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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 


“CNASNMOT, “H “O Aq ydeisojoyg ‘“dnoas joriqug 
‘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 
Ng 


~ ‘ X( i 
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 


“4801 78 UOJ 
SSUIpUNOIINS [eNsN s}t YIM ozIMOUTALY 0} SB OS POTOTOD ST TOIYA “(snlafqoa snwojso.uyUy) [TIMsooddiyM oy T—"ssl “OL 


r an 


{ 
b 


ree. 


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. 


‘qs0U B SULYRUT SIOAVOG—'EOT “OTT 


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 


ne Fa oe 
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. 


yf 


é 
BTS SF FO 


ERYNNIS 
MANITOBA 


75 65 
\ \ 


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- 


WI O4OIB OYA JO S[BUNIUB * (SNLMUSOL SRUMgOPO) SHATBM INVURIYY OYL——"LAL “O17 


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. 


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