ADAPTATION FOR THE INDIVIDUAL
Among the standard books of the classical curriculum in the
denominational college of thirty years ago was a volume which I
suppose has practically disappeared from such courses. It delighted
many of its students for a reason entirely different from that which
the author meant should be its taking feature. It was Paley's "Natural
Theology." The author started with a story of a watch found by a
savage. This child of nature was supposed to examine its mechanism and
to infer that the watch was made for a definite purpose. As I
remember, he was even supposed to discover that its purpose was to
mark time. It was at least to become clear to his savage mind that
this was no chance object, but was the definite product of a designing
mind. Having brought this hypothetical savage to these conclusions,
the author turned himself to savages nearer home who fail to see
design in nature. The book takes up a great many cases of interesting
facts in animals and plants as clearly showing evidences of design as
did the watch our savage picked up. But the inference we were expected
to draw was that the design shown in nature argued clearly for a
Designer above nature; in other words, that nature was unintelligible
without God. Everyone in the class believed in God without this
preliminary, and consequently the book was unnecessary, so far as we
were concerned. We started with the condition of mind which the author
hoped to produce. One effect the book did have; in the absence of any
other reputable course in zooelogy, it gave us an astonishing
collection of interesting facts about animals.
Some of Paley's statements were certainly peculiar. His Malay pig with
its upper teeth wonderfully curved was said to be in the habit of
hanging its head upon a bush while it slept, in order to save the
strain upon its porcine neck. This was too much even for our
credulity. None the less the impression made upon some of us by the
evidence for design in nature has never left us.
Among many scientists to-day it is supposed to be crude to speak of
purpose in nature, and there is reason for their attitude. But the
statement that there is no such plan conveys to the ordinary thinker a
meaning that is far more erroneous than could possibly exist in his
mind should he believe implicitly in design and purpose. As between
design in the universe in the usual sense of the word, and a purely
accidental connection of events in the universe, there can be no
doubt as to the choice. The truth is far better expressed by the word
design than by the chaos which is the alternative idea in the average
mind. In these later years we have come to use a different word. We
now conjure in such connection with the word adaptation. In every
animal and every plant the trained eye sees unending examples of
adaptation; that is, of a fittedness to the work it has to do. The
modern scientist feels sure not only that the animal is fitted to his
work, but that he has been so fitted by the work; that the very use he
makes of his organs has determined their structure. This work has
decided that the structure which he has is the structure that shall
survive and shall produce other structures like itself. Adaptation
therefore does not simply express the idea that the animal is adjusted
to its surroundings, but it further suggests that the animal by
gradual process has become thus adjusted. The word adaptation applies
not simply to the result, but also to the process. The scientist does
not consider the animal a final and complete result. He thinks it
still in a state of flux, and so long as its line lasts it will be in
a state of flux. Change is about it on every side, and it must adapt
itself to this change or it will pass away. It may adjust itself, as
has been previously stated, by moving to another environment in which
it feels more at home, but unless it does this, if there come much
change in its present surroundings, it must either meet the
difficulty by altering itself, or it must give up the struggle. The
alteration is unconscious so far as the animal is concerned. It is
seriously to be doubted whether there is any recognition of the
process on the part of any animal excepting man. But though the
process be unconscious, it is none the less there. Slowly and
gradually the animal and the environment are becoming adjusted to each
other.
While it is exceedingly difficult to lay our hands on any animal which
is at present visibly changing its structure, it is not hard to find
closely related animals. These are nearly alike in structure in most
respects. In a few points, however, they may differ materially, and
these points are often directly concerned with different habits of
life. Considered in this aspect, these adaptations of a single organ
separately examined form an excellent argument in favor of that
gradual alteration of the entire organism which evolution suggests.
The most primitive struggle in which an animal can possibly engage is
the effort to maintain its own life and vigor. This struggle will
result in certain adaptations for the individual, adjustments which
make for the safety of the animal himself. These form the subject
matter of the present chapter.
The farther up the animal kingdom we pass in the study of adaptation,
the more likely we are to find changes which have but little bearing
on the safety of the individual. They work for the good of the entire
species, sometimes to the distinct disadvantage of the individual. The
King Salmon may make its long run to the headwaters of our western
rivers, deposit its eggs, have them fertilized, and then float down to
death. But it does not die before abundant preparation has been made
for the continuance of the race. Such adaptation for the good of the
species will be considered in the next chapter.
The first and most important struggle any animal has to enter is the
never-ending battle for its food. Occasionally there is a similar
straining after the air it breathes. But ordinarily air is
sufficiently abundant, except to animals living in the water, where
the supply is always more or less restricted and easily becomes
exhausted. But food is the constant need of every organism, and most
creatures die for lack of it. In this struggle the animal is pitted
against those of his own kind, rather than against those of other
species. Even his brother is his enemy, for he desires the same food.
In many a nest of birdlings one of them fails to reach its development
simply because the parent either is unable to find or it cannot carry
enough food to satisfy all the hungry mouths in the same nest. Before
the nestlings are ready to take their place in the struggle for life
outside and hunt their own living, one or more of them has succumbed.
After the battle for food comes the struggle for shelter. For most
animals there is no such thing as shelter. They are exposed to the
inclemencies of the weather and to the depredations of their enemies
without the means of retiring into any situation which might protect
them. In the higher animals, especially when they are warmer blooded
and their bodies must be kept at a higher temperature, some form of
covering has come to be almost universal.
Though comparatively few animals are prepared to seek shelter from the
cold, all of them have enemies against whom they must battle. These
foes may wish to eat them or may simply wish to get them out of the
way. In either event this struggle is so persistent and so keen that
after starvation it is probably the source of the largest loss to the
animal kingdom.
Considering first the feeding habits of animals, we find they are
exceedingly varied. Some creatures simply engulf other and more minute
animals, often only microscopic in size, in such quantities as to
satisfy their hunger. Others, feeding upon larger plants or animals,
must have some means of breaking off particles of this food; still
others confine themselves entirely to nutritious fluids, and must have
organs adapted to this particular type of food.
Insects are so common that anyone, who cares to, may easily verify
what is here described. It will take nothing but a clear observant eye
and a little patience to make out what is suggested. Each of our
common insects has one of two clearly defined habits in the matter of
food. Either it eats solid food, which must be made fine before it can
be taken into the mouth, or it feeds upon liquids. These liquids may
be easily accessible like the nectar of flowers, in which case one
sort of mouth will serve; or they may be the juices inside the tissues
of animals and plants, when an entirely different type of mouth must
be employed in their acquisition. Perhaps the most easily found
representative of the biting type of mouth, which breaks up solid
food, will be seen in the common grasshopper. Doubtless each one of my
readers has at some time taken a grasshopper into his hand, and,
holding the tip of his finger against the insect's mouth, has promised
the creature its freedom on condition that it disclosed its
reprehensible habit of chewing tobacco. The grasshopper surely
complied, and I trust the promiser was as good as his word. The
grasshopper's head is so placed that, while it is at the front of its
body, the mouth is directly on the under side of its head, while the
eyes are at the top of the front of its face. Under these
circumstances it cannot see what is going into its mouth, and this
makes an interesting variation of conditions to which it must adapt
itself. The means by which it accomplishes this will be clearer if the
mouth of the grasshopper be compared with our own. Our lips are upper
and lower, but the grasshopper has a front lip and a hind one. The
broad front lip is easily seen at the forward side of the mouth. Just
behind it, serving the purpose of our teeth, is a pair of hard jaws
with horny tips upon them, which serve to break small pieces from its
food. While our jaws and those of all other backboned animals work up
and down, so that we may be said to have an upper and lower jaw, the
grasshopper and all of his insect, crab, or spider relations, which
have jaws at all, have them right and left, and they work from side to
side. Behind these harder mouth parts is found a pair of softer jaws,
each of which has on it a little finger-like feeler. With this pair
the insect holds its food while the hard jaws break it to pieces. The
hind lip follows, and is also provided with short finger-like feelers.
The feelers on the hind lip and on the soft jaw are necessary because
the eyes are so placed as not to be able to see what goes into the
mouth, hence the insect must make up for the loss of sight by the
addition of touch. The same type of mouth as the grasshopper has will
be found among the beetles. Here the males sometimes have the hard
jaws so enormously enlarged that they are known as pinchers and have
given to their owners the name of pinching bugs. All insects with such
jaws as these use them for breaking up solid food.
A glimpse at the mouth of the butterfly captured on an adjoining
flower will show a most remarkable variation from that seen in the
grasshopper. Practically all of the mouth parts mentioned are present
in this insect, and its early ancestors had their organs practically
like those of the grasshopper. Now they are so modified and united
with each other as to be almost unrecognizable. The pair of soft jaws
has become very much elongated, and they lock together in such a way
as to enclose a hollow space between them through which the creature
can suck its fluid food. Not only have these soft jaws joined
together, but, because they have become so much elongated when not in
use, they must be coiled up like a watch spring and laid between two
hairy lip-like processes which correspond in reality to the two
finger-like feelers of the grasshopper's hind lips.
The butterfly, lighting upon the corolla of the flower, uncurls this
long "tongue," and through its hollow center pumps up into its crop
the nectar which the flower has stored in its base. When the butterfly
comes to get the nectar from the flower, it rubs upon its own hairy
body pollen from the stamens of the flower and carries it to the
pistil of the next flower of the same kind which it visits. Most of
us have at some time sucked the nectar from the back of a torn
honeysuckle blossom and approved the taste of the butterfly in this
matter. If the airy creature be watched as it lights upon a flower, it
will not be difficult to see it uncurl this long tongue and probe the
depths of the flower. If the butterfly be taken in the hand and the
tip of a pin inserted in the center of the coiled tongue, it can be
uncoiled without the slightest harm to the butterfly.
Insects which wish to use for their food the juices of other animals
or of plants do not find them so easy to gather. In the mosquito most
of the mouth parts are developed into slender pointed bristles wrapped
in a hind lip. These bristles serve to puncture the skin of the
creature attacked, while the curled lip serves as a tube through which
the blood may be extracted.
If, while sitting on the porch on a warm summer evening, mosquitoes
begin to annoy, let one of them at least serve to show his method of
procedure before he is destroyed. Allow the creature to alight upon
the back of your hand and slowly raise the arm until the eye looking
at near range can see the head of the mosquito, which, by the way, is
sure to be a female. Males in this species are entirely harmless. They
never eat after they have grown up; that is, after they are truly
mosquitoes. But the female is very assiduous. Alternately raising and
lowering her lancets from either side, she pierces, then saws, her way
down through the flesh until she has buried her instruments in her
victim and her head rests against her prey. Now a pumping motion of
the abdomen will be apparent, and this continues its accordion-like
action until it becomes more and more distended. The insect only gives
up its task when the entire abdomen is swollen into a great red ball
of blood. The mosquito will now slowly withdraw its instruments and
retire from the scene, if permitted to do so. If there is any fear of
annoyance from the bite, a drop of ammonia immediately applied will
counteract any irritation which would have been produced by the saliva
of the mosquito. The insect is not intentionally vicious in this
procedure. It is simply gathering its own natural food, though this
does not make it less annoying to us since we are its victims. The
swelling produced after the bite is the result of the action of the
saliva the mosquito injected into the wound. The opening through the
tongue is so small that blood would readily clot inside the tube and
prevent its further usefulness, did not the mosquito inject the
secretion of its salivary glands into the wound. This acts upon the
blood in such a way as to prevent its coagulation.
Anyone who thinks carefully can add numberless specializations for
food getting. For instance, primitive mammals have little pointed
teeth which fit them for feeding on insects. In each of the great
order of mammals a special development of these teeth has occurred.
Among the rodents or gnawing animals the front teeth have become long
and chisel-shaped for nibbling. The horse has formed them for nipping,
and his hind teeth for grinding. In the dog the teeth near the front
have become long for tearing his flesh food, while his hind teeth,
working with the motion of scissors, cut it into pieces.
A second great class of specialization is seen in the changes of habit
that provide the animal with shelter. The home seems so necessary a
part of human life that it is almost impossible to think of an animal
having nothing that in the faintest degree could be called a home. We
at least expect it to have some sheltered place in which it passes
most of its time and to which it returns after its wanderings. The
great majority of all animals have no such home. The place in which we
find them to-day may not be the place in which they will be to-morrow.
All places are alike to them. The ordinary conduct of their daily life
drives them about in the search for food. Their attempt to escape from
their enemies leads them each day into new situations, and they may,
and probably do, have no power to recognize the old location if they
return to it. When we come to the backboned animals there is a little
more tendency to a stationary location. The sun fish may frequent the
same reach of the stream, the trout may haunt the same pool, year
after year, but a great majority of fishes doubtless move
indiscriminately up and down the stream or about the lake or ocean and
are not found two successive days in the same place. The same may be
said of frogs. For a time a particular frog may have a fondness for a
special bend in the stream, but it is only a temporary fondness, I
believe.
Our own need for shelter is the prime motive in leading us to build a
home, and this necessity arises first of all because of our warm
blood. What we are accustomed to call cold-blooded animals are not
truly so. Their blood holds practically the temperature of their
surroundings. As the air or the water in which they live grows warmer
or colder the bodies of these creatures alter with it. Consequently
they are active when the temperature is high and grow more sluggish as
the thermometer falls. When the day grows distinctly cold the animals
may go practically dormant.
Only the birds and mammals have warm blood, and of these the birds are
distinctly the warmer. Whereas the temperature of the mammals runs
from about ninety-eight to a hundred degrees Fahrenheit, that of
birds lies somewhere between one hundred and five degrees and a
hundred and ten. Creatures which are warmer than their surroundings
must have some protection against chilling. Accordingly both mammals
and birds have clothing. In the case of mammals the covering is fur,
in the case of birds feathers. In some of the tropical animals like
the elephant and rhinoceros, or in man, who has learned to protect
himself in cold regions by making clothing for himself, this hair is
very short, and except where serving for ornament is quite scanty, no
longer being of use as a protection. But the great majority of all
mammals are well covered with a dense coat of hair. In many of those
living in the colder regions there is in reality a double coat. The
fur seal of the Alaskan Islands is so provided. A set of long hairs
deeply fastened in the skin forms a covering, which shows on looking
at the seal. Underneath this layer, and set but lightly into the skin,
is a short coat of very much finer hair known as the underpelt. When
the skin is taken from the seal it is split by machinery into a lower
and an upper layer. When so split the deep-seated pits of the long
hairs are cut, and these hairs come out. The fine underpelt thus laid
bare is what is commonly known as sealskin. Fashion has decreed that
this must be dyed a rich brown, although when taken from the animal it
is nearly mouse gray.
The birds have need for better clothing. To begin with, their blood is
much warmer, and hence needs better protection from outside cold. In
addition such of them as fly high must be prepared to stand great
variations in temperature. For these purposes birds need a covering of
the finest type. This clothing, in addition, must be extremely light
because the creature must carry it into the air in flight. All of the
requisite conditions are thoroughly met by the feather, which is the
lightest and warmest clothing known to man. If at night we wish,
regardless of expense, to keep ourselves warm with the lightest and
warmest of covering, we send to the Arctic Sea, and from the breast of
the eider duck we pluck the down which lies between the warm blood of
the duck with its temperature of one hundred and seven degrees and the
water in which the iceberg floats.
Young mammals and birds, before their clothing has well formed, are
naturally susceptible to cold; this leads to the first genuine
approach to a home among animals lower than man. Birds lay their eggs
long before the creatures inside of them are ready to emerge.
Accordingly they have learned to build nests in which to place these
eggs, and to protect them from the outside air; meanwhile the bird
keeps the eggs warm by close contact with its own body. The lowest of
the birds may lay their eggs simply on the ground without any special
protection. As we rise in the scale of the bird world we find nests
provided for the eggs. These nests become increasingly complex and
specialized, until we reach the oriole's home with its wonderfully
woven mass of fiber, which, in spite of its apparent looseness,
supports well the weight of the mother bird and of her eggs. The
robin, not content with making a woven basket, plasters it with clay,
and makes an absolutely impervious nest.
When we remember that both mammals and birds are the modern
descendants of cold and scaly reptiles of an earlier geological time,
it becomes interesting to compare their clothing. Evidently in the
mammals hairs began to come out between the scales. Gradually the
scales became fewer and the hairs more abundant until finally the
scales have all disappeared, except those that remain as the claws on
the toes. The ancestors of the birds, on the other hand, boldly
transformed their scales into feathers.
Another need for shelter arises in connection with the approach of
winter. This problem of withstanding the cold season is complicated by
the presence of two new factors. First and most directly, the cold
itself is a distinct obstacle to the comfort of many of these
creatures; as a secondary result of this cold, the food of many
animals disappears entirely in winter. Most of our birds meet this
difficulty by changing their base of operations. When the north grows
cold these creatures fly to the south. Some of their migrations cover
enormous stretches of country. Our bobolink, so well known and loved
by all watchers of spring migrations, passes twice a year between the
latitude of New York and Rio Janeiro. One of our most careful students
of bird migration says that the Golden Plover makes, twice each year,
the long journey from the Arctic shores of North America to the plains
of La Plata.
Different fur-covered animals have specialized to meet the winter by
any one of three different methods. They may brave it out, hunting for
their food as best they can all winter long. Such a course is pursued
by the rabbit. Again like the squirrel, they may store large
quantities of food during the summer, and on this provender they may
subsist during winter, remaining for most of the time near their
hiding-places, which, however, they may frequently leave upon warm
days. A third method is less common, but very interesting. The
groundhog or woodchuck is the best-known example of the group. It
remains asleep, or, as it is technically known, dormant, during the
winter. This stupor is more profound than ordinary sleep, and from it
these animals awaken with difficulty. It is needless to remark that
the groundhog's behavior on the second of February has no relation
whatever to the weather we are to have later in the season. This is
coming to be pretty generally understood. While the newspapers each
year comment upon the groundhog and his shadow upon that day, year by
year the notice has more of humor in it, and fewer people pay any
attention to it.
As for the backboned animals which are cold-blooded, these must,
unless they are fish, give up the struggle completely, bury themselves
in out-of-the-way places, and go worse than dormant. They often become
absolutely cold and stiff. In the case at least of fish, it is quite
possible for them to be frozen stiff, even to be enclosed in cakes of
ice, and still to recover if the encasement is not too long continued.
But the snakes, the turtles, the toads, the lizards, all are hidden
beneath the ground waiting in absolutely unconscious rest the return
of warmer weather.
After the need for food and shelter comes the continually recurring
necessity on the part of almost every type of animal to escape from
the unwearying persecution of higher creatures which would feed upon
it. The whole creation is a constant network of animals which prey
upon each other. It is the fate of a great majority of all creatures
to fall victim to other animals to whom they serve as food.
Accordingly nature has concocted many devices by which she assists
her favored children in escaping this relentless persecution. Perhaps
the most widespread means which animals have developed in order to
elude their enemies lies in the possession of power to escape their
attention. Two different factors may contribute to this end. The first
of these consists in the practice on the part of many animals of
remaining absolutely quiet in time of danger. This instinct seems to
be nearly universal. The first impulse of most animals upon
discovering danger is to remain absolutely motionless. The eye
detects, with ease, objects in motion. These same objects might
entirely escape attention were they quiet. A mouse could remain in the
corner of a room for a long time without attracting the eyes of the
occupants of the room. Let it but scamper across the corner, and at
once it is discovered. It is quite conceivable that early animals were
divided in the matter; that the impulse of some was to escape from
danger, while others, frightened by the presence of the enemy,
remained absolutely still. Each plan has succeeded. Those which, on
running, ran fast enough to escape became the parents of others like
themselves, led eventually to a line of animals in whose speed lay
their safety. Those, however, which attempted to escape, and failed
because they were not swift enough, had their line cut off, and were
thus less likely to be represented in the following generation. The
constant result of errors along this line would be to destroy the slow
and preserve the swift, and in the course of time it is quite
thinkable that only the swift should remain. As the movements grew
more and more keen, even the slower of these would pass out, thus
tending always to produce the succeeding generation from those who
were most rapid, and hence most likely to transfer to their children a
similar power.
But there is another tendency of animals which leads them when
frightened by their enemies to remain quiet. If this impulse is obeyed
thoroughly enough, it is easy to see how the owner of this habit might
entirely escape detection by his enemy. Any restless animal unable to
restrain his nervous agitation naturally betrays his presence and is
picked off. The result of evolution along this line would be the exact
reverse of the preceding. Those that lay most absolutely quiet would
be the parents of succeeding generations, while those who were slow in
coming to rest, or were indifferent about remaining quiet, were picked
off, and their tendency eliminated from the future of the species. In
this way many animals have come to keep entirely quiet in the presence
of danger. It is not a sign of high intelligence. As a matter of fact,
it is rather a stupid procedure, so far as the animal itself is
concerned, but it is a preserving stupidity, and many animals have it.
The "June Bug" (which is not a bug, but a beetle, and arrives in May)
has this interesting habit of keeping quiet. If in its flight it
strikes the globe of an electric light, it falls at once to the
ground, and remains perfectly quiet for a time. After a short interval
it recovers and starts out to regain its previous activity. But this
recovery is by slow stages, and the whole procedure on its part looks
exceedingly stupid.
The little snake with flattened and expanded head, known as the
blowing viper, or puff adder, is one of the most amusing
representatives of the tendency to "play dead" that could well be
found. If you strike him the faintest blow with the lightest stick, he
at once goes into apparent convulsions, in which he seems to suffer
the greatest agony. Then, throwing himself upon his back, he, to all
appearances, yields up the ghost. If, however, you retire but a slight
distance and keep your eye upon him, you find that his ghost returns
after a comparatively short absence, and he slinks away out of danger.
This is the most effective exhibition of this kind with which I am
acquainted.
As for the habit of "playing 'possum" on the part of our opossum, the
trick would seem to be particularly inane. The truth of the matter is,
what is attributed to an unusual brilliancy on the part of the
creature is positively unusual witlessness. The animal has an
exceedingly small brain, as compared with that of a dog of similar
size, and to anyone who knows brains at all this particular organ
would not be looked upon as furnishing its owner much ability. The
fact is that the opossum has exceedingly small wit, and this little
deserts it in an emergency, as a result of which he grows helpless and
motionless. This is often supposed to indicate great wisdom. There may
be wisdom in it, but it is the wisdom that lies back of all nature. It
certainly is not the wisdom of the opossum.
Man himself possesses to a marked degree this impulse to keep quiet in
danger. The man from the country who is visiting the large city,
suddenly startled by the "honk" of the auto horn, finds his power of
movement promptly arrested, and he is not unlikely to be struck and
injured by the machine from which the city dweller would easily
escape. This is not particularly to the credit of the city dweller,
who, when in the country, may find himself similarly startled by the
sudden appearance of the calf, the pig, or the sheep. The bull, which
a country boy, accustomed to him from childhood, will drive with a
willow switch, is a source of terrified concern to the city man.
While the trick of keeping quiet serves many an animal in time of
danger, there is another device for escaping attention, far more
common and widespread throughout the animal world. The eye does not
easily see an object if it is colored like the background against
which it stands. A host of animals find their main safety in being
indistinguishable in color from the surface on which they live. There
are many biologists who seriously question whether protective
coloration, as Darwin called it, is as effective as he believed it. In
some quarters it is the present fashion to doubt protective coloration
entirely. No one has yet shown any principles which will better
explain the great color scheme of the animal world, and until such
explanation is forthcoming I believe it will not be wise for us to
discard the idea of protective coloration. No doubt it has been
overworked by enthusiastic believers in its efficiency. At the same
time, to overlook it completely, is, I believe, to make a greater
error. I have little doubt that when the broader explanation comes,
which will satisfactorily explain the color scheme of the animal
world, the idea of protective coloration will be found, not so much to
have been wrong, as to have been but partial. It will be included
under the broader principle which takes its place and will not be
supplanted by it.
The idea of protective coloration is that very many animals have
ordinarily come to be colored like the background on which they live.
The process has taken many generations, and is very slow, but is none
the less sure in the end. In most cases the animal is probably
entirely unconscious of this point in its favor, and usually it does
nothing to assist the deception. The result is none the less effective
because the animals themselves are unconscious of the process. The
cabbage worm is green in color like the cabbage. This does not mean
that it got green by eating cabbage or by longing for greennesses.
Through long years the enemies of the cabbage worm have been picking
it off the plants on which it fed. This does not imply that cabbages
as we know them are very old, but cabbage worms doubtless ate the
leaves of the sea-kale long before man had cultivated it into cabbage.
During all these years the enemies of the caterpillars, generally in
the shape of birds, have been assiduously gathering them up.
When we see how much the various members of the same human family may
differ in complexion, how much the various pigs in the same litter may
differ in size and in coloration, it is easy to understand that among
these caterpillars which have eaten the cabbage there must have been
considerable color variations. I do not imagine for a moment that the
birds had any preference for any particular color in their cabbage
worms. They took every caterpillar they saw, but they naturally first
saw those that were least like the background on which they lived.
The only caterpillar which was effectively hidden from his enemy was
the one that was indistinguishable on the leaf. If it escaped in this
way, the probabilities are that it would produce young which would be
at least a little more likely to be green in color than the progeny of
its darker-colored brothers and sisters. By this continued process the
birds steadily weed out the darker-colored specimens. There would
result, in the course of time, a race of caterpillars, whose ancestors
for so many generations back had been light green in color, that there
is little likelihood of any of the older and darker forms turning up
again. In the course of time all dark tendencies will have disappeared
from the family and practically all of the group will be light green.
Any sport or variation in the shape of greater conspicuousness would
fall a quick prey to the enemy and its line be cut off forever.
The same sort of activity has resulted in the peculiar green color of
the katydid. This creature lives chiefly upon the leaves of trees and
shrubs. This insect is so large that, even though it is leaflike in
color, it might still be conspicuous. As a result those katydids whose
wings were most like leaves in form were least likely to be picked up
by the passing bird. This sort of protective appearance is intensified
by exactly the same means as that which brought about protective
coloration. The katydid least leaflike in appearance was eaten first.
Thus those most leaflike remain until the last, and are most likely to
produce young. Again, it was not the fact that they lived among leaves
which made them look leaflike, but it is because they look like leaves
that they escaped being devoured.
The katydid has materially assisted in its own preservation by being
active chiefly at night. In the daytime it keeps comparatively quiet.
Thus seated upon a twig, especially if hidden among the leaves, it is
almost unnoticeable. At night, however, it moves about more freely,
seeking its food and eventually its mate. At such times it becomes
distinctly more conspicuous because its wings are steadily fluttering.
The hind wings are filmy and are very light green. The creature looks
most ghost-like as it flies through the evening air.
A very similar history lies back of the coloring of the ordinary toad.
Though descended from the frog, and originally a creature of the
water, the toad has long since adapted itself to live upon the dry
ground. It still produces its young in the water as it did when a
frog. Whereas the childhood of the frog, that is, its tadpole stage,
is very long and it assumes its adult form comparatively late, just
the reverse is the case of the toad. The young hasten through their
tadpole stage within a few weeks, and assume the shape of the parent
toad when about big enough to cover your little fingernail. Now they
leave the water and seek dry land. Naturally they make the change when
the land is damp, that is, after a warm spring rain. People seeing
these multitudes of little toads hopping around over a bare spot of
ground, and remembering the rain of the night before, insist that it
has rained toads. Of course it never rains down anything which cannot
evaporate up. The stories of showers of toads and of earth worms, with
an occasional fish, or even creatures of larger size, are all pure
myths. There are conceivable tornadoes after which there might be a
shower of such creatures, but at such a time it is likely also to rain
barn roofs and buggies. You may be sure that toads which come down in
the rain are dead after they strike the ground.
The little toads started out, perhaps a hundred at a time, from the
small pool in which their eggs were laid. These creatures find dragons
on every side. The gartersnake comes along and gets his first toll;
the heron follows him and takes such as catch his hungry eye; the
turkey gobbles up his from what are left. By the time the toad-eating
creatures in the neighborhood have taken such as they found, there are
very few remaining. These doubtless have been left for a very good
reason, generally because they were not noticed. This was because they
looked like the ground on which they sat, and because they kept
perfectly quiet while the enemy moved about. This process has gone on
so long that the toad has come to be astonishingly well protected by
its resemblance to the ground. This likeness it intensifies by its
interesting habit not only of keeping entirely quiet, but of dropping
its nose to the ground, instead of sitting high on its front legs, as
it does when not in danger.
I have noticed that if a snake and a toad be placed in the same cage,
when the snake approaches to capture the toad the toad drops into a
squatting position, and is very likely to blow himself up until he is
rounder in outline than he was before. Whether this is a deceptive
trick which makes him the more resemble a stone is more than I can
say. I do not remember having seen our eastern toad do it. I have seen
it happen a number of times in the laboratory of a Colorado
naturalist, and it is quite possible that in the open country more
sparsely covered with vegetation than is our ground in the east this
inflating device may serve the toad more effectually than if it kept
its own outline.
Even among creatures far more active than the toad and the katydid an
inconspicuous color must certainly result in distinctly better
protection. Everyone knows the jay and the cardinal when first he has
seen them, if only he has a slight acquaintance with their pictures.
They are so conspicuous that we recognize them at once. More common in
my region than the jay or the cardinal is the red-eyed vireo. This
creature moves industriously in and out among the leaves of our trees.
It is persistently in motion, is nearly constant in song, and is a
bird of fair size, being larger than our English sparrow, though
smaller than a robin. Many a nature lover will recognize twenty-five
or thirty birds at sight without any difficulty, and not know the
vireo. Yet the vireo is more common than two-thirds of the birds he
knows. There can be but one reason for this; the bird is
inconspicuous. The olive-green of its back, with its light under
parts, serves to hide it completely amid the foliage. Even the
bird-lover learns to find it first by its jerky song, and then by
watching for its movements among the leaves.
One aspect of protective coloration has been brought to our attention
by the artist, Mr. Abbott N. Thayer. He first clearly explained why it
is that animals are usually so much lighter on the under side than
they are upon the upper. Mr. Thayer proves his position by taking some
ordinary cobblestones and painting one of them a uniform color and
placing it upon a board painted the same color. One would think the
stone would be inconspicuous; as a matter of fact, is quite easily
seen. The underside of the stone, turned away from the light, is so
shaded as to mark a distinct boundary between the stone and the board.
Another cobblestone was colored on its upper side like the board, but
the color faded into a lighter and lighter tint until the bottom of
the stone was nearly white. This stone, placed upon the board, was at
a short distance nearly invisible. In other words, although the
pigment was actually lighter on the under side, it was so much less
intensely illuminated, that the result was the same in tint as the
other side under the clear sharp light of the sky.
Many a person, looking down into the water from a bridge, sees nothing
whatever of the fish in the water below, because their backs are
exactly like the bottom of the stream. Suddenly one of the fish, by a
quick movement, turns its lighter under side over in such a way that
it is clearly illuminated from the sky. Immediately a flash as of
silver strikes the eye of the onlooker and makes him aware of the
presence of the fish which had previously been undetected. If rendered
thus suspicious, the observer will carefully examine the bottom of the
water, he may quite likely find dozens of fish which had previously
escaped his attention.
Nature is very versatile. So many of her apparently chance ventures
have proved successful that she has retained many devices by which her
children may be safe. One of these, which is doubtless often quite
effective and may serve to save an animal's life, is that of being
able to emit an odor so nauseating as to offend the enemy's sense of
smell, and doubtless remove the keen edge of his appetite. It is not
uncommon among the group of insects properly known as bugs to possess
an exceedingly unpleasant odor. Anyone who has handled a squash bug
will know exactly what I mean, and there are other members of the
group not so common as the squash bug, which, at least to the human
nose, are distinctly offensive. Some of the beetles also save
themselves by this device.
One of the most interesting developments of this peculiarity is found
in the case of the common skunk. This creature has in each groin a
gland capable of secreting a highly offensive fluid. Ordinarily this
liquid is kept safely within its sac, and for a long time none of it
may escape. When closely cornered, the skunk will turn its tail toward
the enemy and with a quiver and a flip of his tail it can guide the
openings of two little tubes that come out along the root of the tail
in such fashion as to eject the fluid in a fine and foul-smelling
stream against the animal from which the skunk would escape. Once
fairly hit by this fluid, I imagine most animals will drop the skunk.
A dog surely will, and will hate himself for having made the attempt
to capture anything which must be so ignominiously allowed to escape.
If ones clothing is well saturated with it, it is nearly useless to
hope to remove the odor. A dog will carry the smell for several weeks.
For a long time it will be so strong as to make him an unfit denizen
of the house. Even swimming in deep water does not remove it. After
two weeks, although he may seem to be practically free from the odor,
a light rain will bring it all out again and make him nearly as
offensive as before.
Not as prompt in its action, but in the end nearly as effective, is
the protective device which the toad sometimes uses to his distinct
advantage. May I be pardoned a personal account of this particular
feature. It was my good fortune to be for a short time a student in a
class taught by Edward Drinker Cope, one of the most brilliant of our
American biologists. Prof. Cope mentioned in class the fact that the
Batrachians (the group to which the toad belongs) have in many cases
the power to emit from their skin a fluid which is sufficiently
nauseous to deter an animal from eating the creature that secretes it.
Upon such authority as this, I had no hesitancy whatever in repeating
Cope's statement. One morning I had a class in the field studying the
ground ivy, whose dainty blue flowers were lifting themselves out of
the dewy grass. While we were thus engaged, a toad joined the circle.
He came out of his dewy retreat clean and fresh from his morning bath.
I took him in my hands, and made him the subject of an immediate
lesson. I showed to my pupils his eyes and his interesting method of
handling them, his tongue and its strange insertion; showed them how
to look into his mouth and look up his ears to his ear drums, and
pointed out many other interesting facts. Then I told them how Cope
had said that the toad had power to emit from its skin a fluid so
nauseous that many an animal hesitates to eat it. This is the first
peculiarity I had mentioned which I had not myself observed, and a
scientific qualm came over my conscience. Why had I never verified
this statement which I had so frequently repeated? On the impulse of
the moment, with the bright, clean skin of the creature fresh from the
dewy grass, making it less than usually repulsive, I ran my tongue up
its back only to find that it had no taste whatever. I was of course
surprised, but I was not foolish enough to deny, as the result of one
observation, the statement of a good scientist. The observation,
moreover, was one which I naturally did not care to repeat with any
frequency. Of one thing I was sure, toads do not always have an
unpleasant taste.
A year later I had a class down by the side of a neighboring pond. The
pool was not an attractive one, and I had picked from it a more than
commonly unappetizing looking toad, which proved to be a mother which
had not yet laid her eggs. As I held her in my hands and exhibited her
various points to my pupils, I told them of Prof. Cope's statement. I
also told them of my unsuccessful attempt the previous year to verify
the statement. I added, however, that I would not repeat this
experiment on this unappetizing specimen. Hereupon the toad not only
exuded, but squirted, from a gland over her left shoulder blade a
fluid, milky-like in appearance, and forming a jet as thin as a
needle, but ejected with force enough to strike my face, which was at
least fifteen inches away. I moistened my finger on my tongue, lifted
the fluid from my cheek, and tasted it. Cope was right. A toad can
exude a most nauseous fluid. Horsechestnuts extracted and distilled
might possibly provide something as bitter. Why did I not find this in
the preceding case? I have too few observations on which to base a
conclusion, but I have a suspicion as to the reason. In the case of
the toad which spurted the fluid in my face, we had a creature with
whose life were tied up the lives of her many offspring, to be
produced from the eggs she was so soon to lay. Under conditions like
these, nature is more than commonly careful of her children. Whether
this be the reason or not, toads do not always have an unpleasant
taste, but when they do it certainly is most unpleasant.
There remains to be considered the most effective plan yet mentioned
of escaping the enemy, and that is of really escaping. In all the
devices we have considered thus far the enemy is eluded. When the
creature lies quiet, or finds safety in its protective coloration, or
in its bad taste, or unpleasant odor, it still remains in the presence
of the enemy. A more progressive plan altogether is to escape the
enemy by flight. The great advantage of this plan lies in the fact
that the acquisition is valuable for every purpose. The creature then
can escape the enemy, can range widely for food or for a mate. This
gives it an enormous advantage in the struggle for life. The power to
fly, in insects, was doubtless originally gained in the attempt to
escape the enemy. Among many of the lower animals it is nearly the
only purpose that flying serves. Later on it enables the animal to
pass from one food locality to another. In a few creatures it plays an
effective part during the mating season. These last are probably both
derived powers, and the original function was that of escape from the
enemy. The grasshopper has grown its long legs to serve him for
safety, and through them it is helped along, moving about chiefly by
leaps when it wishes to go any material distance. It is only toward
the very end of its life that the grasshopper has wings, and then they
serve probably to aid in the search for a mate. Among the birds flight
began simply in sailing out of the trees, into which the creature,
still half lizard, had crept to escape its enemy. The earliest bird
known to us had comparatively insignificant wings. There was really
more support in its tail than in its wings, and this would distinctly
indicate that it glided more than it flew. It had claws also upon its
wings, and it was probably the case that this creature crept into the
trees, at least in its earliest forms, and sailed down in a manner not
unlike that employed to-day by the flying squirrel. From such simple
beginnings came the wonderful power of flight in the birds.
Among mammals the attempt to escape from the enemy has led to an
interesting development, which will be more fully explained in a later
section when we speak of the history of the horse. The early mammals
walked flat-footed, as we do on our feet and as the raccoon and the
bear do on theirs. Gradually, however, as their enemies became more
fierce and better able to injure the larger mammals, the latter gained
in power of flight, and this gain consisted first in rising from the
toes, lifting the heels completely off the ground. At the same time
the leg and foot were gradually lengthened. Doubtless in this way the
fleet animals, like the deer, the horse and the giraffe, first came by
their long legs. Constant elimination of the short-legged ones, by the
pursuing enemy, resulted in the selection of the long-limbed ones for
breeding purposes, and hence to the ultimate elongation of the legs of
the species.
The method of escape from the enemy involves cowardice. "He who fights
and runs away may live to fight another day," and so it may be the
part of wisdom in the weak creature to escape from his enemy by
flight. It is a far more estimable process, from our standpoint at
least, to stand against the onslaught of the enemy and beat him upon
his own ground. This end is secured in many animals by acquiring horns
or by lengthening certain of the teeth. The horn is a very ancient
instrument of defense. When the reptiles ruled the land horns were not
uncommon. They consisted in those days of hardened scales, which
lengthened and fastened themselves over a core of bone. Such an
old-fashioned instrument, sometimes made of newer materials, still
remains the defense of a number of animals. The rhinoceros has upon
his nose a lengthened projection, which is what might not improperly
be called hair glued into a cone. This enormous horn is a frightful
weapon, both of offense and defense, and, when backed by the terrible
weight of the body of the rhinoceros, it can do as deadly work as
almost any instrument of destruction known to animals below the grade
of man. But, after all, this is an old-fashioned method, and the
rhinoceros is a relic.
Among the carnivorous animals the teeth, which were developed first
chiefly for the tearing of flesh in its consumption, became effective
for their courageous owners. Because these tearing teeth are well
developed in the dog they have come to be known as canine teeth.
Usually where an animal can use its teeth effectively for offense or
defense, it is the canine teeth that are thus modified. The cat has
developed them better than the dog, and one of the cats of a bygone
geological period had canine teeth so magnificently enlarged and so
sharp at the back as to give this frightful creature the name of the
saber-toothed tiger. The long teeth in the upper jaws of the elephant,
commonly known as tusks, are not canine teeth. The elephant has
completely lost his canines. His tusks are his incisors, and they have
developed as have almost no other teeth in the mammals.
These are only a few of the numberless devices nature has evolved for
furthering the success of her children. There are so many others that
to many of us they form almost the chief point of interest in our
study of a new animal, or our closer observation of an old friend.