ADAPTATION FOR THE SPECIES
The strife, as we have described it thus far, is a purely selfish
struggle. Every point gained is a point favorable to the welfare of
the individual animal. But nature is uncommonly careless of the
individual unless the advantage gained is also of use to the species
as a whole. Very often the life of an animal ceases when provision has
been made for its young. The male garden spider may have a long and
dangerous courtship, in which the uncertain temper of his ladylove may
lead her to bite off four or five of his eight legs. But her
ingratitude is not yet complete. He may have barely accomplished his
desperate purpose of fertilizing her eggs at all hazards, when she
ends the process by eating him. The male bumblebee fertilizes the
female in the late summer and then dies. She does not lay her eggs
before the next season. So it happens that no bumblebee ever sees its
own father, and no father bumblebee ever sees his own children. In the
honey bee the male, which has been fortunate enough to fertilize the
queen, pays for his honor by death within the hour. Superfluous
bachelors, among the honey bees, when the bridal season has passed,
are driven from the hive to die of starvation.
An animal need not always be successful himself, but it is more
essential that he hand down his successful traits to those who come
after him. It is more important for the future generation that an
animal should have had it in him to do great things, though he himself
really have never done them, than that he should have learned to do
great things on a meager original endowment. Not what an animal
accomplishes is important to his children, but what he has it in him
to accomplish. Accordingly Nature is full of devices by which those
who have proved their original endowment by winning out in the
struggle shall hand on this endowment to a subsequent generation. In
other words, Nature is anxious that they may successfully mate. Here
we are again on distinctly debatable ground. Darwin himself believed
thoroughly in what he called sexual selection. It is the essence of
this idea that the males and females have grown unlike, more
technically have developed secondary sexual characters, through the
choice of the mating pair. It would usually be the more serious loss
if accident should come to the female, for she may carry fertilized
eggs for some time. Hence, if both sexes may not become attractive, it
is usually the male that develops fine colors, ornamental appendages
or a captivating voice.
An interesting reversal of this process has taken place in civilized
man. His more savage ancestor adorned himself more lavishly than he
permitted his mate to do. With the advance of civilization man has
undertaken to defend his own mate most valorously. The result is it is
safe for her to be beautiful. Under these circumstances, however, it
is more necessary to her welfare that her consort be vigorous rather
than that he be handsome. Hence in the human species beauty has become
the prerogative of the woman, and this is increasingly the case the
higher the civilization. Whether woman suffrage and self-support will
reverse this process remains to be seen. There are indications that
point that way.
There are many biologists who are at present expressing serious doubt
as to the validity of sexual selection. As in the previous cases of
protective coloration, I believe it will be wise for us to retain,
even though with an interrogation point behind it, the idea of sexual
selection until such time as those who object to it have furnished us
with another theory which will more nearly account for the observed
facts. While entirely conscious of the possibility that there is a
weak spot in the theory, we will still tentatively hold to sexual
selection. The fact that beauty in women is so intensely attractive
to man, and that vigor and manliness in man are so attractive to
women, leads us to infer that among the lower animals, although of
course in a vastly less degree, vigor and beauty are also attractive.
The weakest point of the position lies in the fact that it probably
presupposes a higher degree of capacity for appreciation on the part
of lower animals than they possess. Those who deny the truth of the
theory laugh at the idea that a butterfly can see clearly enough and
care enough for what it sees to notice whether its mate has wings of
one type or of another. The size, number and position of the spots on
the wings of many butterflies are so nearly constant that they cannot
of themselves have been entirely determined by the choice of the
insect. Yet this may not preclude the possibility of the fact that,
while the spots were produced through some other agency, certain types
of them were selected by sexual preference.
If attractive coloration is effective anywhere in the animal world, it
will possibly be found among the insects, but it is especially likely
to be found among the birds. Very many field workers in these groups
feel quite sure of the value of attractiveness. When butterflies chase
each other up and down, circling and doubling, following each other
for long distances, it would certainly seem as if they were pleased
with each other's appearance. Some naturalists, especially those who
have worked chiefly in the laboratory, insist that it is the odor, not
the color of these insects, which is attractive, and some experiments
which have been made would seem to point in this direction. But the
creatures experimented upon most carefully were night-flying moths,
and it is quite possible that the sense of sight in the night-flying
moths has lost its vigor.
The great difficulty in understanding sexual attraction in insects, as
based upon beauty, lies in the undoubtedly lower development of their
nervous activity; in other words, in the apparent absence of anything
worth calling mind. I think no one imagines that a butterfly, looking
upon two other butterflies who are competing for her affections,
deliberates between them and determines to admit to the circle of her
friendship the more brilliantly colored male. Moths are so
irresistibly attracted to a light as to fly into it without apparent
power to withstand its influence. They repeat the flight again and
again until they are destroyed. If they react so vigorously to the
stimulus of the light, it seems not impossible that they may also act
vigorously to the stimulus of color pattern, and that the male most
beautifully colored, according to the nervous ideal of the female,
should win her unconscious regard. At least it is certain that, in
very many of the butterflies and moths, the attractive coloration is
chiefly displayed when they are moving actively about; and when they
alight and their enemies can the more easily capture them, they
conceal their brilliant colorings. Most butterflies are very brilliant
on the upper surface of the wings and very much duller on the under
surface. Hence in flight they show their colorings exquisitely, but
when they alight, and are thus more likely to be captured, they fold
the brilliant surfaces together in an upright position. In this way
not only is the dull side of the wings placed outward, but the wings
themselves are placed edgewise to the sky, and it is from this
direction that their enemies, the birds, are most likely to see them.
Once upon the wing these creatures display their beauty with much
greater safety because they can escape the birds very readily by use
of their exceedingly jerky flight. The butterfly's motion is as
irregular as any we have except the bat's. This eccentricity is one
great element in their safety, and makes it less dangerous for them to
display their attractive colorations.
One very large group of the night-flying moths have been named the
"underwings," because of the fact that their hind wings are very much
more brilliant than the front, and in lighting they fold the dull pair
back over the bright, completely concealing them. These creatures are
in the habit of resting in the daytime against walls, or stones, or
the bark of trees. The similarity in color between their front wings,
which alone show while sitting, and the background on which they rest,
is most remarkable. One may pass them again and again, although they
are of considerable size, and not notice them at all. Once let them
display their hind wings and the brilliancy of their color always
attracts immediate attention.
It is among birds, however, that brilliant coloration serves its most
effective purpose. The birds are alert, exceedingly quick of sight,
and are much more discriminating than insects in almost every respect.
It is not so impossible that these creatures might even voluntarily
prefer a distinctly more brilliant mate, though the voluntary
character of the process is not essential to its success. Men
certainly are constantly attracted to women for whose charm it would
puzzle them to account. If this is true with regard to men, it is
certainly probable that birds would be largely influenced by phases of
attractiveness, of which they were observant, but unconscious.
Certain it is that in many birds the males are far more beautiful than
the females. Perhaps the commonest illustration, and, at the same
time, one of the best is found in the so-called red-wing or swamp
blackbird. The male of this creature is a brilliant black, excepting
that upon the angle of the wing, spoken of roughly as his shoulder,
though in reality it is equivalent to our wrist, there appears a
splendid orange patch with a border of lemon yellow. When he folds his
wing he pushes this colored angle of the wing so deftly under the
feathers of his shoulder as almost to conceal it. When in flight the
bird is exceedingly conspicuous, showing, with every bend and twist of
his body, his gorgeous epaulets. Meanwhile, the female is likely to
pass unnoticed. She is dull in color and streaked like the grass among
which she lives. During the mating season the male hovers about her,
swaying from side to side in such a way as certainly to make it appear
as if he realized his good points and was bringing them to bear as
effectively as he knew how. After his mate has nested and is rearing
her young, it would appear that the male uses his brilliancy to lure
the observing enemy away from the nest containing his wife and
children.
Another illustration of the remarkable superiority of the male over
the female, in many parts of the bird world, is seen in the case of
the common barnyard fowl. The rooster is so much more gorgeous than
the hen that anyone reasonably acquainted with these birds cannot have
failed to notice the fact. In some of our modern varieties we have by
breeding colored them nearly alike. The original chicken is colored
much like the common Leghorns. Shades of red and yellow decorate his
neck and back, while the flight feathers of his wings and of his tail
and the sickle feathers which ornament the rear of his back and hang
over his tail are lustrous dark green. The hen meanwhile is very much
less brilliant in her contrasts. I shall speak more fully of this in
discussing polygamy.
The attraction of beauty is not the only lure by which a creature may
win its mate. Sound may captivate as effectively as beauty. This is
true of insects as well as of birds. Certain insects at least advise
their mates of their presence by means of a sound which they emit.
This is particularly noticeable among the group of straight-winged
insects to which the grasshopper, katydid and cricket belong. The
grasshopper has a ridge on the angle of his wing and a roughness on
the side of his leg. When these two are rubbed together the result is
sometimes a fiddling, sometimes a snapping or cracking sound,
differing in different grasshoppers. I doubt not these sounds are
pleasing to the female of the species, for they are always made by the
male. The katydid, instead of fiddling in this way, has a sort of drum
on the angle of his one wing, which he can rub over a tooth in the
corresponding angle of his other wing, thus producing the familiar
"katydid" sound. I have never succeeded in making a dead grasshopper
fiddle, but I have long known how to make a dead katydid say "ka."
Quite recently I have added to my accomplishment in this respect and
can make it say "katy." The "did" part of the song still lies beyond
my power. The crickets produce their sharp notes in much the same
fashion as the katydids.
One observer of the chirping of the cricket says that the pitch of the
song varies with the temperature. He has even worked out a formula by
which one can tell the pitch of the chirp, if he knows the
temperature, or, knowing the temperature, can determine the pitch. Of
course this is too mechanical; yet it indicates that there must be
considerable relation between the two; the warmer the cricket the
happier he is.
It is the males among insects that chirp their love songs. The females
never answer them. There is a peculiar notion that the female katydid,
when thus accused of some offense, replies "katy didn't." The truth of
the matter is that no female katydid ever replied to the accusations
of her lover, if accusation it be. She is absolutely dumb, not having
the drum upon her wings with which to reply. She is provided with ears
wherewith to hear, and, strange to say, she keeps them on her elbow,
as does also the cricket, while the grasshopper has his ears upon the
side of his body.
Everyone who lives in the country, or goes into the country in the
summertime, is sure to know the humming of the so-called locust. It is
an unfortunate fact that the word locust may have several meanings. It
is properly applied to one group of the grasshoppers. The creature
most commonly called a locust is a cicada, or harvest fly. When the
weather gets quite warm the cicada starts his love song. He has two
long flaps to his vest, and under each flap he has a vibrating drum
head. This is set shivering by a muscle on its under side. The female
cicada again is silent.
It is among birds that the love song reaches its finest development.
It may consist simply of a little chirp as in the chippy. It may
consist of two notes of a different pitch repeated steadily, as in the
tufted titmouse. It may attain considerable variation, as in the
robin. But in the choir of our best singers, like the catbird,
thrasher, and mocking bird, there is unending variation of notes. It
seems almost impossible to doubt the charming quality of this voice
upon the mate. It certainly is chiefly confined to the mating season,
and is indulged in almost entirely by the males. This does not mean
that a male does not sing excepting when he wishes to charm his mate.
But the time when he is in his most exquisite feather and most
charming mood is the time when he sings most sweetly, and this is the
time when he is taking to himself a mate. The love joy may so
overcrowd his life that he sings much and often, but the increase in
its amount and character during the mating season seems to proclaim
its purpose beyond a doubt.
In addition to the allurements above described there are certain
peculiar behaviors of the animal during the mating season which are
intensely interesting. Sometimes they consist simply of a wild
delirium of joy, which overpowers the animal completely and makes him
do wonderful things. Birds will fly with impetuous leaps in the air,
mount higher and higher, singing wildly, only to turn suddenly at the
top of the flight and drop promptly to the ground. I have seen such
ecstatic flights in the oven bird and in our rollicking gold finch. I
have seen a catbird on his way to a tree turn three somersaults, much
like those performed by a tumbler pigeon, after which he alighted upon
the bough. None of these acts seemed deliberately performed in front
of the females, but I have seen three or four killdeer parading in
most stately and precise manner, spreading their wings and fluffing
their feathers, performing a sublimated cup-and-cake walk amid a
circle of attracted females.
Even our little English sparrow, as I have previously mentioned,
fluffs himself up and spreads his wings and prances around in front of
his presumably adoring ladylove. But the weirdest performance of this
sort I have ever seen is that shown by the male ostrich. When he
becomes excited, swaying his body from side to side, he sinks slowly
upon his knees, until his body touches the ground, his wings spread on
either side and the feathers fluffed up so as to show every exquisite
plume in all its splendid beauty. The long neck is laid back until the
head, which is doubled sharply forward, is pressed almost against the
back, and in this strange position he sways from side to side,
apparently utterly oblivious, for a time, of everything. After about a
minute of this performance, he seems slowly to come to himself and
rise again to his feet. Now he is particularly likely to make vicious
attack upon anything within reach.
It is not only necessary that the animal should be able to attract a
mate. There may be more than one claimant for the damsel's affection.
In many animals we see provisions whereby the male may effectively
deal with his rivals. This is especially likely to be the case if the
animal be a polygamist. In every species there are produced about as
many males as females. If the polygamous habit leads one male to
gather about him a group of females, with whom he mates, it is evident
that he is displacing an equal number of rivals, and they are not
willingly displaced. Accordingly we find that polygamy is usually
accompanied by a belligerent disposition on the part of the males. In
our ordinary barnyard fowl this trait is very evident. The rooster not
only domineers over the hens, not only struts about among them in
stately fashion and gives vent to his feelings by his sonorous voice,
he must also drive away from the neighborhood any rivals for the
affections of his wives. Hence the rooster attacks upon sight the
neighboring rooster, and battles with him to his entire discomfiture
and sometimes to the death.
Among the members of the deer family this particular phase of the
relation between the sexes has produced in the males, and only very
rarely in the females, the magnificent branching horns. These are
intended not so much as a protection against the enemy as for an
offensive weapon in the battle for the mates.
Beautiful and stately as are these magnificent horns, they last only
for a part of the year. We begin to understand their meaning. When the
wolf is hungriest, toward the close of the bitter winter, the deer is
without horns. When the time for mating comes, the deer within a few
weeks grows his horns, which at first are covered with a plushlike
coating, known as velvet. After a while this dries and he rubs his
horns against the trees until they are clean and smooth. Now he is
ready for the battle royal.
In the case of the fur seals polygamy has carried its specialization
of the males to a remarkable extent. The bull seals are several times
as large as the cows, and are provided with terrific canine teeth.
With these they battle with a violence that very often results in the
death of one of the combatants. A successful bull seal who has
gathered about him a cluster of seal cows is seamed and scarred with
the marks of his annual combats.
One more type of adaptation can be profitably considered. Animals have
developed many devices which serve for the protection of their young.
The wonderful silk spun by the spider was evidently primarily intended
to serve as a covering for the eggs. Probably all of our spiders agree
in using the silk for this purpose. Many of them employ it for
practically no other, though there are half a dozen different uses to
which different spiders may put their silk. Under these conditions we
have a right to infer that silk was primarily developed as a coating
for the eggs. In the case of some of our spiders a little fluffy mass
of silk covers the egg, while a firmly woven sheet of silk covers both
egg mass and fluff, holding it flat against a wall or the trunk of a
tree. In some of the higher spiders, notably our bank spiders, the
silken covering becomes an effective cocoon, spherical in shape, with
a little opening at the top like the neck of a small bottle. The egg
cocoon is woven in a mass of tangled silk between the branches of some
tough weed which will be sure to outlast the winter. Into the egg
cocoon the spider may place one thousand or more eggs. Having thus
provided her children with a snug winter home, the spider dies. When
spring comes with the warm rays of the sun, the eggs hatch and the
cocoon becomes a creeping mass of minute spiders. At the time these
spiders appear there is nothing for them to eat. The obvious way out
of this difficulty is taken. At once there begins a progressive party.
Spider fights with spider, and the prize in each conflict is the body
of the victim, which is promptly eaten. The winners in the first round
pair off again, and a little later, as hunger drives them, another set
of combats comes on, resulting in another halving of the number of
spiders in the cocoon. This process continues until not more than
one-tenth of the original number of spiders remains. By this time they
have gained sufficient strength of leg and jaw, and sufficient
dexterity in the use of both, to make it safe for them to venture out
and try their fortunes among the accidents of a strenuous world. There
can be little doubt after this long process has worked its final
results which tenth remains. Chance plays but small part in this
game. It is the fittest that survive. When this procedure goes on
generation after generation, the result must necessarily be that the
spiders grow fitter and fitter for their work. This method is hard on
the little spider, but it makes good spiders.
Most insects die before their eggs hatch; accordingly they can pay no
attention to their own children. Whatever arrangements are provided
for the safety and strength of these offspring must be provided before
they appear. About the only care the majority of insects take in this
direction is to see that the eggs are placed where the young shall
find food as soon as they emerge. Insects' eggs are very small, and as
a consequence the creatures which emerge from them are likewise
exceedingly minute. As a result they cannot be expected to hunt far
for their food. Different insects use different devices by which to
overcome this difficulty. The katydid, for instance, must die with the
approach of fall. Her children will not appear until the following
year. Her food consists of leaves, but to lay the eggs in such a
situation would be a fatal process, because the leaf will drop off
before the eggs hatch. Accordingly, the katydid lays its shield-shaped
eggs in a double row near the end of a young twig. Next year when the
weather is sufficiently warm to hatch katydids, it is also warm enough
to force the buds on the end of the twigs. When the katydids arrive
their jaws are young and tender, but so are the leaves upon which they
are born. Hence there is little difficulty on the part of the young
katydids in finding an abundance of food. By the time the leaves have
grown tougher, the katydid's jaws are stronger, and the leaves will
still serve as food.
Everyone who is at all familiar with country life and gardening is
familiar with what is called the potato or tomato worm. It is a long,
green, smooth, caterpillar, as long and as fat as your finger and
provided with a horn upon his tail. The gardener may not know that
after a while this creature will burrow into the ground, and there
change into an oblong brown mass with a sort of a pitcher handle at
one side. Next year this pupa will split down the back, and from out
of the brown case will come a hawk-moth, which soon will fly with
rapidly quivering wings and feast upon the nectar of our moon flowers
or on that of the "Jimson" weed. Those who have cleaned these pests
from the potato or tomato vines will often have noticed one of them
covered with what look almost like grains of rice. This appearance
reveals an interesting story. Some time earlier an insect that looked
very much like a dainty wasp with a rather long sting in its tail
hovered over the caterpillar. This is the ichneumon fly. Eventually
lighting upon the caterpillar's back, it punctured the skin with its
sting, and deposited eggs within the caterpillar's body. These eggs
soon hatched and the little grubs worked their way through the body of
its host. The infested victim feeds upon leaves and fills itself with
rich food. These parasites eat the food, and, try as it may, the
caterpillar does not succeed in getting fat. After the grubs have
gotten their full growth, each of them eats its way through a little
hole to the outside of the caterpillar's body. Here it spins around
itself a little white case, and looks like a rice grain. As the
caterpillar moves about, these seeming rice grains are rubbed off and
fall to the ground. Next year there will come up new ichneumon flies
to sting fresh caterpillars and repeat the entire process.
Another remarkable provision for the young on the part of insects is
seen in the behavior of the big sphex wasp, known as the cicada
killer. The cicada, it will be remembered, is what is commonly called
a locust. The cicada killer is a magnificent big wasp, whose body is
nearly an inch long, banded with black and yellow, while the wings are
colored a smoky brown. This muscular wasp digs a long tunnel eight or
ten inches deep, which ends in a slightly larger room. Having provided
the location, he now sallies forth in search of the cicada. The heavy
song of the male probably serves as a guide to the wasp in case of
scarcity of cicadas, but the killer has apparently little difficulty
in finding his prey. The wasp pounces upon the insect, and in spite of
its strength and the thrashing of its vigorous wings punctures it with
his sting again and again. The poison of the sting entering into the
nerve centers gradually paralyzes, but usually does not kill, the
cicada. Now the killer carries its prey home, pushes it to the bottom
of the tunnel and deposits upon it a single egg. The wasp closes up
the hole and leaves the place. When the egg hatches and the grub of
the wasp emerges, it finds a big cicada just at hand, upon which it
feeds. By the time the cicada is completely devoured, the wasp grub
has obtained its full growth. After a short period of development a
new sphex wasp is ready to work its way out of the tunnel, find a
mate, dig a hole, and safely provide for its own children.
Still more remarkable adaptations for the care of the young appear
among the birds. Here the eggs are not to be deserted, but are to be
cared for until the young appear. These again must have attention
until such time as they are quite able to take care of themselves. The
birds are warm-blooded animals, and even their young, while they are
developing in the egg, are warm-blooded. Consequently the temperature
of the egg must be maintained evenly and uniformly, or there will be
no development.
The fish may drop its eggs carelessly upon the bottom of the stream. A
frog may deposit them in a mass of jelly and leave them forever. A
turtle may bury its eggs in a sand bank and abandon them to their
fate. The warm blood of the young bird demands more attention than
this. Accordingly, the parent bird has learned to make for itself some
sort of nest, in which the young may be kept properly warm until they
are developed. The ancestral bird, who was to be the progenitor of the
entire bird class, must have had some very simple method of providing
a place in which its eggs might be hatched. As the descendants of this
original bird have passed into new situations, the various lines have
taken upon themselves different shapes until we have the multiform
birds of to-day. The habits of the birds have also varied. Each has
adapted itself to the situation in which it found itself, and no
adaptation has been more varied and effective than the adjustment of
the nesting site. Nests are found upon the ground, in the bushes, on
the lower limbs, in the crotches of the trees, in the trunks of the
trees, upon their very summits, and on the tops of inaccessible crags.
To every sort of situation some bird has been enabled to adapt itself.
This has made it possible for very many more birds to thrive than
could have found a place in the world, had they all lived upon the
same plan.
In the case of the bank swallow his nest may be a very simple
contrivance, consisting only of a tunnel running back into a bank, and
widening at the back. Some material that will soften the bed upon
which eggs are to be laid must be placed in this cavity. The whole
home is a very simple and crude affair. But little better is the
arrangement which the woodpecker calls a home. This has been cut into
the dry wood of a defective tree. No woodpecker can make his home in
absolutely solid sapwood. Hence the first labor of the woodpecker must
consist in finding a place in which it can dig. If there is an old
stump of a limb sticking up, the problem is readily solved. Such wood
has no sap in it, and is brittle enough to be easily dug out. But, if
there be no such stub, the woodpecker will find a suitable place in
most trees. At some time or other almost every tree loses a big limb.
When such accident occurs there will always be in the old trunk a
region through which sap once went to this limb. This region, deprived
of its function, goes completely dry, like the heartwood of the tree,
and it is into such material as this that the woodpecker succeeds in
drilling his well-protected home.
As birds rise higher in the scale the nest-building becomes a more
complicated affair, and after a while we find a well-woven substantial
nest, through which even the air will not chill the eggs enough to
prevent their hatching, while the warmth is supplied by the mother's
body. It is often a matter of surprise to many people that a bird
should contrive to build a nest so exquisitely circular. The trick,
after all, is not quite so difficult as it looks. The robin gathers up
a few sticks and places them as the beginning of the platform. More
and more are brought and woven into each other, making a framework
altogether too big for the nest. Then mud is brought and plastered
inside of this. With the plastering of this mud the careful
circularity of the work begins. Every time a little material has been
added the robin sits down in the nest and revolves her body, in this
way shaping the interior much as the potter shapes a pot. In the case
of the artisan, it is the pot that revolves. In the case of the robin,
the bird itself revolves. The effect is the same in both cases--a
circular vessel is produced. A little lining added to the interior of
the nest softens it for the reception of the eggs. In this exquisite
home the robin lays her eggs, and sits upon them until they are
developed enough to hatch, and then feeds the young until they are old
enough to feed themselves.
Far more remarkable than any of the devices thus far described are the
wonderful developments which have come in the class of animals known
as the mammals. Here the most wonderful protection is made for the
care and feeding of the young. But this is to be the subject of a
separate chapter.
As long as we thought of each sort of animal as being a separate
species shaped in the beginning by the hands of the Creator, each of
these devices seemed to us a new manifestation of the Divine
Providence, whose fertile planning had conceived so many methods of
providing for his children. Unconsciously we thought of God acting as
man acted. Each animal seemed a purely separate invention purposely
designed for an especial place. Now we understand the plan in creation
better, and see that each animal has come from another not quite like
itself, some distance back, and this from still another. Our
admiration for these devices as they arise through evolution is no
less, but takes on another form.