the function of intel-

959 ANIMAL LIFE

lect or reason. While its excessive development in man
obscures its close relation to instinct, both shade off by
degrees into reflex action. Indeed, no sharp line can be
drawn between unconscious and subconscious choice of
reaction and ordinary intellectual processes. |

Most animals have little self-consciousness, and their
reasoning powers at best are of a low order; but in kind,
at least, the powers are not different from reason in man.
A horse reaches over the fence to be company to another.
This is instinct. When it lets down the bars with its teeth,
that is reason. When a dog finds its way home at night by
the sense of smell, this may be instinct; when he drags a
stranger to his wounded master, that is reason. When a
jack-rabbit leaps over the brush to escape a dog, or runs in
a circle before a coyote, or when it lies flat in the grass as a
round ball of gray indistinguishable from grass, this is in-
stinct. But the same animal is capable of reason—that is,
of a distinct choice among lines of action. Not long ago a
rabbit came bounding across the university campus at Palo
Alto. As it passed a corner it suddenly faced two hunting
dogs running side by side toward it. It had the choice of
turning back, its first instinct, but a dangerous one; of
leaping over the dogs, or of lying flat on the ground. It
chose none of these, and its choice was instantaneous. It
ceased leaping, ran low, and went between the dogs just as
they were in the act of seizing it, and the surprise of the
dogs, as they stopped and tried to hurry around, was the
same feeling that a man would have in like circumstances.

On the open plains of Merced County, California, the
jack-rabbit is the prey of the bald eagle. Not long since a
rabbit pursued by an eagle was seen to run among the
cattle. Leaping from cow to cow, he used these animals
as a shelter from the savage bird. When the pursuit was
closer, the rabbit broke cover for a barbed wire fence.
When the eagle swooped down on it, the rabbit moved a
few inches to the right, and the eagle could not reach him

INSTINCT AND REASON 253

through the fence. When the eagle came down on the
other side, he moved across to the first. And this was con-
- tinued until the eagle gave up the chase. It is instinct
that leads the eagle to swoop on the rabbit. It is instinct
again for the rabbit to run away. But to run along the line
of a barbed wire fence demands some degree of reason. If
the need to repeat it arose often in the lifetime of a single
rabbit it would become a habit.

The difference between intellect and instinct in lower
animals may be illustrated by the conduct of certain mon-
keys brought into relation with new experiences. At one
time we had two adult monkeys, “ Bob” and “Jocko,” be- _
longing to the genus Macacus. Neither of these possessed
the egg-eating instinct. At the same time we had a baby
monkey, “ Mono,” of the genus Cercopithecus. Mono had
never seen an egg, but his inherited impulses bore a direct
relation to feeding on eggs, just as the heredity of Macacus
taught the others how to crack nuts or to peel fruit.

To each of these monkeys we gave an egg, the first that
any of them had ever seen. The baby monkey, Mono,
being of an egg-eating race, devoured his egg by the opera-
tion of instinct or inherited habit. On being given the
egg for the first time, he cracked it against his upper teeth,
making a hole in it, and sucked out all the substance.
Then holding the egg-shell up to the light and seeing that
there was no longer anything in it, he threw it away. All
this he did mechanically, automatically, and it was just as
well done with the first egg he ever saw as with any other
he ate. All eggs since offered him he has treated in the
same way.

The monkey Bob took the egg for some kind of nut.
He broke it against his upper teeth and tried to pull off
the shell, when the inside ran out and fell on the ground.
He looked at it for a moment in bewilderment, took both
hands and scooped up the yolk and the sand with which it
was mixed and swallowed the whole, Then he stuffed the

254 ANIMAL LIFE

shell itself into his mouth. This act was not instinctive.
It was the work of pure reason. Evidently his race was
not familiar with the use of eggs and had acquired no in-
stincts regarding them. He would do it better next time.
Reason is an inefficient agent at first, a weak tool; but
when it is trained it becomes an agent more valuable and
more powerful than any instinct.

The monkey Jocko tried to eat the egg offered him in
much the same way that Bob did, but, not liking the taste,
he threw it away.

The confusion of highly perfected instinct with intellect
is very common in popular discussions. Instinct grows
weak and less accurate in its automatic obedience as the
intellect becomes available in its place. Both intellect and
instinct are outgrowths from the simple reflex response to
external conditions. But instinct insures a single definite
response to the corresponding stimulus. The intellect has
a choice of responses. In its lower stages it is vacillating
and ineffective; but as its development goes on it becomes
alert and adequate to the varied conditions of life. It
grows with the need for improvement. It will therefore
become impossible for the complexity of life to outgrow
the adequacy of man to adapt himself to its conditions.

Many animals currently believed to be of high intelli-
gence are not so. The fur-seal, for example, finds it way
back from the long swim of two or three thousand miles
through a foggy and stormy sea, and is never too late or too
early in arrival. The female fur-seal goes two hundred
miles to her feeding grounds in summer, leaving the pup
on the shore. After a week or two she returns to find him
within a few rods of the rocks where she had left him.
Both mother and young know each other by call and by
odor, and neither is ever mistaken, though ten thousand
other pups and other mothers occupy the same rookery.
But this is not intelligence. It is simply instinct, because
it has no element of choice in it. Whatever its ancestors

INSTINCT AND REASON 255

were forced to do the fur-seal does to perfection. Its in-
stincts are perfect as clockwork, and the necessities of
migration must keep them so. But if brought into new
conditions it is dazed and stupid. It can not choose when
different lines of action are presented.

The Bering Sea Commission once made an experiment
on the possibility of separating the young male fur-seals,
or “killables,” from the old ones in the same band. The
method was to drive them through a wooden chute or run-
way with two valve-like doors at the end. These animals can
be driven like sheep, but to sort them in the way proposed
proved impossible. The most experienced males would
beat their noses against a closed door, if they had seen a
_ seal before them pass through it. That this door had been
shut and another opened beside it passed their comprehen-
sion. They could not choose the new direction. In like
manner a male fur-seal will watch the killing and skinning
of his mates with perfect composure. He will sniff at their
blood with languid curiosity ; so long as it is not his own
it does not matter. That his own blood may flow out on
the ground in a minute or two he can not foresee.

Reason arises from the necessity for a choice among ac-
tions. It may arise as a clash among instincts which forces
on the animal the necessity of choosing. A doe, for ex-
ample, in a rich pasture has the instinct to feed. It hears
the hounds and has the instinct to flee. Its fawn may be
with her and it is her instinct to remain and protect it.
This may be done in one of several ways. In proportion as
the mother chooses wisely will be the fawn’s chance of sur-
vival. Thus under difficult conditions, reason or choice
among actions rises to the aid of the lower animals as well
as man.

141. Mind—The word mind is popularly used in two
different senses. In the biological sense the mind is the
collective name for the functions of the sensorium in men
and animals. It is the sum total of all psychic changes,

956 ANIMAL LIFE

actions and reactions. Under the head of psychic functions
are included all operations of the nervous system as well as
all functions of like nature which may exist in organisms
without specialized nerve fibers or nerve cells. As thus de-
fined mind would include all phenomena of irritability, and
even plants have the rudiments of it. The operations of
the mind in this sense need not be conscious. With the
lower animals almost all of them are automatic and uncon-
scious. With man most of them must be so. All func-
tions of the sensorium, irritability, reflex action, instinct,
reason, volition, are alike in essential nature though differ-
ing greatly in their degree of specialization.

In another sense the term mind is applied only to con-
scious reasoning or conscious volition. In this sense it is
mainly an attribute of man, the lower animals showing it
in but slight degree. The discussion as to whether lower
animals have minds turns on the definition of mind, and
our answer to it depends on the definition we adopt.

Fig. 155.—A ‘‘ pointer” dog in the act of “ pointing,” a specialized instinct.
(Permission of G. O, Shields, publisher of Recreation.)

CHAPTER XV

HOMES AND DOMESTIC HABITS

142. Importance of care of the young.—The nest-building
and domestic habits of animals are adaptations, but adapta-
tions of behavior or habit rather than of structure, and are
based on instinct, intelligence, and reason. These instincts
and habits are among the most important shown by animals,
because on them depends largely the continuance of the
species. Of primary importance in the perpetuation of the
species is the possession by animals of adaptations of struc-
ture and behavior, which help the individual live long enough
to attain full development and to leave offspring. But in
the case of many animals a successful start in life on the
part of the offspring depends on the existence in the par-
ents of certain domestic habits concerned with the care and
protection of their young. The young of many animals de-
pend absolutely, for a part of their lifetime, on this parental
care. In these cases it is quite as necessary for the continucd
existence of the species that the habits that afford this care
be successful as that the parent should come successfully to
mature development and to the production of offspring.

143. Care of the young, and communal life-——The nest-
building or home-making habits and the continued per-
sonal care of the young for a part of their lifetime are most
highly developed and widespread among the birds, mam-
mals, and insects; and it is both among the insects and
the higher vertebrates that we find most developed those
social and communal habits discussed in Chapter IX. The
principal activities of an animal community have to do

18 257

958 ANIMAL LIFE

with the protection and sustenance of the young, and the
care of the young is undoubtedly a chief factor in the de-
velopment of communal life.

144. The invertebrates (except spiders and insects).—
Among the lower invertebrates parental aid to the young is
confined almost exclusively to exhibitions of pure instinct.
And this is true of many of the higher animals also. Eggs
are deposited in sheltered places, and in such places and
under such circumstances that the young on hatching will
find themselves close to a supply of their natural food. The
depositing of eggs in water by parents with terrestrial hab-
its whose young are aquatic, is an example. The toad,
which lives on land, feeding on insects, has young which
live in water and feed on minute aquatic plants (alge).
The dragon fly, that hawks over the pond or brook with
glistening wings, has young that crawl in the slime and
mud at the bottom of the pool. With most animals, after
laying eggs, the parents show no further solicitude toward
their offspring. The eggs are left to the vicissitudes of
fortune, and the parents know nothing of their fate. In
many cases the parent dies before the young are hatched.
The mammals and birds are the only two great groups ex-
cepted, although there are numerous specific exceptions
scattered among the lower invertebrates, fishes, batrachians,
and higher invertebrates, notably the insects.

There are no instances of care of the young after hatch-
ing among the sponges, polyps, worms, or star-fishes and
sea-urchins, and but few among the crustaceans and mol-
lusks. But there are in some of these groups a few cases
of nest or home building in a crude and simple way. Cer-
tain sea-urchins (Fig. 156) and worms and mollusks bore
into stones, and remain in the shelter afforded by the cay-
ity. A shell-fish (Zima hiams) cements together bits of
coralline, shells, and sand to form a crude nest or hiding-
place. The cray-fish digs a cylindrical burrow in the ground
in which it can hide.

HOMES AND DOMESTIC HABITS 259

145. Spiders.—Most spiders spin silken cocoons or sacs
in which to deposit their eggs. Some spiders carry this
egg-filled cocoon about with them for the sake of protect-
ing the eggs. After hatching, the spiderlings remain in the
cocoon a short time, feeding on each other! Thus only the

Fig. 156.—Sea-urchins living in holes bored into rocks of the seashore below high-
tide line.

strongest survive and issue from the cocoon to earn their
living in the outer world. With certain species of spiders
the young after hatching leave the cocoon and gather on
the back of the mother and are carried about by her for
some time. In connection with their webs or snares many
spiders have silken tunnels or tubes in which to lie hidden,
a sort of sheltering nest. Those spiders that live on the
ground make for themselves cylindrical burrows or holes
in the ground, usually lined with silk, in which they hide
when not hunting for food. Especially interesting among
the many kinds of these spider nests are the burrows of
the various trap-door spiders. These spiders are common
in California and some other far Western States. The bur-

260 ANIMAL LIFE

row (Fig. 157) or cylindrical hole is closed above by a silken,
thick, hinged lid or door, a little larger in diameter than
the hole and neatly beveled on the edge, so as to fit tightly
into and perfectly cover the hole when closed. The upper
surface of the door is covered with soil, bits of leaves, and
wood, so as to resemble very exactly the ground surface
about it. We have found these trap-door nests in Cali-
fornia in moss-covered ground, and here the lids of the nests
were always covered with green, growing moss.

An English naturalist who studied the habits of these
trap-door spiders found that if he removed the soil and bits
of bark and twigs, or the moss, from the upper surface of
the lid the spider always re-covered it. It is, of course,
plain that by means of this covering the nest is perfectly
concealed, the surface of the closed door not being dif-
ferent from the surrounding ground surface. This natu-
ralist finally removed the moss not only from the surface
of a trap-door, but also from all the ground in a circle of a
few feet about the nest. The next day he found that the
spider had brought moss from outside the cleared space
and covered the trap-door with it, thus making it very con-
spicuous in the cleared ground space. The spider’s instinct
was not capable of that quick modification to allow it to do
what a reasoning animal would have done—namely, coy-
ered the trap-door only with soil to make it resemble the
cleared ground about it.

Another interesting nest-making spider is the turret-
spider, that builds up a little tower (Fig. 158) of sticks and
soil and moss above its burrow. The sticks of which this
burrow are built are an inch or two in length, and are
arranged in such manner as make the turret five-sided.
The sticks are fastened together with silk, and the turret
is made two or three inches high. This turret-building
spider is one of those that carry about their egg-cocoon
with them. <A female of this spider in captivity was ob-
served to pay much attention to caring for this cocoon.

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262 ANIMAL LIFE

“Tf the weather was cold or damp, she retired to her tunnel ;
but if the jar in which she lived was set where the sun
could shine upon it, she soon re-
appeared and allowed the cocoon
to bask in the sunlight. If the
jar was placed near a stove that
had a fire in it, the cocoon was
put on the side next the source
of warmth; if the jar
was turned around, she
lost no time in moving
the cocoon to the warmer
side. Two months after
the eggs were laid the
young spiders made their
appearance and immediately
perched upon their mother, many
on her back, some on her head,
and even on her legs. She car-
ried them about with her and fed
them, and until they were older
they never left their mother for
Sa a moment.”
Fig, 158.—Nest of the turret- 146. Insects—So much space
case has already been devoted to an
account of the elaborate nest-making and domestic habits
of the bees, ants, and termites (see Chapter IX), that we
need in this place merely refer to that account. It is
among these social insects that the most interesting and
highly specialized habits connected with the care of the
young and the building of homes are found.

Many insects make for themselves simple burrows or
nests in the ground or in wood. The young or larve of
certain moths burrow about in the soft inside tissue of
leaves, and the whole life of the moth except its short adult
stage is passed inside the leaf. These larve are called leaf-

HOMES AND DOMESTIC HABITS 263

miners. The larve of some moths and of many hymenop-
terous insects live in galls on live plants. These galls are
simply abnormal growths of plant tissue, and are caused by
the irritating effect on the tissue of the larve which hatch
from eggs that have been thrust into the soft plant sub-
stance by the female insects. In the familiar galls on the
golden-rod live the larve of a small moth, and in the vari-
ous kinds of oak galls live the young of the numerous spe-
cies of Cynipida, the hymenopterous gall insects. The tiny
larvee of some of the midges live in small galls on various
plants. To this last group of gall-making insects belongs
the dreaded Hessian fly, the most destructive insect pest of
wheat.

Among the bees and wasps only a few species, compara-
tively, are communal or live in communities. But nearly
all the wasps and bees, whether social or solitary in habit,
build nests for their young and provide the young with
food, either by storing it in the nest or by hunting for it
and bringing it to the nest as long as the young are in the
larval condition. The “mud-daubers” or thread-waisted
wasps make nests of mud attached to the lower surface of
flat stones, to the ceiling of buildings, or in other out-of-
the-way and safe places. These nests usually have the form
of several tubes an inch or so long placed side by side. In
each of the tubes or cells an egg is laid, and with it a
spider which has been stung so as to be paralyzed but
not killed. When the young wasp hatches from the egg
as a grub or larva, it feeds on the unfortunate spider.
Others of the solitary wasps make nests in the ground
or in wood, and all of them provision their nests with
some particular kind of insect or spider. Some use only
caterpillars, some plant-lice, and some grasshoppers. Simi-
larly the solitary bees make nests in the ground as do the
mining-bees, or in wood as do the carpenter-bees, or by
cutting and fastening together leaves, as do the leaf-cutting
bees. The bees provision their nests, not with paralyzed

264 ANIMAL LIFE

insects, but with masses of pollen or pollen mixed with
nectar.

147. The vertebrates.—It is among the vertebrates, espe-
cially in the higher groups, the birds and mammals, that
we find the care of the young most perfectly undertaken
and most widespread.

Among the fishes, the lowest of the vertebrates, most
species content themselves with the laying of many eggs in
a situation best suited for their safe hatching. But some
species show interesting domestic habits. The female cat-
fish swims about with her brood, much as a hen moves
about with her chickens. Some of the larger ocean cat-
fish of the tropics receive the eggs or the young within the
mouth for safety in time of danger. Certain sunfishes care
for their young, keeping them together in still places in the
brook. They also make some traces of a nest, which the
male defends. The male salmon scoops out gravel to make
a shallow nest, in which the female deposits her eggs. The
male then covers the eggs. The males of the species of
pipe-fish and sea-horses receive the eggs of the female into
a groove or sac between the folds of skin on the lower part
of the tail. Here they are kept until the little fishes are
large enough to swim about for themselves. The brave
little sticklebacks build tiny nests about an inch and a half
or two inches in diameter, with a small opening at the top.
The eggs are laid in this nest, and the young fish remain in
it some time after hatching. The male parent jealously
guards the nest, and fights bravely with would-be intruders.

The batrachians and reptiles rarely show any care for
their young. The eggs of most batrachians are laid in the
water and left by the female. The males of the Surinam
toad receive the eggs in pits of the spongy skin of the back,
where they remain until the young hatch. The eggs of
snakes are laid under logs or buried in the sand, and no
further attention is given them by the parents.

Among the birds, on the other hand, nest-building and

HOMES AND DOMESTIC HABITS 265

care of the young are the rule, and a high degree of devel-
opment in these habits is reached. All of us are familiar
with many different kinds of nests, from the few twigs
loosely put together by the mourning-dove to the firm,
closely knit, wool or feather lined nest of the humming-
bird (Fig. 159), and the basket-like hanging nest. of the

Fig. 159.—Nest and eggs of the Rufus humming-bird (7rochilus rufus). Photograph
by J. O. Snyper, Stanford University, California,

266 ANIMAL LIFE

oriole (Fig. 161). Not all birds make nests. On the rocky
islets of the northern oceans, where thousands of puffins
and auks and other maritime birds gather to breed, the
eggs are laid on the bare rock. At the other extreme is
the tailor bird of India, which sews together leaves by
means of fibrous strips plucked from a growing plant to

Fig. 160.—Nest and young of the Rufus humming-bird ( 7vochilus rufus). Photograph
by J. O. SNYDER, Stanford University, California.

HOMES AND DOMESTIC HABITS 267

CET bie Sak Bateer, 3 FN > Carter

Fie. 161.—Baltimore orioles and nest; the male in upper left-hand corner of figure.

form a long, bag-like nest (Fig. 162). In the degree of
care given the nestlings there is also much difference. The
robin brings food to the helpless young for many days, and

268 ANIMAL LIFE

finally teaches it to fly and to hunt for food for itself.
Young chickens are not so helpless as the nestling robins,
but are able to run about, and under the guiding
care of the hen mother to pick up food for
themselves.

Among the mam-
mals the young are
always given some
degree of care. Ex-
cepting in the case
of the duck-bills, the
lowest of the mam-
mals, the young are
born alive—that is,
are not hatched from
eggs laid outside the
body—and are nour-
ished after birth for
a shorter or longer
time with milk
drawn from the
body of the mother.
Before birth the
young undergoes a
longer or shorter
period of development and growth in the body of the
mother, being nourished by the blood of the mother. The
nests or homes of mammals present varying degrees of
elaborateness, from a simple cave-like hole in the rocks
or ground to the elaborately constructed villages of the
beavers with their dams and conical several-storied houses
(Fig. 163). The wood-rat piles together sticks and twigs
in what seems, from the outside, a most haphazard fashion,
but which results in the construction of a convenient and
ingenious nest. The moles and pocket-gophers (Fig. 165)
build underground nests composed of chambers and gal-

Fig. 162.—Tailor-bird (Ornithotomus sutorius)
and nest.

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270 ANIMAL LIFE

BS meee Ys Peer tee enreme wit i Seite! ase %
Fig. 164.—Nest of the Californian bush-tit (Psaltriparus minimus). Photograph by
G. O. SNYDER, Stanford University, California.

leries. The prairie-dogs make burrows in groups, forming
large villages.

The devotion to their young displayed by birds and
mammals is familiar to us. The parents will often risk or

HOMES AND DOMESTIC HABITS oT1

suffer the loss of their own lives in protecting their off
spring from enemies. Many mother birds have the instinct
to flutter about a discovered nest crying and apparently
broken-winged, thus leading the predatory fox or weasel to

Za =

Fig. 165.—Nest and run-way of the pocket-gopher.

fix his attention on the mother and to leave the nest un-
harmed. This development of parental care and protec-
tion of the young reaches its highest degree in the case of
the human species. The existence of the family, which is
the unit of human society, rests on this high development
of care for the young.

CHAPTER XVI
GEOGRAPHICAL DISTRIBUTION OF ANIMALS

148. Geographical distribution Under the head of dis-
tribution we consider the facts of the diffusion of organ-
isms over the surface of the earth, and the laws by which
this diffusion is governed.

The geographical distribution of animals is often known
as zodgeography. In physical geography we may prepare
maps of the earth which shall bring into prominence the
physical features of its surface. Such maps would show
here a sea, here a plateau, here a range of mountains,
there a desert, a prairie, a peninsula, or an island. In po-
litical geography the maps show the physical features of
the earth, as related to the states or powers which claim
the allegiance of the people. In zoégeography the realms
of the earth are considered in relation to the types or
species of animals which inhabit them. Thus a series of
maps of the United States could be drawn which would
show the gradual disappearance of the buffalo before the
attacks of man. Another might be drawn which would
show the present or past distribution of the polar bear,
black bear, and grizzly. Still another might show the
original range of the wild hares or rabbits of the United
States, the white rabbit of the Northeast, the cotton-tail of
the East and South, the jack-rabbit of the plains, the snow-
shoe rabbit of the Columbia River, the tall jack-rabbit of
California, the black rabbits of the islands of Lower Cali-
fonia, and the marsh-hare of the South and the water-hare
of the canebrakes, and that of all their relatives. Such a

272

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7 TEL

307

LEREMA
ACCIUS

Fie. 166.—Map showing the distribution of the clouded Skipper butterfly (Lerema
accius) in the United States. The butterfly is found in that part of the country
shaded in the map, a warm and moist region.—After SCUDDER.

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75 65
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GULF of MEXICO
95 85
\

J

Fie. 167.—Map showing the distribution of the Canadian Skipper butterfly (Zrynnis
manitoba) in the United States. The butterfly is found in that part of the
country shaded in the map. This butterfly is subarctic and subalpine in dis-
tribution, being found only far north or on high mountains, the two southern
projecting parts of its range being in the Rocky Mountains and in the Sierra
Nevada Mountains.—After ScupDER.

19

274 ANIMAL LIFE

map is very instructive, and it at once raises a series of
questions as to the reasons for each of the facts in geo-
graphical distribution, for it is the duty of science to sup-
pose that none of these facts is arbitrary or meaningless.
Each fact has some good cause behind it.

149. Laws of distribution—The laws governing the dis-
tribution of animals are reducible to three very simple
propositions. Every species of animal is found in every part
of the earth having conditions suitable for its maintenance,
unless—

(a) Its individuals have been unable to reach this re-
gion, through barriers of some sort; or—

(0) Having reached it, the species is unable to maintain
itself, through lack of capacity for adaptation, through
severity of competition with other forms, or through de-
structive conditions of environment; or—

(c) Having entered and maintained itself, it has become
so altered in the process of adaptation as to become a spe-
cies distinct from the original type.

150. Species debarred by barriers—As examples of the
first class we may take the absence of kingbirds or meadow-
larks or coyotes in Europe, the absence of the lion and
tiger in South America, the absence of the civet-cat in New
York, and that of the bobolink or the Chinese flying-fox in
California. In each of these cases there is no evident rea-
son why the species in question should not maintain itself
if once introduced. The fact that it does not exist is, in
general, an evidence that it has never passed the barriers
which separate the region in question from its original
home.

Local illustrations of the same kind may be found in
most mountainous regions. In the Yosemite Valley in
California, for example, the trout ascend the Merced River
to the base of a vertical fall. They can not rise above this,
and so the streams and lakes above this fall are destitute
of fish.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 975

151. Species debarred by inability to maintain their ground.
—Examples of the second class are seen in animals which
man has introduced from one country to another. The
nightingale, the starling, and the skylark of Europe have
been repeatedly set free in the United States. But none of
these colonies has long endured, perhaps from lack of adap-
tation to the climate, more likely from severity of competi-
tion with other birds. In other cases the introduced species
has been better fitted for the conditions of life than the
native forms themselves, and so has graduallv crowded out
the latter. Both these cases are illustrated among the rats,
The black rat, first introduced into America from Europe
about 1544, helped crowd out the native rats, while the
brown rat, brought in still later, about 1775, in turn practi-
cally exterminated the black rat, its’ fitness for the condi-
tions of life here being still greater than that of the other
European species.

Certain animals have followed man from land to land,
having been introduced by him against his will and to the
detriment of his domestic animals or crops. To many of
these the term vermin has been applied. Among the ver-
min or “animal weeds” are certain of the rodents (rats,
mice, rabbits, etc.), the mongoose of India, the English
sparrow, and many kinds of noxious insects. Of all the
vermin of this class few have caused such widespread de-
struction of property as the common European rabbit intro-
duced into Australia. The annual loss through its presence
is estimated at $3,500,000.

- It often happens that man himself so changes the en-
vironment of a species that it can no longer maintain it-
self. Checking the increase of a species, either by actually
killing off its members or by adverse change in its sur-
roundings, is to begin the process of its destruction. Cir-
cumstances become unfavorable to the growth or reproduc-
tion of an animal. Its numbers are reduced, fewer are
born each year, and fewer reach maturity, it grows rare,

276 ANIMAL LIFE

is gone, and the final step of extinction may often pass
unnoticed.

But a few years ago the air in the Ohio Valley was dark
in the season of migration with the hordes of passenger
pigeons. The advance of a tree-destroying, pigeon-shooting
civilization has gone steadily on, and now the bird which
once crowded our Western forests is in the same region an
ornithological curiosity. The extinction of the American
bison or “buffalo,” and the growing rarity of the grizzly
bear, the wolf, and of large carnivora generally, furnishes
cases in point. When Bering and Steller landed on the
Commander Islands in 1741, the sea-cow, a large herbivo-
rous creature of the shores, was abundant there. In about
fifty years the species, being used for food by fishermen,
entirely disappeared. In most cases, however, a species
that crosses its limiting barriers, but is unable to main-
tain itself, leaves no record of the occurrence. We know, as
a matter of fact, that stray individuals are very often found
outside the usual limit of a species. A tropical bird may
be found in New Jersey, a tropical fish on Cape Cod, or a
bird from Europe on the shores of Maine. Of course,
hundreds of other cases of this sort must escape notice;
but, for one reason or another, the great majority of these
waifs are unable to gain a new foothold. For this reason,
outside of the disturbances created by man, the geographical
distribution of species changes but little from century to
century; and yet, when we study the facts more closely,
evidences of change appear everywhere.

152. Species altered by adaptation to new conditions.—
Of the third class or species altered in a new environment
examples are numerous, but in most cases the causes in-
volved can only be inferred from their effects. One class
of illustrations may be taken from island faune. An island
is set off from the mainland by barriers which species of
land animals can very rarely cross. On an island a few waifs
of wave and storm may maintain themselves, increasing in

Vie. 168—The manatee, or sea-cow (7vrichechus latirostris). A living species of sea-
cow related to the now extinct Steller’s sea-cow.

278 ANIMAL LIFE

ah i }
se Pe ¥

Fia. 169.—On the shore of Narborongh Island, one of the Galapagos Islands, Pacific
Ocean, showing peculiar species of sea-lions, lizards, and cormorants. Drawn
from a photograph made by Messrs. SNopeRraAss and HELLER.

numbers so as to occupy the territory; but in so doing
only those will survive that can fit themselves to the new
conditions. Through this process a new species will be
formed, like the parent species in general structure, but
having gained new traits adjusted to the new environment.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 979

The Galapagos Islands are a cluster of volcanic rocks
lying in the open sea about six hundred miles to the west
of Ecuador. On these islands is a peculiar land fauna, de-
rived from South American stock, but mostly different in
species. Darwin noted there “twenty-six land birds; of
these, twenty-one, or perhaps twenty-three, are ranked as
distinct species. Yet the close affinity of most of these
birds to American species is manifest in every character, in
their habits, gestures, and tones of voice.”

Among land animals similar migrations may occur, giv-
ing rise, through the adaptation to new conditions, to new
species. The separation of species of animals isolated in
river basins or lakes often permits the acquisition of new
characters, which is the formation of distinct species in
similar fashion. On the west side of Mount Whitney, the
highest mountain in the Sierra Nevada of California, there
is a little stream called Volcano Creek. In this brook is a
distinct species or form of trout, locally called golden
trout. It is unusually small, very brilliantly colored, its
fins being bright golden, and its tiny scales scarcely over-
lap each other along its sides. This stream flows over a
high waterfall (Agua Bonita) into the Kern River. The
Kern River is full of trout, of a kind (Salmo gilberti) to
which the golden trout is most closely allied. There can
not be much doubt that the latter is descended from the
former. With this assumption, it is easy to suppose that
once the waterfall did not exist, or that through some
agency we can not now identify certain fishes had been
carried over it. Once above it, they can not now return,
nor can they mix with the common stock of the river.
Those best adapted to the little stream have survived.
The process of adaptation has gone on till at last a distinct
species (or sub-species*) is formed. In recent times the

* In descriptive works the name species is applied to a form when
the process of adaptation seems complete. When it is incomplete, or

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Fie. 171.—Three species of jack-rabbits, differing in size, color, and markings, but
believed to be derived from a common stock. ‘The differences have arisen
through isolation and adaptation. The upper figure shows the head and fore legs
of the black jack-rabbit (Zepus insularis), of Espiritu Santo Island, Gulf of
California ; the lower right-hand figure, the Arizona jack-rabbit (Lepus alleni),
specimen from Fort Lowell, Arizona; and the lower left-hand figure is the San
Pedro Martir jack-rabbit (Lepus martirensis), from San Pedro Martir, Baja
California,

282 ANIMAL LIFE

hand of man has carried the golden trout to other little
mountain torrents, where it thrives as well as in the one
where its peculiarities were first acquired.

Other cases of this nature are found among the blind
fishes of the caves in different parts of the world (Fig. 172).
In general, caves are
formed by the ero-
sion or wearing of
underground rivers.
These streams are
either clear and cold,
‘and when they issue
to the surface those
fishes which like cold
and shaded waters
are likely to enter
them. But to have
eyes in absolute dark-

Fie. 172.—Fishes showing stages in the loss of eyes ness, in which no use

and color. A, Dismal Swamp fish ( Chologaster
avetus), ancestor of the blind fish; B, Agassiz’s can be made of them,

cave fish (Chologaster agassizi); C, cave blind jg q disadvantage in

fish (Typhlichthys subterraneus). the stru g gl e for life.
Hence the eyed species die or withdraw, while those in which
the eye grows less from generation to generation, until its
function is finally lost, are the ones which survive. By such
processes the blind fishes in the limestone caves of Ken-
tucky, Indiana, Tennessee, and Missouri have been formed.

rather when specimens showing intergradation of characters are known,
the word sub-species is used. The word variety has much the same
meaning when used for a subdivision of a species, but it is a term
defined with less exactness. Thus the common fox (Vulpes pennsyl-
vanicus) is a distinct species, being separate from the arctic fox or the
gray fox or the fox of Europe. The cross fox ( Vulpes pennsylvanicus
decussatus) is called a sub-species, as is the silver fox (Vulpes pennsyl-
vanicus argentatus), because these intergrade perfectly with the common
red fox.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 982

To processes of this kind, on a larger or smaller scale,
the variety in the animal life of the globe is very largely
due. Isolation and adaptation give the clew to the forma-
tion of a very large proportion of the “new species” in
any group.

153. Effect of barriers.—It will be thus seen that geo-
graphical distribution is primarily dependent on barriers or
checks to the movement of animals. The obstacles met
in the spread of animals determine the limits of the spe-
cies. Each species broadens its range as far as it can. It
attempts unwittingly, through natural processes of increase,
to overcome the obstacles of ocean or river, of mountain or
plain, of woodland or prairie or desert, of cold or heat, of
lack of food or abundance of enemies—whatever the bar-
riers may be. Were it not for these barriers, each type or
species would become cosmopolitan or universal. Man is
pre-eminently a barrier-crossing animal. Hence he is found
in all regions where human life is possible. The different
races of men, however, find checks and barriers entirely
similar in nature to those experienced by the lower animals,
and the race peculiarities are wholly similar to characters
acquired by new species under adaptation to changed con-
ditions. The degree of hindrance offered by any barrier
differs with the nature of the species trying to surmount it.
That which constitutes an impassable obstacle to one form
may be a great aid to another. The river which blocks the
monkey or the cat is the highway of the fish or the turtle.
The waterfall which limits the ascent of the fish is the
chosen home of the ouzel. The mountain barrier which
the bobolink or the prairie-dog does not cross may be the
center of distribution of the chief hare or the arctic blue-
bird.

154. Relation of species to habitat.—The habitat of a
species of animal is the region in which it is found ina
state of Nature. It is currently believed that the habitat
of any creature is the region for which it is best adapted.

284 ANIMAL LIFE

But the reverse of this is oftentrue. There are many cases
in which a species introduced in a new territory, through
the voluntary or involuntary influence of man, has shown a
marvelous adaptation and power of persistence. The rapid
spread of rabbits and pigs as wild animals in Australia, of
horses and cattle in South America, and of the English
sparrow in North America, of bumble-bees and house-
flies in New Zealand, are illustrations of this. Not one
of these animals has maintained itself in the wild state
in its native land as successfully as in these new countries
to which it has been introduced. The work of introduc-
tion of useful animals illustrates the same fact. The shad,
striped bass, and cat-fish from the Potomac River, intro-
duced into the Sacramento River and its tributaries by the
United States Fish Commission, are examples in point.
These valued food-fishes are nowhere more at home than in
the new waters where no species of their types had ever
existed before. The carp, originally brought to Europe
from China, and thence to the United States as a food-
fish, becomes in California a nuisance, which can not be
eradicated, destroying the eggs and the foodstuff of far
better fish.

In all mountain regions waterfalls are likely to occur,
and these serve as barriers, preventing the ascent of trout
and other fishes. On this account in the mountains of Cali-
fornia, Colorado, Wyoming, and other States, hundreds of
lakes and streams suitable for trout are found in which no
fishes ever exist. In the Yellowstone Park this fact is es-
pecially noticeable. This region is a high volcanic plateau,
formed by the filling of an ancient granite basin with a vast
deposit of lava. The streams of the park are very cold and
clear, in every way favorable for the growth of trout; yet,
with the exception of a single stream, the Yellowstone
River, none of the streams was found to contain any fish
in that part of it lying on the plateau. Below the plateau
all of them are well stocked. The reason for this is ap-

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286 ANIMAL LIFE

parent in the fact that the plateau is fringed with cataracts
which fishes can not ascend. Each stream has a cafion or
deep gorge with a waterfall at its head, near the point
where it leaves the hard bed of black lava for the rock
below (Fig. 173). So for an area of fifteen hundred square
miles within the Yellowstone National Park the streams
were without trout because their natural inhabitants had
never been able to reach them. When this state of things
was discovered it was easy to apply the remedy. Trout of
different species were carried above the cascades, and these
have multiplied with great rapidity.

The exception noted above, that of the Yellowstone
River itself, evidently needs explanation. An abundance
of trout is found in this river both above and below the
great falls, and no other fish occurs with it. This anomaly
of distribution is readily explained by a study of the tribu-
taries at the head waters of the river. When we ascend
above Yellowstone Lake to the continental divide, we find
on its very summit that only about an eighth of a mile of
wet meadow and marsh, known as Two Ocean Pass (Fig.
174), separates the drainage of the Yellowstone from that
of the Columbia. A stream known as Atlantic Creek flows
into the Yellowstone, while the waters of Pacific Creek on
the other side find their way into the Snake River. These
two creeks are connected by waterways in the wet meadow,
and trout may pass from one to the other without check.
Thus from the Snake River the Yellowstone received its
trout, and from the Yellowstone thcy have spread to the
streams tributary to the upper Missouri.

This case is a type of the anomalies in distribution of
which the student of zodgeography will find many. But
each effect depends upon some cause, and a thorough study
of the surroundings or history of a species will show what
this cause may be. In numerous cases in which fishes have
been found above an insurmountable cascade, the cause is
seen in a marsh flooded at high water, connecting one

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288 ANIMAL LIFE

drainage basin with another. An example of this is found
in Lava Creek in Yollowstone Park. Aboye Undine and
Wraith Falls, both insurmountable, are found an abun-
dance of trout. A marsh dry in summer connects Laya
Creek with Black Tail Deer Creek, a tributary of the
Yellowstone and without waterfall. From the Yellow-
stone through this creek and marsh the trout find their
way into Lava Creek. In California numerous anomalies
have been noted, as the occurrence of Tahoe trout in
Feather River and in the Blue Lakes of Amador, which are
on the other side of the main crest of the Sierra Nevada
from Lake Tahoe, and the occurrence of the Whitney
golden trout in Lone Pine Creek, another similar instance.
In each case naturalists have found the man who actually
carried the species across the divide. If this matter had
been investigated a generation later, these cases would have
been unexplainable anomalies in geographical distribution.
Real causes are almost always simple when they are once
known.

The ways in which species may cross barriers in a state
of Nature are as varied as the creatures themsclves, and far
more varied than the actual barriers. By the long-con-
tinued process of adjustment to conditions with the inces-
sant destruction of the unadapted, the various organisms
have become so well fitted to their surroundings that the
casual observer may well suppose that each inhabits the
region best fitted for it. Men have even thought that the
conditions of life have been fitted to the creatures them-
selves, so perfect is this relation.

155. Character of barriers to distribution.—Taking the
animal kingdom as a whole, the two great barriers modify-
ing distribution are the presence of the sea and changes in
temperature. It is only in rare cases that any land ani-
mals can cross either of the great oceans, and these rare
cases relate chiefly to the arctic regions. For this reason
the land faunsz of Africa, South America, and Australia

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 989

have developed almost independently of one another. To
the fresh-water fishes the sea forms equally a barrier, and

(Permission of

Photograph by Dr. R. T. Morris.

rfall,
G. O. Shields, publisher of Recreation.

A Labrador salmon clearing an 18-foot wat

175.—

Fia.

even the shore-fishes very rarely pass across great depths.

Relatively few of the shore-fishes of Cuba, for example, ever

cross the deep Florida Straits, and none of those of Cali-
20

290 ANIMAL LIFE

fornia ever reach Honolulu, nor are Hawaiian shore-fishes
ever seen on the coast of California. For these reasons
natural boundaries of the great realms of distribution are
found in the sea.

The other great check to distribution is found in heat
and cold. Most of the tropical animals can not endure
frost. The arctic animals, however fierce or active, are
enfeebled by heat. The timber line, north of which and
above which frost occurs the year round, therefore serves

Fie. 176.—Alligators ; animals found only in the warm waters of tropical and sub-
tropical regions.

as a boundary of limitation. Another equally marked is
the frost line. Even the fishes of the tropics are extreme-
ly sensitive to slight cold. Off Florida Keys the cutlass-
fish is sometimes seen stiff and benumbed on the water,
where the temperature is scarcely below 60° Fahr. A
“norther” on the Gulf of Mexico will sometimes bring
fishes which live in considerable depths to the surface,
through chilling the water. These barriers are rarely
crossed by localized species, but many forms, especially
birds, keep within a relatively uniform temperature through
migration. The summers are spent in the north or in the
mountains, the winters in districts that are warmer.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS) 99]

The climate, as distinct from the temperature, also
greatly influences many species. Ia the Eastern United
States and in the extreme Northwest, as in Europe and
much of Asia, the atmosphere is humid all the year long.
Rains occur at intervals in the summer, and rain or snow in
the winter. The green season is from spring to fall, and the
resting of plants is in the winter. To this condition the
native animals adapt themselves, and this would seem to
be the natural order of things.

But as we pass the Western plains of Nebraska, Kan-
gas, and Texas this condition is materially changed. For
part of the year rainfall is practically unknown. The air
becomes dry, and under the cloudless sky the greater part
of the vegetation ripens its seed and perishes. This is the
arid climate. When the rainfall is very scant the region
is never covered with verdure, and is known as desert.
Such great desert tracts are found in parts of Wyoming,
Utah, Nevada, Idaho, Colorado, Arizona, New Mexico, Cali-
fornia, as well as in the northern parts of Mexico. In some
cases the deserts are exposed to great heat, forming an
ultra-torrid region, as in Death Valley in California and in
certain parts of Arizona.

But the arid region is not as a whole desolate. In many
parts rain falls more or less heavily for part of the year,
bringing a rank growth of annual grasses and of verdure
in general. In California this rainfall is in the winter, the
coldest part of the year, and the country is green from
November or October to June or May. In Mexico and
northward to Colorado the chief rainfall is in midsummer,
the warmest part of the year,and the summer is the time
of verdure.

To all these conditions the plant life must adapt itself
and with this the animal life. But the species that have
become fitted to the arid habitat have undergone some
change in the process and may have become different spe-
cies. It is, then, not easy for them to recross the barrier

292 ANIMAL LIFE

of climate to compete with those forms already adapted.
For this reason a marked change of climate like a marked
change of temperature forms a natural barrier to distribu-
tion and serves to circumscribe a natural fauna.

Closely associated with climate is the nature of forest
growth, the growth of grass, and in general the development
of conditions which serve for food or shelter to animals,
These conditions depend in part on soil, partly on climate
and topography, and partly on the acts of man. The for-
est and forest soils, acting like a great sponge, retain the
waters of the rainy season, and thus regulate the size of
the streams. The stream that changes least in volume is
most favorable to the life of fishes, frogs, and water ani-
mals generally. The destruction of forests on the moun-
tain sides acts adversely to the life of these creatures as
well as to the interests of the farmer below whose lands
the streams should water. When the forests are destroyed,
the great host of wood creatures, the bears, squirrels, war-
blers, various birds, beasts, and insects of the woods can no
longer maintain themselves, and grow rare and disappear.
For reasons that are obvious the conditions that produce
forest, prairie, canebrake, sage-desert, cactus-desert, and
the like are potent in regulating the distribution of the
species.

Still another set of conditions depends on the food sup-
ply. The planting of orchards tends to multiply greatly
the number of individuals of those species which prey upon
fruit. When food is abundant the severity of the struggle
for life is relaxed and individuals increase in number. A
species may be put to great stress by the disappearance of
the animal or plant on which it has depended. Each
change made by man among the wild animals or plants
may have far-reaching effects upon others. The coyote or
prairie-wolf destroys sheep in the ranges of the West. It
is thinned out by means of the bounty upon its scalp.
Then the jack-rabbit, on which it also feeds, greatly in-

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 993

creases in abundance, injuring fruit trees and grain fields.
It is then necessary to pay for its destruction also.

To destroy hawks or owls because they catch chickens
may increase the numbers and destructiveness of field-mice
on which they also prey. To shoot robins, linnets, and
other birds that destroy small fruits is likely to increase
greatly the insect pests on which these birds also feed.
The inter-relations of species and species are so close that
none should be exterminated by man unless its habits and
relations have been subjected to careful scientific study.
Still less should any new ones be introduced without the
fullest consideration of the possible results. For example,
the mongoose, a weasel-like creature, was introduced from
India into Jamaica to kill rats and mice. It killed also the
lizards, and thus produced a plague of fleas, an insect which
the lizards kept in check. The English sparrow, intro-
duced that it might feed on insects inhabiting shade-trees,
has become a nuisance, crowding out better birds and not
accomplishing the purpose for which it was brought to the
United States.

To most kinds of animals a mountain range must act as
a barrier to distribution. In a region having high moun-
tains a species will become in time split up into several,
because the individuals in one valley will be isolated from
those of another. The fauna of California furnishes many
illustrations of this, as among its mountain chains are
many deep valleys shut off from each other and having
different peculiarities of temperature. For this reason two
counties of California differ much more widely in their
fauna than do two counties in Illinois. But Illinois as a
whole has more different kinds of animals than California,
because no barrier anywhere prevents their entrance. The
State has, we may say, its doors wide open to immigrants
from all quarters. The same is true of Iowa or of Kansas
or Kentucky. Illinois has a richer fauna than Iowa, be-
cause its extension is north and south, and it therefore

294 ANIMAL LIFE

covers a wider range of climate. Kentucky has a richer
fauna than Iowa because it includes a greater variety of
conditions. New England was called by Professor Agassiz
a “ zodlogical island,” because of the relatively small num-
ber of its native animals, especially of species inhabiting its
rivers. The cause of this is found in its isolation, being
shut off from the Middle States by mountain ranges, while
it is bounded on two sides by the sea.

156. Barriers affecting fresh-water animals.—The animals
inhabiting fresh-water streams are affected by differences in
temperature and elevation much as land animals are. They
tend to spread from stream to stream whenever they can
find their way. An isolated stream is likely to have its
peculiar fauna just as island life is likely to differ from
that of the mainland. The same species wanders widely
within the limits of a single river basin. If a kind of fish
establishes itself anywhere in the Mississippi Valley, it may
find its way to every stream in the whole basin. If it likes
cold spring water, as the rainbow-darter does, we may look
for it in any cold spring. If, like the long-eared sun-fish,
it frequents deep pools in the brooks, we may look for it
under roots of stumps and in every “swimming hole.” If,
like the channel-cat, it chooses the ripples of a river, we
may fish for it wherever ripples are. The larger the whole
river basin the more species find their way into it, and
therefore the greater the number of species in any one of
its streams.

Each species finds its habitat fitted to its life, and then
in turn is forced to adapt itself to this habitat. Any other
kind of habitat then appears as a barrier to its distribu-
tion. Thus to a fish of the ripples a stretch of still water
becomes a barrier. A species adapted to sandy bottoms
will seldom force its way through swift waters or among
weeds or rocks. The effect of waterfalls as barriers is else-
where noticed. In some streams the dam made by a colony
of beavers has the same effect. Mill-dams and artificial

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 995

waterfalls have checked the movements of many species,
while others have been helped by artificial channels or
canals. Streams that run muddy at times are not favor-
able for animal life. Still less favorable is the condition
frequent in the arid region in which streams are full to
the banks in the rainy season and shrunk to detached
pools in the dry months.

The stream that has the greatest variety of animals in it
would be one (1) connected with a large river, (2) in a warm
climate, (3) with clear water and (4) little fluctuation from
winter to summer, (5) with little change in the clearness of
the water, (6) a gravelly bottom, (7) preferably of lime-
stone, and (8) covered in its quiet reaches and its ripples
with water-weeds. These conditions are best realized in
tributaries of the Ohio, Cumberland, Tennessee, and Ozark
Rivers among American streams, and it is in them that the
greatest number of species of fresh-water animals (fishes,
cray-fishes, mussels, etc.) has been recorded. These streams
approach most nearly to the ideal homes for animals of the
fresh waters. The streams of Wisconsin, Michigan, and the
Columbia region have many advantages, but are too cold.
Those of Illinois, Iowa, northern Missouri, and Kansas are
too sluggish, and sometimes run muddy. Those of Texas
and California shrink too much in summer, and are too
isolated. The streams of the Atlantic coast are less iso-
lated, but none connect with a great basin, and those of
New England run too cold for the great mass of the spe-
cies. For similar reasons the fresh-water animal life of
Europe is relatively scanty, that of the Danube and Volga
being richest. The animal life of the fresh water of South
America centers in the Amazon, and that of Africa in the
Nile, the Niger, and the Congo. The great rivers of Si-
beria, like the Yukon in Alaska and the Mackenzie River
in British America, have but few forms of fresh-water ani-
mals, though those kinds fitted for life in cold, clear water
exist in great abundance.

296 ANIMAL LIFE

157. Modes of distribution —The means and modes of mi-
gration and distribution are obvious in the case of animals
that can fly or swim or make long journeys on foot. An
island can be visited and become peopled by birds from the
nearest mainland. Fishes and marine mammals can travel
from ocean to ocean. But many animals have no means
of crossing watery barriers. “Oceanic islands, that have
been formed de novo in mid-ocean and are not detached
portions of pre-existing continents, are almost invariably
free from such animals as are incapable of traversing the
sea. If sufficiently distant from any continent, oceanic
islands are generally without mammals, reptiles, and am-
phibia, but have both birds and insects and certain other
invertebrates which are transported to them by involuntary
migration.”

As suggested in the last sentence, migration may be
passive or involuntary. For example, those minute ani-
mals that can become dried up and yet retain the power
of renewing their active life under favorable conditions are
sometimes carried in the dried mud adhering to the feet of
birds, and may thus become widely distributed. Parasites
are carried by their hosts in all their wanderings. Some
animals, as rats and mice, are carried by ships and railway
trains and thus widely distributed.

158. Fauna and faunal areas.—The term fauna is applied
to the animals of any region considered collectively. Thus
the fauna of Illinois comprises the entire list of animals
found naturally in that State. It includes the aboriginal
men, the black bear, the fox, and all its animal life down
to the Ameba. The relation of the fauna of one region
to that of another depends on the ease with which bar-
riers may be crossed. Thus the fauna of Illinois differs
little from that of Indiana or Iowa, because the State con-
tains no barriers that animals may not readily pass. On
the other hand, the fauna of California or Colorado differs
materially from that of adjoining regions, because a moun-

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 997

tainous country is full of barriers which obstruct the diffu-
sion of life. Distinctness is in direct proportion to isola-
tion. What is true in this regard of the fauna of any region
is likewise true of its individual species. The degree of
resemblance among individuals is in strict proportion to the
freedom of their movements. Variation within the limits
of a species is again proportionate to the barriers which
prevent equal and free diffusion.

159. Realms of animal life—The various divisions or
realms into which the land surface of the earth may be
divided on the basis of the character of animal life have
their boundary in the obstacles offered to the spread of the
average animal. In spite of great inequalities in this regard,
we may yet roughly divide the land of the globe into seven
principal realms or areas of distribution, each limited by
barriers, of which the chief are the presence of the sea and
the occurrence of frost. There are the Arctic, North Tem-
perate, South American, Indo-African, Lemurian, Patago-
nian, and Australian realms. Of these the Australian
realm alone is sharply defined. Most of the others are sur-
rounded by a broad fringe of debatable ground that forms
a transition to some other zone.

The Arctic realm includes all the land area north of the
isotherm of 32°. Its southern boundary corresponds closely
with the northern limit of trees. The fauna of this region
is very homogeneous. It is not rich in species, most of the
common types of life of warmer regions being excluded.
Among the large animals are the polar bear, the walrus, and
certain species of “ice-riding ” seals. There are a few spe-
cies of fishes, mostly trout and sculpins, and a few insects.
Some of these, as the mosquito, are excessively numerous
in individuals. Reptiles are absent from this region and
many of its birds migrate southward in the winter, finding
in the arctic only their breeding homes. When we consider
the distribution of insects and other small animals of wide
diffusion we must add to the arctic realm all high moun-

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GEOGRAPHICAL DISTRIBUTION OF ANIMALS 999

tains of other realms whose summits rise above the timber
line. The characteristic large animals of the arctic, as the
polar bear or the musk-ox or the reindeer, are not found
there, because barriers shut them off. But the flora of the
mountain top, even under the equator, may be character-
istically arctic, and with the flowers of the north may be
found the northern insects on whose presence the flower
depends for its fertilization. So far as climate is concerned
high altitude is equivalent to high latitude. On certain
mountains the different zones of altitude and the corre-
sponding zones of plant and insect life are very sharply
defined (Fig. 178).

The North Temperate realm comprises all the land be-
tween the northern limit of trees and the southern limit of
frost. It includes, therefore, nearly the whole of Europe,
most of Asia, and most of North America. While there
are large differences between the fauna of North America
and that of Europe and Asia, these differences are of minor
importance and are scarcely greater in any case than the
difference between the fauna of California and that of our
Atlantic coast. The close union of Alaska with Siberia
gives the arctic region an almost continuous land area from
Greenland to the westward around to Norway. To the
south everywhere in the temperate zone realm the species
increase in number and variety, and the differences between
the fauna of North America and that of Europe are due in
part to the northward extension into the one and the other
of types originating in the tropics. Especially is this true
of certain of the dominant types of singing birds. The
group of wood-warblers, tanagers, American orioles, vireos,
mocking-birds, with the fly-catchers and humming-birds so
characteristic of our forests, are unrepresented in Europe.
All of them are apparently immigrants from the neotropical
realm where nearly all of them spend the winter. In the
same way central Asia has many immigrants from the Indian
realm to the southward. With all these variations there

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GEOGRAPHICAL DISTRIBUTION OF ANIMALS) 30]

is an essential unity of life over this vast area, and the rec-
ognition of North America as a separate (nearctic) realm,
which some writers have attempted, seems hardly practi-
cable.

The Neotropical or South American realm includes
South America, the West Indies, the hot coast lands of
Mexico, and those parts of Florida and Texas where frost
does not occur. Its boundaries through Mexico are not
sharply defined, and there is much overlapping of the north
temperate realm along its northern limit. Its birds espe-
cially range widely through the United States in the sum-
mer migrations, and a large part of them find in the North
their breeding home. Southward, the broad barrier of the
two oceans keeps the South American fauna very distinct
from that of Africa or Australia. The neotropical fauna is
richest of all in species. The great forests of the Amazon
are the dreams of the naturalists. Characteristic types
among the larger animals are the snout or broad-nosed
(platyrrhine) monkeys, which in many ways are very distinct
from the monkeys and apes of the Old World. In many of
them the tip of the tail is highly specialized and is used as
a hand. The Edentates (armadillos, ant-eaters, etc.) are
characteristically South American, and there are many
peculiar types of birds, reptiles, fishes, and insects.

The Jndo-African realm corresponds to the neotropical
realm in position. It includes the greater part of Africa,
merging gradually northward into the north temperate
realm through the transition districts which border the
Mediterranean. It includes also Arabia, India, and the
neighboring islands, all that part of Asia south of the limit
of frost. In monkeys, carnivora, ungulates, and reptiles
this region is wonderfully rich. In variety of birds, fishes,
and insects the neotropical realm exceeds it. The monkeys
of this district are all of the narrow-nosed (catarrhine)
type, various forms being much more nearly related to
man than is the case with the peculiar monkeys of South

302 ANIMAL LIFE

America. Some of these (anthropoid apes) have much
in common with man, and a primitive man derived from
these has been imagined by Haeckel and others. No
creature of this character is yet known, but that it may
have once existed is not impossible. To this region be-
long the elephant, the rhinoceros, and the hippopotamus,
as well as the lion, tiger, leopard, giraffe, the wild asses,
and horses of various species, besides a large number of
ruminant animals not found in other parts of the world.
It is, in fact, in its lower mammals and reptiles that its
most striking dis-
tinctive characters .
- are found. In its
fish fauna it has
very much in com-
mon with South
America.

The Lemurian
realm comprises
Madagascar alone.
It is an isolated di-
vision of the Indo-
African realm, but
the presence of
many species of
lemur and an un-
specialized or
primitive type of
lemur is held to

Fie. 179.—A lemur (Lemur varius). justify its recogni-

tion as a distinct

realm. In most other groups of animals the fauna of Mada-
gascar is essentially that of neighboring parts of Africa.

The Patagonian realm includes the south temperate

zone of South America. It has much in common with the
neotropical realm from which its fauna is mainly derived,

GEOGRAPHICAL DISTRIBUTION OF ANIMALS 303

but the presence of frost is a barrier which vast numbers of
species can not cross. Beyond the Patagonian realm lies
the Antarctic continent. The scanty fauna of this region
is little known, and it probably differs from the Patagonian
fauna chiefly in the absence of all but the ice-riding species.

The Australian realm comprises Australia and the
neighboring islands. It is more isolated than any of the
others, having been protected by the sea from the invasions
of the characteristic animals of the Indo-African and tem-
perate realms. It shows a singular persistence of low or
primitive types of vertebrate life, as though in the process
of evolution the region had been left a whole geological
age behind the others. It is certain that if the closely
competing fauna of Africa and India could have been able
to invade Australia, the dominant mammals and birds of
that region would not have been left as they are now—mar-
supials and parrots.

It is only when barriers have shut out competition that
simple or unspecialized types abound. The larger the land
area and the more varied its surface, the greater is the
stress of competition and the more specialized are its char-
acteristic forms. As part of this specialization is in the
direction of hardiness and power to persist, the species from
the large areas, as a whole, are least easy of extermination.
The rapid multiplication of rabbits and foxes in Australia,
when introduced by the hand of man, shows what might
have taken place in this country had not impassable barriers
of ocean shut them out.

160. Subordinate realms or provinces.— Each of these great
realms may be indefinitely subdivided into provinces and
sections, for there is no end to the possibility of analy-
sis. No school district has exactly the same animals or
plants as any other, as finally in ultimate analysis we find
that no two animals or plants are exactly alike. Shut off
one pair of animals from the others of its species, and its
descendants will differ from the parent stock. This differ-

304 ANIMAL LIFE

ence increases with time and with distance so long as the
separation is maintained. Hence new species and new
fauna or aggregations of species are produced wherever
free diffusion is checked by any kind of barrier.

161. Faunal areas of the sea.—In like manner, we may
divide the oceans into faunal areas or zones, according to
the distribution of its animals. For this purpose the fishes
probably furnish the best indications, although results very
similar are obtained when we consider the mollusks or the
crustacea. The fresh-water fishes are not considered here,
as in regard to their faunal areas they agree with the land
animals of the same regions. Perhaps the most important
basis for primary divisions is found in the separation from
the localized shore-fishes of the cosmopolitan pelagic species,
and the scarcely less widely distributed bassalian species or
fishes of the deep sea.

The pelagic fishes are those which inhabit the open sea,
swimming near the surface, and often in great schools.
Such forms are mainly confined to the warmer waters.
They are for the most part predatory fishes, strong swim-
mers, and many of the species are found in all warm seas.
Most species have special homing waters, to which they
repair in the spawning season. Often there will be special
regions to which they never resort, either for breeding or
for food. At other times a certain species will appear in
numbers in regions where it has hitherto been unknown.
For example, the frigate-mackerel (Auais thazard), homing
in the East Indies and the Mediterranean, appeared in
great numbers in 1880 off the coast of New England. Typ-
ical pelagic fishes are the mackerel, tunny, dolphin, flying-
_fish, opah, and some species of shark. This group shades
off by degrees into the ordinary shore-fish, some being partly
pelagic, venturing out for short distances, and some are
pelagic for part of the year only. To the free-swimming
forms of classes of animals lower than fishes, found in the
_ open ocean, the name Plankton is applied.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS) 305

The dassalian fauna, or deep-sea fauna, is composed of
species inhabiting great depths (2,500 feet to 25,000 feet)
in the sea. At a short distance below the surface the
change in temperature from day to night is no longer felt.
At a still lower depth there is no difference between winter
and summer, and still lower none between day and night.
The bassalian fishes in-
habit a region of great
cold and inky darkness.
Their bodies are subjected
to great pressure, and the
conditions of life are prac-
tically unvarying. There
is therefore among them
no migration, no seasonal
change, no spawning sea-
son fixed by outside con-
ditions, and no need of
adaptation to varying en-
vironment. Asa result, all
are uniform indigo-black
in color, and all show more
or less degeneration in
those characters associated
with ordinary environ-
ment. Their bodies are
elongate, from the lack of Fre. 180.—A erinoid (hizocrinus loxoten-

specialization in the ver- sis). A deep-sea animal which lives,
€ fixed plant like, at the bottom of the
tebre. The flesh, being ocean.

held in place by the great

pressure of the water, is soft and fragile. The organs of
touch are often highly developed. The eye is either exces-
sively large, as if to catch the slightest ray of light, or else
it is undeveloped, as if the fish had abandoned the effort
to see. In many cases luminous spots or lanterns are de-
veloped by which the fish may see to guide his way in the

21

2

a SOR ey ANIMAL LIFE

sea, and in: some forms these shining appendages are highly

- developed. ‘In one form (thoprora) a luminous body cov-
ers tlie end of the nose, like the head-light of an engine.

In another (/pnops) the two eyes themselves are flattened
out, covering the whole top of the head, and are luminous
in life. Many of these species have excessively large teeth,
and some have been known to swallow animals actually
larger than themselves. Those which have lantern-like
spots have always large eyes.

The deep-sea fishes, however fantastic, have all near rel-
atives among the shore forms. Most of them are degener-
ate representatives of well-known species—for example, of
eels, cod, smelt, grenadiers, sculpin, and flounders. The
deep-sea crustaceans and mollusks are similarly related to
shore forms.

The third great subdivision of marine animals is the
littoral or shore group, those living in water of moderate
depth, never venturing far into the open sea either at the

‘surface or in the depths. This group shades into both

the preceding. The individuals of some of the species are
excessively local, remaining their life long in tide pools or
coral reefs or piles of rock. Others venture far from home,
and might well be classed as pelagic. Still others ascend
rivers either to spawn (anadromous, as the salmon, shad,
and striped bass), or for purposes of feeding, as the robalo,
corvina, and other shore-fishes of the tropics. Some live
among rocks alone, some in sea-weed, some on sandy shores,
some in the ‘surf, and some only in sheltered lagoons. In
all seas there are fishes and other marine animals, and
each creature haunts the places for which it is fitted.

CLASSIFICATION OF ANIMALS *

In this diagram of classification every animal] referred to in this book, either by its
vernacular or its scientific name, is assigned to its proper class and branch. Of the
species mentioned by their scientific names, only the genus name is given in this list.

KINGDOM ANIMALIA

BRANCH I. PROTOZOA

Ciass I, Rhizdp’oda.
Ama’ ba, Globigeri’nex, Radiola ria.
Cuass II. Mycétozd'a.
Cuass III. Mastigdph'ora.
Volvocin'ex, Gé'niwm, Paindori'na, Eudori'na, Vol'véx.
Crass IV, SpG6rozd'a. —
Grégari na.
Ciass V. Infusd’ria.
Péirame' cium, Vorticél'la.

BRANCH IL. PORIF’ERA

Cuass I. Porifera.
Sponges, Cdlcolijn'thus, Prophysé'ma, Spongil'la, Spon'gia,
Cli ona.

BRANCH III. CQ2LEN'TERA’TA (sé-lén-te-ra'-ta)

Crass I. Hydrozd'a.
Hi dra, Eucd'pe, Siphondph'ora, Phisdph' ora, Obé'lia, sea-aném'-
one, pol'yp, Phisa'lia, Péirapdg'urus.
Cuass II. Seyphozd’a (si-f6-z6'-a).
Jelly-fish, Ltz'zia.
‘Crass III. Actinozd’a.
Cor'als, Métri' dium.
Ciass IV. Cténdph’ora (tén-dph’-o-ra).

* The arrangement of branches (or phyla) and classes here used is that adopted in
Parker and Haswell’s Text-Book of Zodlogy (1897).
307

308 ANIMAL LIFE

BRANCH IV. PLATYHELMIN’THES

Cuass I. Turbella’ria.
Plana ria.
Cuass I]. Trématd'da.
Cuass III. Cé&std’da.
Tape-worm, Te'nia, Lig'ula, flat-worm.
APPENDIX TO PLATYHELMINTHES—CLASs Nemertin ea.

BRANCH V. NEMATHELMIN’THES

Cuiass I, Nématd’da.
Sin'gamus, round-worm, Tvrichi'na, Bothriocéph'alus, pup-
worm, Uncind'ria.
Cuass I]. Acanthocéph’ala.
Ciass III, Cheetdg’natha (ké-tdg’-na-tha).

BRANCH VI. TROCHELMIN’THES

Cuass I. Rotif’era.
Rotatd’ria.

Cuass II. Dinophi'lea.

Cuass III. Gastrdt’richa.

BRANCH VII. MOLLUSCOI'DA

Ciass I. Pédlyzd'a.
Cuass Il. Phord'nida.
Cuass III. Brachidp’oda.

BRANCH VIII. ECHINODER’MATA

Cuass I. Asteroi' dea,
Starfish.

Crass II. Ophiuroi’ dea.

Cass III. Bchinoi’dea.
Sea-urchin. :

Ciass IV. Hédlothuroi dea.
Sea-cucumber.

Cuass V. Crinoi’dea.
Crinoid, Rhizicri'nus.

Cass VI. Cy¥stoi' dea.

Cuass VII, Blastoi’'dea.

CLASSIFICATION OF ANIMALS 309

BRANCH IX. ANNULA'TA

Cuass I, Cheetdp'oda (ké-tdp’-o-da),
Earth- worm.
APPENDIX TO THE CH&TOPODA—CLASS Myzostém ida.
Cuass II. Géphyré'a (jéf-e-ré'-a).
Cuass III. Archi-annéliida,
Cuass IV, Hirudin’ea.

BRANCH X. ARTHROP’ODA

Cuass I, Crusta’cea.

Lobster, cray-fish, crab, barnacle, Lé'pas, hermit-crab, Pag'urus,
pea-crab, Pinnothé'res, E'pizodn'thus, fish-lice, whale-lice, Saceu-
li'na, Lerneo' cera, prawn, Pené us.

APPENDIX TO CrustacEA—C.uass Trilobi ta.
Cuass II. Onychiph ora.
Cuass III. Myriap’oda.
Cén’tipéd.
Cuass IV. Inséc'ta.

Water-beetle, water-bug, canker-worm moth, bee, white ant,
cockroach, mosquito, weevil, grasshopper, caterpillar, butterfly,
katydid, beetle, Dip'tera, Lépiddp'tera, monarch butterfly, And'sia,
Cu'lex, Meldn'oplus, May-fly, locust, cottony-cushion scale, Jcé’rya,
lady-bird, Véda'lia, praying-horse, Mdn'tis, Sér'phus, Cécro'pia,
gall insect, An'dricus, mole-cricket, Gryllotal'pa, Hydréph'ilus,
Prid'nus, Campond'tus, plant-lice, Aph’ide, Coéc'cide, Aphis-lion,
ant, He'iton, termite, bumble-bee, carpenter-bee, André'na, Halic'-
tus, yellow-jacket, hornet, Vés'pa, wasp, At'ta, bird-lice, Malloph’-
aga, flea, louse, Pedic'ulus, Lipeu'rus, Himendp'tera, ichneumon
fly, Thalés'sa, horn-tail, Tré'mex, Polis'tes, Stylép'ide, Sty'lops,
red orange-scale, toad-bug, Gdl'gulus, inch-worm, span-worm,
geometrid, walking-stick, Diapherdm’era, PAgl'lium, meadow
brown, Grdap'ta, Kal'lima, sphinx-moth, tomato-worm, Phlége-
thon 'tius, puss-moth, Cerii'ra, viceroy butterfly, Basilar'chia, Da-
na'ide, Helicén'ide, Piér'ide, Papilidn'ide, Syr’phide, flower-flies,
tree-hopper, Membric'ide, Hemip'tera, Saw'ba (saw'-ba), carrion-
beetle, Callosa'’mia, promé'thea, cricket, cicada, dragon-fly, Cynip’-
ide, Hessian-fly, mud-dauber, Leré'ma, Erin'nis, skipper butterfly,
Schistocér' ca.

Cuass V. Aradch’nida.

Tardig’rada, bear-animalecule, scorpion, Lycds'ide, tick, itch-

mite, Sarcdp'tes, spider, trap-door spider, turret-spider, Cteni'za.

310 ANIMAL LIFE

BRANCH XI. MOLLUS'CA

Crass I. Pélecyp’oda.
Clam, pond-mussel, i'ma.
Cuass II. Amphineu'ra.
Cuass III. Gastrdp’oda.
Pond-snail, Lymnz'us, whelk.
APPENDIX TO THE GASTROPODA—CLAssS Scaphdp’oda anp Rh36’dope.
Ciass IV. Cé&phaldp’oda.

BRANCH XII. CHORDA'TA

Sup-BrancH I. Adélochdr'da. Ciass Adelochorda.
Sus-srancH II, Urochor'da. Cuiass Urochorda.

Sea-squirts, Tunica 'ta.

Susp-Brancu III. Vertebrata. Division A. Acrania. Cass Aocra-
nia. Division B, Crania ta.

Crass I, Cyclostém ata.

Cuass I], Pis'ces (pis’-séz).

Codfish, sculpin, skate, lady-fish, Al'bula, sword-fish, Xtph'ias,
flounder, Platéph'rys, Salanx, Cot'tus, blob, miller’s- thumb,
conger-eel, Rém'ora, Exoce'tus, flying-fish, Cypselu'rus (sip-se-lu -
rus), deep-sea angler, lantern-fish, Coryndl'ophus, Echtos'toma,
AGthéph' ora, nokee, scorpion-fish, Emmydrich'thys, mad-tom,
Schilbeddes, cat-fish, horned pout, toad-fish, sting-ray, globe-fish,
porcupine-fish, torpedo, electric eel, electric cat-fish, star-gazer, elec-
tric ray, Urol'ophus, Di'odon, Narci'né, Ra'ja, black-fish, mud-fish,
trout, Sdl’mo, chub, horned dace, Echeneidide Amphiprion, No'-
meus, hag-fish, Myxi'ne, Heptatré’ma, Polistotré’ma, lamprey,
Oligocot'tus, mouse-fish, lava-fish, Pterophrijne, pipe-fish, Phyllép'-
teryx, anglers, Lé'phius, Antennd'rius, Ceratiide, minnow, mack-
erel, sucker, salmon, shad, alewife, sturgeon, striped bass, quin’nat,
eel, sun-fish, stickle-back, carp, cutlass-fish, rainbow darter, chan-
nel-cat, Aw'xis, tunny, dolphin, opah, shark, thoprd'ra, Tp'nops,
cod, smelt, grenadier, rdb’alo, corvi'na, Chélogds'ter, Tiphlich’' this,
blind-fish.

Cuass III. Amphib’‘ia.
Toad, frog, salamander, tree-frog, H7j'la.
Cuass IV. Reptil'ia.

Tortoise, snake, horned toad, Phrijndsd'ma, rattlesnake, lizards,
An’'olis, chameleon, Gila monster, Hélodér'ma, Hlaps, coralil'los,
Léimpropél tis, Oscedla, alligator.

CLASSIFICATION OF ANIMALS 311

Ouass V. A’ves.

Bird-of-paradise, peacock, pheasant, robin, pigeon, chicken, eagle,
vulture, guil’‘lemot (gil’-e-mdt), murre (mir), auk, ful’mar, pét’rel,
sparrow, bluebird, woodpecker, owl, Coliim'ba, pelican, Melanér' pes,
cormorant, meadow-lark, warbler, turkey, blue jay, Aythya, Cya-
nocitta, Uria, cow-bird, cuckoo, parrot, ptarmigan, whippoor-
will, Antrés'tomus, gull, tern, fly-catcher, bittern, mocking-bird,
shrike, bobolink, goose, humming-bird, oriole, puffin, tailor-bird,
king-bird, nightingale, starling, skylark, passenger-pigeon, ouzel
(ooz'-el), linnet, tanager, vireo, wood-warbler, Phdlacrd'coraa,
Troch'ilus, Ornithot omus, Psaltrip' arus.

Cuass VI, Mammalia.

Horse, ram, fur-seal, rabbit, cat, ox, tiger, lion, sheep, elephant,
whale, bear, wolf, squirrel, lém’ming, fox, dog, weasel, stoat, rein-
deer, otter, ant-eater, giraffe’, skunk, porcupine, hedgehog, arma-
dillo, Callorhi'nus, sea-lion, deer, buffalo, kangaroo, Méc'ropus,
duck-bill, Mén’otréme, monkey, gopher, elk, bison, prairie-dog,
big-horn, hare, antelope, black-tail deer, hound, mole, hyena, mice,
rodent, woodchuck, jack-rabbit, Macd'cus, Cércopith'ecus, beaver,
wood-rat, pocket-gopher, coyo'te, civet-cat, flying-fox, mén’goose,
sea-cow, Viil'pes, walrus, musk-ox, ape, rhindc’eros, hippopdt’amus,
leopard, ass, 1é’mur, 7rich'échus, manatee, Lé' pus, Oddbe'nus,
Lé mur.

GLOSSARY

[Only those terms are defined in this glossary that are not explained in the text.
In the case of the terms defined or explained in the text, reference is made to the
number of the paragraph in which the explanation occurs. The pronunciation of the
vernacular and scientific names of the animals mentioned in the text is given in the
Classification.]

Aboma’sum: 42.

Adaptation: 67, 74.

Albuminous: said of substances containing albumen,

Alimén'tary canal: 42.

Alluring coloration: 111.

Altric'ial : 79.

Altruis'tic instinct: 129.

Ameoe’boid: having the changing form of an Amceba.

Andd'romous: said of fishes that go from the sea up rivers to lay their
eggs.

Anatomy : 39.

Animal'cule: an animal of microscopic smallness.

Antén’nez : the “feelers,” the most anterior pair of appendages of in-
sects and insect-like animals; situated on the head, and the seat of
organs of special sense.

An'thropoid : man-like.

A’nus: 42.

Appén dix vermifor'mis: 82.

Artificial selection : 72.

Assim ilate : to receive food and transform it into a homogenous part
of the body substance.

At6ll' : a ring-shaped coral island nearly or quite inclosing a lagoon.

Atrophy : a stoppage of the growth or development of a part or organ.

Auditory : referring to the sense of hearing.

Autédm atism: the state of being automatic; involuntary action.

313

314 ANIMAL LIFE

Bassa lian: 160.

Bid! ogist : student of animals and plants.

Blas'to6derm : 50.

Blas'tula: 50.

Budding: the process of reproduction among animals in which a small
part of the body substance of an animal grows out from the sur-
face, separates from the parent, and develops into a new individual.

Ce’cum (sé-kum): 42.

Carniv orous: flesh-eating.

Cat’arrhine: nostril downward; said of the narrow-nosed Old-World
monkeys.

Cell: 2.

Cél'lulose: a peculiar compound insoluble in all ordinary solvents,
forming the fundamental material of the structure of plants, and
also contained in the mantle of tunicates. .

Chitin: 57.

Chld'rophyll: 13.

Chrd'matophore : a color-bearing granule or sac.

Chr6’mosome: 2.

Chrysalis : 57.

Chyle: 42.

Cilia: 5.

Cleavage: 50.

Cd'lon: 42.

Commén’'salism: 90.

Cdm'munal: 83.

Conjugation : 5.

Contractile vacuole: a vacuole that dilates and contracts regularly,
and is supposed to have an excretory function.

Cyst: 98.

Cytoplasm : 2.

Degeneration: 95.

Development : 46.

Differentiation : the setting apart of special organs for special work ;
progressive change from general to special ; specialization.

Digestion: the process of dissolving and chemically changing food so
that it can be assimilated by the blood and furnish nutriment to
the body.

Dimor’phism : 24,

Divertic’ulum: a blind pouch arising from another larger pouch. 44.

Duodé’num: 42.

GLOSSARY 315

Ec'tdblast : 50.

Ec'téderm: 20.

Egg-cell: 20.

Egois'tic instinct: 129.

Embrydl ogy : 39.

Embryonic: 49.

En'ddblast : 50.

En dédderm : 20.

Environment: an organism’s surroundings taken collectively.

Ex'cretory : referring to excretion, as excretory organs, the organs
which get rid of waste matter in the animal body.

Fau’na (fawna): 157.
Fertilized egg: 20.
Fission: 4.
Flagél'la: 13.
Function: 37.

Ganglion (pl. ganglia): a nerve-center composed of an aggregation of
nerve-cells,

Gas'trula: 50.

Gém'mule: 20.

Generalization: 41.

Gé6l ogist: student of the structure and history of the earth.

Gréga rious: 87.

Growth: 46,

Habit: 139.

Herbiv’ orous: plant-eating.

Heréd'ity : 54.

Hermaphroditiic: 35.

Hibernation: passing the winter in a death-like sleep.
Hodmogé'neous: of the same composition or structure throughout.

T'leum: 42.

Inorganic: not being nor having been a living organism ; not organic.
Insectiv orous: insect-eating.

Instinct: 128.

Intellect : 139.

Intercél'lular: outside of and between the cells.

Jeju'num: 42.

316 ANIMAL LIFE

Lagoon’: a pool or lake; the still water inclosed within an atoll.
Lar'va: 57.

Lépiddp'terous : referring to the Lepidoptera, or moths and butterflies,
Lit'toral: 160.

Lu'men: the cavity of a tubular organ. 44.

Medu'sa: 24.

Még'alops: 59.

Més'oderm: 20.

Métab'olism: the act or process by which dead food is built up into
living matter, and living matter is broken down into simpler prod-
ucts within a cell or organism.

Métamor’ phosis: 56.

Migration: 70.

Millimeter: about one twenty-fifth of an inch; a term used in the
metric system of measure.

Mimicry: 112.

Mind: 140.

Molt: 57.

Méndg'amous: said of animals in which a male mates with only a
single female.

Multiplication: used in the text usually synonymously with repro-
duction.

Myrmecdph ilous: said of insects which are found inhabiting the nests
of ants.

Natural selection: 70.

NG'tochord: an elastic rod, or row of cells, formed in the early embryo
of chordate animals (including all the vertebrates and some others),
which lies below the dorsal nervous tube and above the ventral ali-
mentary tube.

Nucleus (pl. nuclei): 2.

CEsdph'agus: 42.

Olfactory: referring to the sense of smell.

Oma’'sum: 42,

Organ: 37.

Organic: referring to the matter of which animals and plants are com-
posed.

Organism: a living being, plant or animal.

Orienta'tion: 95.

O'tolith: 121.

GLOSSARY 317

Papil'la (pl. papillee): a small nipple-like process, as the papille of the
skin or tongue.

Parasite: 93.

Parthénogén esis: 35.

Pélag'ic: inhabiting the surface of mid-ocean. 160.

Phar’ ynx: 42.

Physidl ogy : 39.

Plat'yrrhine: broad-nosed ; said of the New-World monkeys.

Plu'teus: 59.

Polyg amous: said of animals in which a single male mates with sev-
eral females: 35.

Pdlymor’ phism : 24,

Pdlyp: 21.

Post-embryonic: 49

Preecd'cial (pré-co’-shal) : 79.

Prédatory : feeding on other animals.

Prd'polis: 84.

Protective resemblance: 107.

Prd toplasm : 2.

Prdventric'ulus: 42.

Pseu dopod: 4.

Psy'chic: 140.

Pu'pa: 57.

Reason: 139.

Recognition mark: 77, 115.
Réc'tum: 42.

Reflex action: 127.
Reproduction: 67.
Respiration: 4.
Retic’'ulum: 42.

Rétina: 128.

Rumen: 42.

Ru'minant: 42.

Saliva: 42.

Sensation : 67.

Sensorium: 126.

Silica: the mineral of which quartz, sand, flint, etc., are composed.
Spawn, v.: to lay eggs.

Specialization: 41.

Species: 151.

318 ANIMAL LIFE

Sperm cell: 20.

Spir’acle: one of the breathing openings of an insect situated on the
side of the abdomen or thorax.

Spon’gin: 20.

Stimulus (pl. stimuli): that which excites action in plant or animal
tissue.

Stra'ta: layers, usually said of rocks.

Stridula' tion: 122.

Sub-species: 151.

Symbid'sis : 90.

Tactile: referring to the sense of touch: 118.

Tadpole: 58.

Tén'tacle: a protruding flexible process or appendage, usually of the
head of invertebrate animals, being used as an organ of touch,
prehension, or motion.

Térmitdph ilous : said of insects inhabiting the nests of termites,

Vac'uole: a minute cavity containing air, water, or a chemical secre-
tion of the protopiasm, found in an organ, tissue, or cell.

Vestig'ial: 82.

Vis'cera: the organs in the great cavities of the body, commonly used
for the organs in the abdominal cavity.

Volk (yok): 48,

Zoé'a: 59.

Zoogedg raphy: 147.

ZO oid: one of the more or less independent members of a colonial or
compound organism.

Z6d! ogist : a student of animals.

INDEX

Abomasum, 68.

Actinocephalus oligacanthus (ill.),
14.

Adaptations, 118, 123; classifica-
tion of, 123 ; concerned with sur-
roundings, 143; degree of struc-
tural change in, 146; for defense
of young, 137; for rivalry, 135 ;
for self-defense, 128; for secur-
ing food, 125; origin of, 123.

Agassiz’s cave-fish (illus.), 282.

Albula vulpes, metamorphosis of
(illus.), 98.

Alimentary canal, 66; of cock-
roach (illus.), 73; of earthworm
(illus.), 71; of flatworm (illus.),
70; of Holothurian (illus.), 70;
of mussel (illus.), 72; of Obelia
(illus.), 69; of ox (illus.), 67; of
Planaria (illus.), 70; of sea-
cucumber (illus.), 70.

Alligator (illus.), 290.

Alluring coloration, 216.

Alternation of generations, 42.

Altricial, 140.

Ameba,5; multiplication of (ill.) 53.

Ameba polypodia (illus.), 8.

Anatomy, 64.

Andrena, nest of (illus.), 160.

Andricus californicus, galls of
(illus.), 148.

Angler, deep-sea (illus.), 124.

Animals, life of simplest, 1; many-
celled, 2; one-celled, 2; slightly
complex, 24.

Anosia plexippus, metamorphosis
of (illus.), 92; mimicked by Ba-
silarchia archippus (illus.), 219.

Antenna of cray-fish (illus.), 233 ;
of leaf-eating beetle (illus.), 280.

Antenne, specialized, of prome-
thea moth (illus.), 281.

Antrostomus vociferus (illus.), 203.

Ants (illus.), 155.

Anus, 68.

Appearance, terrifying, 212.

Arctic realm, 297.

Area, faunal, 296.

Artificial selection, 120.

Auditory organ of cray-fish (illus.),
233; of cricket (illus.), 234; of
grasshopper (illus.), 284; of mol-
lusk (illus.), 283; of mosquito
(illus.), 235.

Auditory organs, 232.

Australian realm, 303.

Aythya (illus.), 1387.

Barbadoes earth, 19.
Barnacle, adult and larva (illus.),
195; metamorphosis of (illus.),

101. 319

320

Barrier, mountains a, to distribu-
tion, 293; sea a, to distribution,
288; temperature a, to distribu-
tion, 290.

Barriers affecting fresh-water ani-
mals, 294; effect of, 283; species
debarred by, 274; to distribu-
tion, character of, 288.

Basilarchia archippus mimicking
Anosia plexippus (illus.), 219.

Bassalian fauna, 305.

Beavers, nest of (illus.), 269.

Beetle, larva of (illus.), 146.

Beetles, lady-bird (illus.), 214.

Bird, egg of (illus.), 79.

Bird-louse (illus.), 188.

Bird of paradise (illus.), 58.

Birds, nest-making habits of, 264.

Birth, 78.

Bittern, nestlings of (illus.), 246,
247.

Blastoderm, 82.

Blastula, 82.

Blue jay (illus.), 138.

Brain, 241.

Budding, 13.

Bumble-bee, 159, (illus.), 161.

Bush-tit, nest of California (illus.),
270.

Butterfly, egg of (illus.), 79; mon-
arch, metamorphosis of (illus.),
92.

Ceecum, 68.

Calcolynthus porimigenius (illus.),
33. :

Calf, taste buds of (illus.), 229.

Callorhinus alascanus (illus.),
136.

Camponotus (illus.), 155.

Canadian skipper butterfly, distri-
bution of (illus.), 273.

ANIMAL LIFE

Canal, alimentary, 66.

Cankerworm-moth (illus.), 59.

Care of young of mammals, 268.

Carpenter-bee, nest of (illus.), 160.

Caterpillar parasitized (illus.), 189,
(illus.), 190.

Cave blind-fish (illus.), 282.

Ceanothus (illus.), 141.

Cell, animal, 2; egg, 21,56; plant,
2; products, 3; wall, 3.

Cells, brood, of honey-bee (illus.),
152; nerve, 240; reproductive,
29, 55.

Cellulose, 24, 27.

Centiped (illus.), 180.

Cerura, larva of (illus.), 216.

Chalk, 18.

Chick, embryonic stages of (illus.),
87.

Chitin, 91.

Chlorophyll, 24.

Chologaster agassizi (illus.), 282.

Chologaster avetus (illus.), 282.

Chromatophore, 24.

Chromosome, 3.

Chrysalid of butterfly, showing
protective resemblance (illus.),
206.

Chrysalis, 93.

Chyle, 68.

Cilia, 9.

Cleavage, 82.

Climate, influencing distribution,
291; instincts of, 248.

Clouded skipper butterfly, distri-
bution of (illus.), 273.

Coccidium oviforme (illus.), 14.

Cockroach, alimentary canal of
(illus.), 73; egg case of (illus.),
140.

Cocoon of Cecropia moth (illus.),
141.

INDEX

Colon, 68.

Colonial jelly-fishes, 45; Protozoa,
24.

Colony, 31.

Color, 222.

Coloration, alluring, 216.

Colors, warning, 212.

Commensalism, 172, 173.

Communal life, 168; advantages
of, 170.

Communities, animal, 149.

Conditions, primary, of animal
life, 106.

Conjugating cells, 28.

Conjugation, 11, 27, 55.

Contractile vacuole, 10.

Coral, brain, 45; island (illus.),
44; organ-pipe (illus.), 45; red,
45.

Corals, 37-43.

Corynolophus reinhardti (illus.),
124,

Cottony cushion scale (illus.), 142.

Courtship, instincts of, 248.

Crab, metamorphosis of (illus.), 97;
with sea-anemone (illus.), 177.
Cray-fish, auditory organ of (illus.),

233.

Cricket, auditory organ of (illus.),
234,

Cricket, mole (illus.), 146.

Crinoid (illus.), 305.

Crop, 71.
Crowd of animals, 114.
Crustaceans, adults and larve

(illus.), 195.
Cteniza californica, nest of (illus.),

261.
OCyanocitta cristata (illus.), 188.
Cycle, life, 78.
Cypselurus (illus.), 131.
Cytoplasm, 3.

22

321

Dead-leaf butterfly (illus.), 211.

Death, 103.

Deep-sea angler (illus.), 124.

Deer, horns of (illus.), 148.

Defense of the young, 137.

Degeneration, causes of, 197, 198;
human, 200; through quiescence,
193.

Desiccation, 104.

Development, 78; continuity of,
83; divergence of, 84; embry-
onic, 80; first stages in (illus.),
81; laws of, 86; metamorphic,
90; of flounder (illus.), 100; of
locust (illus.),91; of vertebrates,
(illus.), 8%; post-embryonic, 80;
significance of facts of, 89.

Diapheromera femorata (illus.),
209.

Differentiation, 41; of structure,
64,

Dimorphism, 42; sex, 58.

Diodon hystriz (illus.), 134.

Dismal Swamp fish (iilus.), 282.

Distribution, character of barriers,
to, 288; geographical, 272; in-
fluenced by climate, 291; laws
of, 274; modes of, 296; moun-
tains a barrier to, 293; of Cana-
dian Skipper butterfly (illus.),
273; of clouded Skipper butter-
fly (illus.), 273; of Hrynnis mant-
toba (illus.), 273; sea a barrier
to, 288; temperature a barrier
to, 290.

Diverticula, ‘74.

Division of labor, 22, 168.

Dog, pointer (illus.), 256.

Dragon-fly, eye of (illus.), 239.

Duck, family (illus.), 137.

Duodenum, 68.

Duration of life, 101.

322

Earthworm, alimentary canal of
(illus.), 71.

Ectoblast, 82.

Ectoderm, 33.

Egg case of Californian barn-door
skate (illus.), 140; cockroach (il-
lus.), 140.

Egg cell, 21, 56.

Egg, fertilized, 35; of bird (illus.),
79; of butterfly (illus.), 79; of
fish (illus.), 79; of katydid (il-
lus.), 79; of skate (illus.), 79; of
toad (illus.), 79.

Electric ray, (illus.), 135.

Embryology, 64.

Embryonic development, 80; of
the pond snail, 81.

Emmydrichthys vulcanus (illus.),
132.

Endoblast, 82.

Endodern, 33.

Environment, instincts of, 248.

Epizoanthus paguriphilus, with
sea-anemone (illus.), 177.

Erynnis manitoba, distribution of
(illus.), 273.

Eucope (illus.), 42.

Eudorina, 27.

Eudorina elegans (illus.), 28.

Exocetus (illus.), 131.

Eye of dragon-fly (illus.), 239; of
jelly-fish (illus.), 238.

Fauna, 296; bassalian, 305 ; littoral,
306; pelagic, 304.

Faunal areas of the sea, 804.

Feeding habit of Californian wood-
pecker (illus.), 128,129; of Wela-
nerpes formiciworus bairdii
(illus.), 128, 129; instincts of,
244,

Female, 57.

ANIMAL LIFE

Fish, egg of (illus.), 79; embry-
onic stages of (illus.), 87; -louse
(illus.), 188.

Fishes, man-of-war (illus.), 175;
nest-making habits of, 264.

Fission, 9; binary, 54.

Flagella, 25.

Flagellata, 24.

Flatworm, alimentary canal of
(illus.), 70. ;

Flounder, development of (illus.),
100; wide-eyed (illus.), 100.

Flying fishes (illus.), 131.

Food, adaptations for securing,
125; necessary to animal life,
106.

Form, primitive, 20.

Fossil, 18.

Fresh-water animals, barriers af-
fecting, 294.

Function, 63.

Fur seal (illus.), 186.

Galapagos Islands, animals of
(illus.), 278; locusts of (illus.),
280.

Gall, giant, of white oak (illus.),
1438.

Galls, insect, on leaf (illus.), 144.

Gapes, worm which causes, 60.

Gastrula, 82.

Gemmule, 35.

Generalization, 66.

Generation, spontaneous, 51.

Generations, alternation of, 51.

Geographical distribution, 272.

Geometrical larva on branch(illus.),
209.

Gerrhonotus scincicauda (illus.),
204.

Giraffe (illus.), 126.

Gizzard, 71.

INDEX

Globigerina-ooze, 18.

Globigerine, 16.

Goniwm, 25-30.

Gonium pectorale (illus.), 25.

Grasshopper, auditory organ of
(illus.), 234.

Green-leaf insect (illus.), 210.

Gregarina, 18, 182.

Gregarina polymorpha (illus.), 14.

Gregarinide, 14.

Gregariousness, 163.

Growth, 78.

Gryllotalpa (illus.), 146.

Habit, 251.

Habitat, relation of species of, 288.

Habits, domestic, 257.

Habits, nest-making, of birds, 264;
of fishes, 264; of insects, 262; of
invertebrates, 258; of spiders,
259; of vertebrates, 264.

Hearing, sense of, 232.

Heliosphera actinota (illus.), 19.

Heredity, 89.

Hermaphroditism, 60.

Hermit-crab, with the sea-anemone
(illus.), 176.

Hibernation, 103.

Hiving honey-bees (illus.), 154.

Holothurian, alimentary canal of
(illus.), 70.

Homes, 257.

Honey-bee (illus.), 150; adult and
larva (illus.), 88; leg of (illus.),
151: life history of, 149.

Honey-bees, hiving a swarm of
(illus.), 154.

Host, relation of parasite to, 179.

Human degeneration, 200.

Humming-bird, nest of
(illus.), 265, 266.

Hydra, 37.

rufus

323

Hydra vulgans (illus.), 38.
Hydrophilus (illus.), 146.
Hyla regilia (illus.), 145.

Icerya and Vedalia, 121.

Icerya purchasi (illus.), 142.

Tleum, 68.

Individual, 31.

Indo-African realm, 301.

Inorganic matter, 112.

Insect galls on leaf (illus.), 144.

Insects, metamorphosis of, 90; nest-
making habits of, 262; parasitic,
188.

Instinct, 242.

Instincts, altruistic, 248; classifi-
eation of, 248; concerned with
care of the young, 250; egoistic,
243; of climate, 248; of court-
ship, 248; of environment, 248 ;
of feeding, 244: of play, 247; of
reproduction, 249: of self-de-
fense, 245; variability of, 251.

Intellect, 254.

Intestine, 68.

Invertebrates, nest-making habits
of, 258.

Irritability, 8, 240.

Island, coral (illus.), 44.

Itch-mite (illus.), 192.

Jack-rabbits,
(illus.), 281.

Jay, Canada (illus.), 138.

Jejunum, 68.

Jelly-fish, eye of (illus.), 238.

Jelly-fishes, 37; colonial, 45.

showing variation

Kallima (illus.), 211.
Kangaroo (illus.), 139.
Katydid, egg of (illus.), 79.
Lady-bird beetles (illus.), 214.

324

Lady-fish, metamorphosis of (illus.),
98.

Larva, 92; of the mosquito, 98;
of butterfly pupating (illus.), 94;
of the honey-bee, 152.

Leaf-cutting ant mimicked by tree-
hoppers (illus.), 220.

Lemur (illus.), 302. ,

Lemur vartus (illus,), 802.

Lemurian realm, 302.

Lepas, adult and larva (illus.), 195 ;
metamorphosis of (illus.), 101.
Terema accius, distribution of

(illus.), 273.

Lerneocera (illus.), 188.

Life cycle, 78.

Life, communal, 168; duration of,
101; primitive, 21; processes,
21; social, 149.

Light, influence of, on animals,
237,

Lipeureus densus (illus.), 188.

Littoral fauna, 306.

Lizard, alligator (illus.), 204.

Lizzia koellikeri, eye of (illus.),
238.

Locust, post-embryonic develop-
ment of (illus.), 91.

Locusts of Galapagos
(illus.), 280.

Louse, sucking (illus.), 188.

Islands

Macropus rufus, 139.

Mad Tom (illus.), 182.

Male, 57.

Mammals, care of young of, 268.

Manatee (illus.), 277.

Man-of-war, Portuguese (illus.),
1%.

Mantis (illus.), 127.

Many-celled animal, 2.

Marine Protozoa, 15.

ANIMAL LIFE

» Marks, recognition, 22, 129, 223.

Meduse, 41.

Megalops, 97.

Melanerpes formicivorus bairdii,
feeding habit of (illus.), 128, 129.

Membracide mimicking Sauba
ant (illus.), 220.

Mesoderm, 33.

Metamorphosis, 90; of Albula vul-
pes (illus.), 98; of Anosia plex-
tppus (illus.), 92; of barnacle
(illus.), 101; of butterfly (illus.),
92; of crab (illus.), 97; of in-
sects, 90; of lady-fish (illus.),
98; of Lepas (illus.), 101; of
mosquito (illus.), 93; of sea-ur-
chin (illus.), 96: of sword-fish
(illus.), 99; of toad, 94, (illus.),
95; of Xiphias gladius (illus.),
99.

Metazoa, 32.

Metridium dianthus (illus.), 48.

Micro-organism, 16.

Migration of lemming, 118; of lo-
cust, 118.

Mimickry, 218.

Mind, 255.

Mining-bee, nest of (illus.), 160.

Molt, 91.

Monarch butterfly (illus.), 219;
mimicked by Viceroy butterfly
(illus.), 219.

Monogamy, 135.

Mosquito, auditor organ of (illus.),
235; head of (illus.), 127; meta-
morphosis of (illus.), 93; young
stages of (illus.), 147.

Moth, cankerworm (illus.), 59.

Mountains as barriers to distribu-
tion, 293.

Mouth parts of mosquito (illus.),
127.

INDEX

Mouse-fish in gulf-weed (illus), 208.

Mt. Orizaba (illus.), 300.

Multiplication, 50; of animals,
114; simplest method of, 53;
slightly complex methods of, 54.

Murres, Pallas’s (illus.), 165.

Mussel, alimentary canal of (illus.),
72.

Mutual aid, 163.

Narcine brasiliensis (illus.), 185.

Natural selection, 117.

Neotropical realm, 301.

Nerve cells, 240.

Nerve fibers, 240.

Nest-making habits of birds, 264;
of fishes, 264; of insects, 262;
of invertebrates, 258 ; of spiders,
259; of vertebrates, 264.

Nest of Baltimore oriole (illus.),
267; of beavers (illus.), 269; of
Californian bush-tit (illus.), 270;
of Cleniza californica (illus.),
261; of pocket- gopher (illus.),
271; of rufus humming- bird
(illus.), 265, 266; of tailor-bird
(illus.), 268; of trap-door spider
(illus.), 261; of turret - spider
(illus.), 262.

Nokee (illus.), 1382.

Nomeus gronovit (illus.), 175.

North Temperate realin, 299.

Nuclear membrane, 3.

Nucleus, 3.

Obelia, alimentary sac of (illus.),
69.

(Esophagus, 67.

Omasum, 67.

One-celled animals, 2.

Organ, 63; auditory, of cray-fish
(illus.), 233; of cricket (illus.),

325

234; of grasshopper (illus.), 234;
of mosquito (illus.), 235; of mol-
lusk (illus.), 233.

Organic matter, 112.

Organs, auditory, 232; of smell,
229; of sound-making, 235; of
taste, 228; of touch, 226; spe-
cialization .of, 66; vestigial, 147.

Oriole, Baltimore, nest of (illus.),
267.

Ornithotomous sutorius, nest of,
268.

Osprey Falls (illus.), 285.

Otolith, 232.

Ox, alimentary canal of (illus.) 67.

Oxygen, necessary to animal life,
107.

Pagurus (illus.), 176.

Pandorina, 26.

Pandorina sp. (illus.), 26.

Pandorina moruwm (illus.), 27.

Papilio, chrysalid of (illus.), 205.

Papilla, tactile, of skin of man
(illus.), 227.

Papille, 67.

Paramecium, 9.

Paramecium aurelia (illus.), 10.

Paramecium caudatum (illus.), 11.

Paramecium plutorinum (illus.),
ie

Parasite of caterpillar (illus.), 190;
relation to host, 179.

Parasites, simple structure of, 181.

Parasitism, 179; kinds of, 180.

Parthenogenesis, 60.

Patagonian realm, 302.

Pediculus (illus.), 188.

Pelagic fauna, 304.

Pelican, brown (illus.), 125.

Peneus, adult and larva (illus.),
195,

326

Pharynx, 71.

Phlegethontius carolina, larva of
(illus.), 215.

Phrynosoma blainvillet
131.

Phyllium (illus.), 210.

Phyllopteryx (illus.), 212.

Physalia (illus.), 175.

Physiology, 64.

Physoptora (illus.), 47.

Pocket-gopher, nest of (illus.), 271.

Pointer dog (illus.), 256.

Polistes, parasitized by Stylops
(illus.), 192.

Polygamy, 59.

Polymorphism, 42.

Polyps, 37.

Polystomella strigillata (illus.), 17.

Porcupine-fish (illus.), 134.

Post-embryonic development, 80.

Pigeon horn-tail (illus.), 191.

Pipe-fish (illus.), 212.

Planaria, alimentary canal of
(illus.), 70.

Plankton, 304.

Plants, difference between animals
and, 111.

Platophrys lunatus (illus.), 100.

Play, instincts of, 247.

Pluteus, 96.

Precocial, 140.

Prawn, adult and larva (illus.), 195.

Praying-horse (illus.), 127.

Pressure, a condition of animal
life, 109.

Primitive form, 20.

Primitive life, 21.

Prionus, larva of (illus.), 146.

Processes, life, 21.

Promethea moth (illus.), 231.

Prophysema primordiale (illus.),
34.

(illus.),

ANIMAL LIFE

Protective resemblance, 201.

Protoplasm, 3; chemical constitu-
tion of, 4; physical constitution
of, 4.

Protozoa, 1; colonial, 24; marine,
15.

Psaltriparus minimus, nest of
(illus.), 270.

Pseudopod, 5.

Pterophryne histrio in Sargassum
(illus.), 208.

Pupa, 93.

Puss moth, larva of (illus.), 216.

Quiescence, degeneration through,
193.

Rabbit, embryonic stages of (illus.),
87.

Radiolaria, 16.

Radiolaria-ooze, 19.

Raja binoculata (illus.), 140.

Realm, Arctic, 297; Australian,
3803 ; Indo-African, 301; Lemu-
rian, 302; Neotropical, 301;
North Temperate, 299; of ani-
mal life, 297; Patagonian, 302;
South American, 301.

Realms, subordinate, 303.

Reason, 251.

Recognition marks, 22, 129, 223.

Rectum, 68.

Reflex action, 241.

Remora (illus.), 173.

Remora remora (illus.), 125.

Reproduction, 9, 50; instincts of,
249.

Reproductive cells, 28, 55.

Resemblance, aggressive, 202; gen-
eral protective, 202; protective,
201; special protective, 207; va-
riable protective, 204,

INDEX

Respiration, 7.

Resting spore, 28.

Reticulum, 67.

Rhizocrinus loxotensis (illus.), 305.
Rivalry, adaptations for, 135.
Rookeries, fur-seal (illus.), 169.
Rumen, 67.

Sacculina (illus.), 187.

Sacculina, adult and larva (illus.),
195.

Salamander, embryonic stages of
(illus.), 87.

Saliva, 67.

Salmon leaping (illus.), 289.

Salmo viridens (illus.), 145.

Sarcoptes (illus.), 192.

Sauba, ant mimicked by Membra-
cide (illus.), 220.

Seale, red orange (illus.), 196.

Schilbeodes furtosus (illus.), 182.

Schistocerca (illus.), 280.

Scorpion (illus.), 127.

Scorpion-fish (illus.), 132.

Sea, a barrier to distribution, 288.

Sea, faunal areas of, 304.

Sea-anemone, 37; with alge in
body (illus.), 178.

Sea-cow (illus.), 277.

Sea-cucumber, alimentary canal of
(illus.), 70.

Sea-squirt (illus.), 194.

Sea-urchin, metamorphosis of (il-
lus.), 96.

Sea-urchins (illus.), 259.

Seal, fur (illus.), 186; pups killed
by parasite (illus.), 186; rook-
eries (illus.), 169.

Selection, artificial,120; natural,117.

Self-defense, adaptations for, 128;
instinets of, 245.

Sensation, 8.

327

Senses, special, 224; of the simplest
animals, 225.

Sensorium, 241.

Serphus (illus.), 141.

Sex,57; object of, 57; dimorphism,
58.

Shark-clinging fish (illus.), 125.

Sheep, bighorn (illus.), 167; Rocky
Mountain (illus.), 167.

Sight, sense of, 237.

Simplest animals, life of, 1.

Siphonophora, 46.

Skate, egg case of California barn-
door (illus.), 140; egg of (illus.),
79.

Skin of man, tactile papilla of
(illus.), 227.

Smell, sense of, 229.

Smelling organs, 229.

Smelling pits of leaf-eating beetle
(illus.), 230.

Social life, 149.

Sound-making, 235; organs, 285.

South American realm, 301.

Specialization, 66; of organs, 66.

Special senses, 224.

Species, altered by adaptation to
new conditions, 276; debarred
by barriers, 274; debarred by in-
ability to maintain their ground,
275; definition of, 279; relation
of, to habitat, 283.

Sperm cell, 35, 56.

Sphinx moth, larva of (illus.), 215.

Spicules, sponge, 33.

Spiders (illus.), 212; nest-making
habits of, 259.

Sponges, 32.

Spongin, 36.

Spontaneous generation, 51.

‘Spore, 15, 52; resting, 28.

Sting-ray (illus.), 183.

328

Structure,63; differentiation of, 64.

Struggle for existence, 116.

Stylops parasitizing Poltstes (illus.),
192.

Sub-species, definition of, 282.

Surroundings, adaptations con-
cerned with, 143.
Swallow-tail butterfly, chrysalid

of (illus.), 205.
Sword-fish, metamorphosis of (il-
lus.), 99.
Symbiosis, 172, 175.
Syngamus trachealis (illus.), 60.
Systematic zodlogy, 64.

Tactile organs, 226.

Tactile papilla of skin of man (il-
lus.), 227.

Tadpole, 94.

Tenia solium (illus.), 183.

Tailor-bird, nest of (illus.), 268.

Tape-worm (illus.), 183.

Taste buds of calf (illus.), 229.

Taste organs, 228.

Taste, sense of, 228.

Temperature a barrier to distribu-
tion, 290; a condition of animal
life, 108.

Tentacle, 37.

Termite, 158, (illus.), 159.

Terrifying appearances, 212.

Thalessa lunator (illus.), 191.

Toad, egg of (illus.), 79; horned
(illus.), 131; metamorphosis of,
94, (illus.), 95.

Torpedo (illus.), 135.

Tortoise, embryonic stages of (il-
lus.), 87.

Touch, sense of, 226.

Trap-door spider nest (illus.), 261.

Tree-hoppers mimicking leaf-cut-
ting ant (illus.), 220.

ANIMAL LIFE

Tree-toad (illus.), 145.

Tremex columba (illus.), 191.

Trichechus latirostris (illus.), 277.

Trichina spiralis (illus.), 184.

Tripoli, 19.

Trochilus rufus, nest of (illus.),
265, 266.

Trout, rainbow, head of (illus.),
145.

Two Ocean Pass (illus.), 287.

Tunicate (illus.), 194.

Turret-spider, nest of (illus.), 262.

Typhlichthys subterraneus (illus.),
282.

Uncinaria, killing fur-seal pups
(illus.), 186.

Uria lomvia arra (illus.), 165.

Urolophus goodet (illus.), 133.

Vacuole, 10; contractile, 10.

Variety, definition of, 282.

Vedalia and Icerya, 121.

Vertebrates, early stages in devel-
opment of (illus.), 87; nest-mak-
ing habits of, 264; parasitic, 193.

Vespa (illus.), 162; nest of (illus.),
163.

Vestigial organs, 147.

Viceroy butterfly mimicking Mon-
arch butterfly (illus.), 219.

Voice, 236.

Volvocine, 24.

Volvoz, 28.

Volvox globator (illus.), 29.

Volvox minor (illus.), 29.

Vorticella, 12.

Vorticella microtoma (illus.), 12.

Walking-stick insect (illus.), 209.
Warning colors, 212.
Walrus, Atlantic (illus.), 298,

INDEX

Wasps, social, 161.

Water - beetle (illus.), 146; bug,
giant (illus.), 141.

Whippoorwill (illus.), 203.

Woodpecker, Californian, feeding
habit of (illus.), 128, 129.

Xiphias gladius, metamorphosis
of (illus.), 99.

329

Yellow-jacket (illus.), 162.

Yolk, 80.

Young, adaptations for defense of,
137; care of the, 250, 257; num-
ber of, 61.

Zoea, 97.
Zodgeography, 272.
Zodlogy, systematic, 64.

THE END

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