Chapter IX: Section V: Natural Selection (1)

NATURAL SELECTION.

In briefly outlining the transmutations of living forms that took place during the Geologic Ages, we have said that changing conditions of life—through an ever-shifting environment—have compelled modifications in the form, structure, and habits of living creatures; and that those creatures which were unable to _adapt_ themselves, through _useful variations_, to the altering conditions of life have perished, while those that _did_ adapt themselves lived and progressed in organization. What, then, is the great agency through which some life forms have been _eliminated_ during the ages, while others have been _selected_ to continue through these ages? This agency is =Natural Selection=. The phrase “natural selection” is simply a convenient, condensed statement of observable and easily verifiable facts, viz.: that animals and plants are so situated in this world that they can only secure their food and mates by work, by effort, by struggles, whether consciously or unconsciously, and whether directly or indirectly; and that in these struggles those that are best equipped for their life duties are the ones that are most naturally successful in living and procreating their kind. The survival of those best adapted to their environment may be spoken of, in the language of Spencer, as the =Survival of the Fittest=. Darwin’s phrase—=Natural Selection=—has precisely the same significance and means that those creatures which are best fitted to their surroundings are the survivors. The working of Natural Selection may be made clearer by a brief reference to the =Artificial Selection by Man= of various animals and plants. Variations frequently take place in domesticated animals and plants. Some of these variations appeal to man as being of practical value, others as beautiful, and others again as curious or interesting. He selects those individuals whose variations he wishes to preserve, breeding only them together, and in this way accentuates those variations he desires to perpetuate. In course of time the accumulation of these differences becomes so marked as to make the animals differ greatly from the original stock from whence they came. A well-known illustration of this process is the pigeon. All our domestic pigeons, forming a large number of well-marked races, such as the fantail, the tumbler, the pouter, etc., have been produced from the ordinary wild rock pigeon of Europe. The bird fancier, noticing individual differences in the offspring of the wild rock pigeon, selected the peculiar individuals, and bred only them together. By this simple process of artificial selection and isolation of the chosen or selected individuals, all the races of pigeons have been produced. Plate I shows the wild rock pigeon _Columba livia_ and the domesticated pouter. The forms of these pigeons are very different, yet the wild rock pigeon has been transmuted into the pouter through the agency of Artificial Selection. The same is the case with our pigs, dogs, cats, apples, grapes, and other domesticated animals and plants. Fig. 16 shows the domesticated pig that has been derived from the wild boar by Artificial Selection. By this process, such distinct races as the Newfoundland, the Skye Terrier, and the Bulldog have been produced—creatures that have all come from common ancestors, yet so different looking, one from the other, that if they had been found in the wild state, they would not only have been ranked as distinct species, but as even distinct genera. The same method has given us different races of horses, cows, sheep, flowers, grains, etc. The swiftest horses, for instance, are selected to breed together; then the fleetest offspring of these, time after time, until horses are produced whose speed far surpasses that of the originally selected pair from whence they were derived. Darwin has taught us that what man does on a small scale, in a comparatively short time, Nature has been doing on a vast scale for long ages and has thus given rise, from simple forms, to the infinite variety and complexity of animal and plant life, as we behold it on the globe to-day. The selection by man of useful variations in domesticated creatures being appropriately called Man’s Selection, or Artificial Selection; the vastly greater selection by nature of animals and plants with useful variations and on an infinitely grander scale, through inconceivably long ages, is most fittingly called =Natural Selection=. The struggles—of animals, for instance—that necessarily lead to the survival of the fittest, are intensified and made exceedingly acute and severe by the fact that _all animals tend to increase in a geometrical ratio, and by the further fact that the food and place for animals are limited_. In other words, the population of the animals in a given area tends greatly to outrun the means of subsistence. And since animals are constantly varying in many directions and are as plastic in the hands of Nature as clay under the chisel of the modeler, those that possess any useful variations, whether congenital or acquired, that give them any advantages in this great battle of life, will most likely come out of the struggle as victors.

=Multiplication of Animals.= Even in the slow-breeding elephant, the offspring tend to increase threefold in each generation. Some animals tend to increase twenty or thirty fold in each generation, while still others tend to increase a thousand fold or even ten thousand fold. If all the offspring of the elephant lived, in eight hundred years there would be over nineteen million elephants alive. If the eight million eggs which the roe of certain fishes, such as the cod or the eel, contains, were to develop into adult forms, the ocean would quickly become a solid mass. The aphis or plant louse is so very prolific that it has been estimated that the tenth brood of one female alone would contain more ponderable matter than all the population of China,—estimating this population at five hundred millions. Yet, in spite of this tendency on the part of animals and plants to increase in numbers at such a stupendous rate, it is found that, in any given area, the conditions of which are not changing, the number of the animals and plants remains fairly constant. This is because of the fact that, _along with the stupendously large birth rate, there is an equally stupendous death rate_. This high death rate is to a large extent indiscriminate, for it involves those that are physically fit to live, as well as those that are unfit to live. At the edge of a coral-reef, free-swimming, active embryos are found in immense numbers. After a while some of these settle at too great a depth in the water or on the muddy bottom, and die; others get into a more suitable position and live. Again, whole nests of bees are destroyed by the badger; tongue loads of ants are engulfed at one gulp by the ant-bear; hundreds of thousands of fry are destroyed by the Greenland whale at one swallow. In all these cases, the destruction is indiscriminate,—the good, bad, and indifferent are alike decimated, but in spite of this wholesale and indiscriminate destruction, keeping down, as it does, the stupendous birth rate of living creatures, _more animals and plants are born than are required to keep up the normal number of individuals that can be supported in a given area. Among these creatures there arises a struggle for existence, a struggle for food and place, and those that are the best fitted to live come out of the contest as conquerors._ In this struggle those creatures that emerge as victors, on account of having been best adapted to their conditions of life, may be spoken of as having been selected by Nature; or they may be spoken of as chosen by Natural Selection. In this great struggle, over which Natural Selection presides as some _inexorable, ever-watchful, sharp-eyed task-master_, the victory is to the cunning instead of the stupid; the race is to the swift instead of the slow; and the battle is to the strong. The wolves of keenest scent, the tigers of more supple spring and sharper sight, secure their prey and thrive, while the weaker members fail to get their food and starve. During migration the birds that are strongest on the wing reach the land whither they are flying, while the weaker perish on their course. Thus Natural Selection acts in two ways, _eliminating the unfit_ and _selecting the fit_, and there are, besides, two special modifications of Natural Selection which are called =Sexual Selection= and =Insect Selection=.

The following analysis of Natural Selection may be useful to the reader, viz.:

{ Physical and Climatic Causes.
{ Elimination { Enemies.
{ { Competition.
{
Natural Selection { { Natural Selection Proper.
{ {
{ { { By Preferential
{ { { Mating.
{ Selection { Sexual Selection. {
{ { By Battle.
{ Insect Selection.

=Elimination of the Unfit.= The elimination of the unfit takes place through the agency of physical and climatic causes and also through enemies, and competition of members of the same species. Elimination through the action of surrounding physical and climatic conditions is shown by the following facts: if certain tropical animals be transferred to sub-Arctic or even temperate regions they are unable to adapt themselves to the requirements of the new climatic conditions, and die sooner or later; many animals are killed if the fresh-water lake in which they live be invaded by the waters of the ocean; fishes which live at great depths in the sea and are, therefore, subject to great pressure, are killed when they are brought to the surface, on account of the expansion of the gases in their tissues; if the water where corals are living becomes too fresh, too muddy, or too cold, they will die. The change of climate to a much colder temperature at the close of the Jurassic Ages was probably the cause of the extinction of the huge reptiles that took place at that time. In the winter of 1854-5, four-fifths of the birds in Darwin’s grounds perished on account of the severity of the cold.

As to elimination by enemies, it is well known to naturalists that throughout nature battle within battle is continually recurring with varying success. When weaker animals are preyed upon by stronger ones, and self-defense is useless, the bulky and slow animals are eliminated, while the swift and agile ones escape; the stupid are destroyed, while the cunning often survive. As to elimination by competition, the stronger animals kill the weaker ones, and then quarrel and fight with one another over the prey, the strongest, etc., getting the food. While weaker animals are being preyed upon by various enemies, and are thus eliminated, these enemies are also competing with one another for the prey. At the same time that the stupid and slow creatures are being destroyed by their captors, thus leaving the more cunning and agile animals in possession of their habitat, the stupider and less active captors are gradually eliminated by competition, through failing to capture their more agile and cunning prey.

The agency of Natural Selection in bringing about the innumerable adaptations of animals and plants, and, therefore, causing transmutations of living creatures, can most interestingly and instructively be illustrated by a study of the _coloration of animals and plants_.

As a correct idea of the mode of operation of Natural Selection, and the pronounced results attained thereby, is very desirable, it will well repay the reader to study _in extenso_ the relations of _color-patterns_ to _environment_. The whole subject is nothing but repeated illustrations of one principle, viz.: Natural Selection. This fact and the interest of the subject will justify the numerous details.

=The Coloration of Animals and Environment.= The colors of animals often harmonize most wonderfully with their surroundings. Thus green is a common color of animals in the evergreen forests of the tropics; white is the prevailing color in the arctic regions; and a yellowish hue in desert places. In the evergreen forests of tropical America, whole groups of birds are found whose fundamental color is green; there the parrots and fruit-eating pigeons are commonly green; the bee-eaters, leaf-thrushes, and many other birds, have so much green in their color as to add greatly to their concealment in the dense green foliage. In the desert places, the lion, the desert antelope and the camel harmonize with the color of the rocks and sand among which they live. In the Arctic regions the polar bear, the Greenland falcon, and the American polar hare are white. We have further the dusky hue of creatures that haunt the night, such as mice, moles, and bats; and the gorgeous tints of fishes that swim among the coral reefs. These local color adaptations of animals are of great use to them, either enabling them to escape the notice of their enemies or to come upon their prey with the least risk of being detected. Certain groups of animals have a local color-adaptation, and may be noticed under the heading of _Protective Coloration_; others have acquired a wonderful resemblance to surrounding inanimate objects, such as leaves, twigs, bird-droppings, flowers, etc., and may be described under the heading of Protective Imitation of particular objects, or _Protective Resemblance_; closely allied to the latter are those resemblances to surrounding objects which are not so much for the purpose of protecting the animals from enemies as for attracting their prey, and these resemblances, therefore, will be described under the title of _Alluring Coloration_. Other animals have such color patterns as to be very conspicuous in their surroundings; these animals are usually very poisonous or possess other deleterious qualities that cause beasts of prey to avoid them; hence these animals may be said to possess _Warning Coloration_. Many animals that are very desirable food for carnivorous creatures have acquired, in past ages, a remarkable resemblance to these dangerous animals with warning coloration, and are treated of under the title of _Mimicry_. There are still other groups of animals whose peculiar coloration enables the member of a herd or flock which may have become separated from the herd to readily recognize its companions at a distance as friends, and thus distinguish them from enemies. The color patterns of these animals may be classed under the heading of _Recognition Marks_.

=Protective Coloration, or Local Color Adaptation.= In forest-haunting animals of large size, such as forest-cats, and forest-deer, rounded spots are frequently noticed. Animals like the tiger, that spend a great deal of their time among high grasses and reeds, are striped vertically. The combined artistic effects of these spots and stripes, in connection with the lights and shades of the forest and the reeds, are such as quite effectually to conceal the animals from view: for the black stripes of the tiger, for instance, correspond with the black shadows of the reeds or grasses; and his yellow stripes with the yellow of the reeds. In like manner the rounded spots of the forest-deer harmonize with the spotty shadows of the leaves in the forest. An experienced tiger hunter has stated that in following up a wounded tiger the natives saw the animal at a distance of about twenty meters, under a tree among the reeds, and pointed out the animal to him; but the color effects of the stripes of the tiger so harmonized with the artistic effect of the light and shade of the reeds as to effectually prevent his seeing the animal for a minute or so. The zebra, which is such a conspicuous animal in our zoölogical gardens, on its native soil and in a bright star-light night, may be so close to one as to be heard breathing, and yet cannot be seen, so completely are its color patterns in harmony with its native habitat. Marine organisms that float on the surface of the water are beautifully tinged with blue in harmony with the color of the ocean as it appears to their enemies above, the birds; while, looked at from below, they are white, thus harmonizing with the white clouds and the foam as seen by enemies from below.

There are many animals that are very conspicuous when removed from their native haunts, yet when in their proper environment are invisible or detected with the greatest difficulty. Such a large animal as the giraffe is effectually concealed by its form and color when standing among the broken and dead trees that exist on the edges of the thickets where it may be seeking its food. The odd shape of the head, with its horns that resemble broken branches, and the blotchy spots on the skin, so harmonize with its surroundings that even the keen eyes of the natives sometimes mistake giraffes for trees and trees for giraffes.

There is a bat (_Kerivoula picta_) found in the island of Formosa that has a very conspicuous black and orange color. The body of this bat is of an orange color, and its wings are black and orange-yellow. When resting it suspends itself, head downwards, from the branches of an evergreen tree. During all the year some part of the foliage of this tree is undergoing decay, so that many of the leaves assume tints of orange and black. When the bat is suspended among such decaying leaves its colors and those of the leaves so harmonize that the animal is perfectly concealed, and thus eludes its enemies.

The sunbirds of Africa are very conspicuous when out of their natural environment, being brilliant and gorgeously colored. These birds find their main food supply among plants that have very conspicuous flowers; the aloe-blossoms, especially, which they frequent, are brilliantly colored. The colors of these birds so completely harmonize with the gay colors of the blossoms that even the keen eye of the hawk is unable to detect them. One species of these birds, the black sunbird, is never absent from a forest tree known as the Kaffir boom. This tree has not a single green leaf on it, but consists of a great mass of purplish-black and scarlet blossoms. A dozen of the black sunbirds may be feeding in this tree, and their notes may be heard among its branches all the day, yet their color adaptation to their environment is so complete that they are seen with the greatest difficulty or are entirely invisible.

Those birds whose colors have varied in such a way that they have not harmonized with their surroundings,—that have not become adapted to their environment,—are seen by the hawks and are exterminated; but those whose colors were more favorably arranged and more in harmony with their surroundings have been most often undetected by their enemies, the hawks, and have lived and transmitted their useful color patterns by heredity to their offspring. This process going on, age after age, has developed such perfect adaptations of the sunbirds to their environment as the naturalist observes at present.

These examples are illustrations of the fact that those creatures that are most in harmony with their surroundings are the ones that live and procreate their kind. They are cases of the survival of those animals that are best adapted to their environment,—the survival of the fittest; the selection by nature of favored creatures; in short, Natural Selection.

Among our native birds, the woodcock and snipe have such tints and markings as strikingly to harmonize them with the dead marshy vegetation which constitutes their native haunts. The ptarmigan in winter has a light coloration in harmony with the environment of snow, while in its summer plumage it is tinted and mottled in harmony with the color effects of the lichen-covered stones among which it spends a great deal of its time. Young unfledged plovers are spotted in such a way as very accurately to resemble the beach pebbles among which they remain for protection.

The white-headed fruit pigeon (_Ptilopus cinctus_) is a very conspicuous bird when taken from its native haunts. It has a pure white neck and head, black back, yellow belly, and well-marked, deeply-curved black band across the breast, and black wings. It is a handsome as well as very conspicuous bird. It frequents trees that are a species of Eucalyptus in the island of Timor, and which have very open foliage and yellowish or whitish bark. The pigeons may be sitting motionless on exposed branches of the tree during the glaring heat of the day. The yellow and white bark of the tree, the deep blue sky seen through the openings of the leaves, with the intense tropical sunlight casting black shadows of one branch upon another, make color effects that harmonize so completely with the color patterns of the pigeons that they are entirely invisible.[8] Here again the complete color adaptation of the pigeons to their environment has been accomplished through the agency of Natural Selection,—Nature selecting those pigeons most frequently that varied most in the direction of useful color patterns, by concealing them from their enemies, the hawks.

There are some birds that are not protectively colored, which therefore do not possess a local color adaptation. The common raven, which inhabits the Arctic regions, is black instead of being white. The raven is found as far north as any known bird or mammal. It is a powerful and fearless bird, and needs no protective coloring; and since it feeds on dead animals it needs no concealing coloration to enable it to secure its food.

The bright and conspicuous coloration of many birds’ eggs have often been looked upon as a difficulty on the theory of adaptive coloration. But Wallace thinks that a careful consideration of the subject in all its bearings shows that in a great number of cases these colorations are instances of protective coloration. He further thinks that when we cannot see the meaning of the particular colors, we may suppose that in some ancestral forms they have been protective, and, being harmless, have persisted under changed conditions which rendered the protection needless. He states, in illustrating the protective coloration of eggs, that “the beautiful blue or greenish eggs of the hedge-sparrow, the song-thrush, the blackbird, and the lesser redpole, seem at first sight especially calculated to attract attention; but it is doubtful whether they are really so conspicuous when seen at a little distance among their surroundings. For the nests of these birds are either in evergreens, as holly or ivy, or surrounded by the delicate green tints of the early spring vegetation, and may thus harmonize very well with the colors around them. The great majority of the eggs of our smaller birds are so spotted or streaked with brown or black on variously tinted grounds, that when lying in the shadow of the nests and surrounded by the many colors and tints of bark and moss, of purple buds and tender green or yellow foliage, with all the complex glittering lights and mottled shades produced among these by the spring sunshine and by the sparkling raindrops, they must have a quite different aspect from that which they possess when we observe them away from their natural surroundings.” We have here, probably, according to Wallace, a similar case, of general protective harmony, to that of the green caterpillars with beautiful white or purple bands and spots, which, though gaudily conspicuous when seen alone, become practically invisible among the complex lights and shadows of the foliage they feed upon.

Eggs that are not protectively colored are usually white or of some uniform pale color, and are concealed by the birds in covered nests or in holes in trees or in the ground. Many birds lay white eggs in open nests, but even here the devices for concealing them are very effective and interesting. In some cases, such as the partridge, the goat-sucker, and others, the birds have the habit of sitting close and almost continuously on the eggs. These birds are protectively colored. Ducks, pheasants, and other birds have the habit of covering their white eggs with dead leaves or other material when they leave the nest. There are some large and powerful birds that lay conspicuous white eggs in open nests. Such are the cormorants, herons, storks, pelicans, and others. But they guard their nests carefully, and are able to drive away any enemies.

It is thus seen that there are many devices by which birds’ eggs are protected, and these have all been developed through the agency of Natural Selection. For example, those exposed eggs of timid birds that varied most in the direction of protectively colored ones were the ones that were most likely to be overlooked by egg-eating creatures, and those that were least protectively colored, and therefore the most conspicuous, were the ones most likely to be destroyed. The protectively colored eggs being thus, in the nature of things, the ones that have suffered less destruction from enemies, have developed into birds that have transmitted, by heredity, their characteristics to succeeding generations of birds’ eggs. And thus in the course of time have many of the colorations of eggs been developed from simple to elaborate patterns.

Many snakes, frogs, butterflies, caterpillars, and so on, are colored green in harmony with the foliage of the trees among which they live. By being colored green these animals are more or less effectually hidden from their enemies, such as carnivorous birds. If they were not concealed by protective coloration they would quickly be exterminated by hungry and greedy enemies. The green frogs are greatly protected by their colors from their enemies, the green snakes. If they were not so colored, they would quickly be exterminated by these snakes. Even protectively colored as they are, large numbers of them are caught by the snakes, which are also protectively colored and keener witted. It may be stated that the protectively colored frogs are hunting for their food, the protectively colored insects; but, at the same time, they try to avoid their protectively colored enemies, the snakes, though they are often caught; the protectively colored snakes are hunting for their food, the frogs, but, in their green surroundings, endeavor to avoid their enemies, the reptile-eating birds, though often unsuccessful. And so on, from day to day, the tragedies of the animal world are enacted.

It may be well to pause a moment here and ask why it is that animals are not completely exterminated which are thus perpetually being persecuted and preyed upon by stronger foes. Why, for instance, are any frogs left in a green locality, when they are so eagerly sought for by their enemies, the snakes? What are the factors that permit the average or normal number of them to exist in the given locality? The most important of them have already been referred to separately, but it will be useful to recapitulate, briefly, a few of them now. One of the most important of them is the tendency of the frogs to increase in geometrical ratio. In the breeding season great numbers of young are brought into the world, thus reinforcing the depleted ranks of the adults. Another factor is the instinctive effort of each frog to avoid its enemies, and their protective coloration greatly facilitates their concealment and escape. Another factor is the circumstance that the snakes also have enemies. They must be wary and careful in hunting prey lest they be unduly exterminated by their foes, the snake-eating animals. Then another factor is the alternating day and night, by which the warfare, offensive and defensive, must be periodically checked. Then again, the hibernating season, winter, checks or rather suspends the life-destroying crusades. It may thus be recognized that many complex factors occur to explain the circumstance that a given locality generally contains the normal number of individuals that constitute the species of frogs.

The complex factors at work in these relations and inter-relations of animals and plants are so nicely balanced that with a comparatively stable environment the number of species of the different classes of animals may remain more or less constant. But suppose one of these factors is profoundly diminished, such, for instance, as a great diminution in the number of the snake-eating birds. Then the snakes, not being much persecuted by their enemies, will be in a most favorable environment; they will increase greatly in numbers; they will be eagerly on the look-out for their food, the frogs; the frogs, being thus unusually persecuted and hard-pressed, will diminish greatly in numbers. Now, this will be a golden opportunity for the insects; their environment becomes much more favorable. The snakes do not bother the insects, but destroy their enemies, the frogs. Therefore, the insects increase greatly in numbers and profoundly affect the vegetation upon which they live and thrive. The injury to the vegetation may thus seriously affect an altogether different class of animals—the grass-eating or foliage-eating animals, like the ruminants.

It may thus be understood how profoundly complex are the relations and correlations of living creatures, and how a disturbance of some of the links in these living chains may very extensively affect the other links. The different creatures referred to above are probably the simplest illustrations that can be given of protectively colored animals. For here we have green animals adapted to a green environment—the green foliage of trees or grass. This adaptation has been brought about by Natural Selection. Suppose, for instance, that the habitat of the _non-protectively_ colored _ancestors_ of the frogs was for any reason unduly crowded by their enemies, the snakes. The frogs would be hard pressed and much persecuted. Suppose that among the young of these frogs there were some that varied in the direction of grass-green or leaf-green colors, while others did not so vary, or possibly varied in the direction of even conspicuous colors, such as black or white. It is evident that the latter groups would be easily detected by the snakes and destroyed, while the green group would frequently escape notice and would thrive and procreate their kind. This process being repeated generation after generation, there would come a time when the given habitat would contain none but frogs with a protective coloration of green. All of these protective colorations of snakes, butterflies, and caterpillars, as well as of frogs, are adaptations. They are illustrations of the survival of the fittest; the survival of those best adapted to their environment—in short, Natural Selection.

=Protective Imitation of Particular Objects.= Insects often exhibit a very great amount of detailed resemblance to the leaves, flowers, and twigs of plants among which they live. Those that live on grass are striped longitudinally, while those that feed on ordinary leaves have an oblique striation. There is a larva of a Georgia butterfly (_Sphinx fuciformis_) which feeds on a plant having small blue flowers and linear, grass-like leaves. This larva has a blue head and a green body striated like the leaves. The resemblance of the insect to its environment is very striking. There is another species that feeds on a plant with small red flowers situated in the axils of the leaves, and this larva has a row of seven red spots of unequal size, which corresponds quite closely with the size and color of the flowers. There is a caterpillar in Borneo that resembles a piece of moss with two exquisite pink-white seed-capsules. Its general hue is greenish, with two little pink spots on its upper surface, and it is covered with hair. Its movements are very slow, and when eating it withdraws its head beneath a mobile fleshy hood that it possesses, so that its motions in feeding are not noticeable. When living in its native haunts it is all but impossible to detect it, so completely does it resemble the surrounding moss. Other insects resemble green or dead leaves in all their varieties of form and color, and to show what a great protection such resemblance affords to insects in concealing them from view the following observation of a naturalist in Nicaragua may be related. In that country there are armies of foraging ants that devour every insect they can catch. Among a multitude of these ants he observed a locust that looked very much like a green leaf. The ants, many of them, were continually running over the body and legs of the insect without detecting its character. In many parts of the world there are many butterflies (_Kallima_, for example) the under surfaces of whose wings very closely resemble dead leaves. They frequent dry forests and are rapid flyers. They are rather large, and the upper surfaces of their wings are quite showy, having bluish and orange colors (Plate II). It is their habit always to settle on some twig where there are decaying or dead leaves. In doing so it folds its wings together over the back, thus concealing the gay upper surfaces and presenting the protectively colored under surfaces. The resemblance to a dead leaf is much more striking from the fact that the short tails of the hind wings just touch the branch on which the insect rests and look very much like the stalk of a leaf. From this stalk a dark curved line extends to the elongated tip of the upper wings, thus imitating the midrib of a leaf. On both sides of this midrib are oblique lines that are partly markings and partly nervures, which give the appearance of a leaf with its veining. The head and antennæ fit in such a way between the closed upper wings as not to interfere with that irregular outline which is characteristic of the withered and dry leaves. Often the closed wings are covered with small black dots gathered into circular groups that exactly resemble the minute fungi found on decaying leaves, and it is sometimes difficult to believe that the insects themselves are not attacked by a fungus. Wallace states that this wonderful imitation is most complete, and that in Sumatra he has often seen a butterfly enter a bush and then disappear as if by magic. He states that once he was so fortunate as to see the exact spot on which the insect settled, but even then lost sight of it for some time, and was able to discover it close to his eyes only after persistent and careful search.

The curious and interesting leaf-insects of Java are veined and colored in such a way that, with the leaf-like expansions from various parts of the body, not one person in a dozen can detect them when they are resting upon their food-plants right under one’s eyes. Other insects resemble pieces of stick (Plate III), with all the minute details of branches and knots. An eminent naturalist has stated that after being a practical entomologist for thirty years, he was deceived by one of these stick insects and took out his pruning knife to cut from a plum-tree what seemed to him to be a projecting spur. This spur proved to be the caterpillar of a geometer-moth, about two inches in length. He placed a portion of the plum-tree on a table, and showed it to several members of his family, designating a space of several inches in which the caterpillar was to be found, but none of them could detect the insect until it was pointed out to them. These protective resemblances of living creatures to inanimate objects are beautiful illustrations of adaptation to environment through Natural Selection.

Beautiful illustrations of protective resemblance to particular objects are furnished by leaf-hoppers (insects) in Central America. They resemble the thorny and prickly growths of the plants on which they presumably live. Some of them also resemble gall growths on the plants (Plate IV).

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=Alluring Coloration.= Besides those insects which secure protection from enemies by their resemblance to the inert objects among which they live, there are others whose adaptive resemblance, and therefore concealment, is for the purpose of securing their food,—for alluring their prey. A most interesting case of alluring coloration is that furnished by a wingless insect of India, the mantis. Its color and form are such as to closely resemble such a fantastic flower as the pink orchis. The insect rests motionless among the bright green foliage, being very conspicuous on account of its pink color, and looking so much like a flower that it allures and captures the butterflies which settle upon it.

There is a species of spider (_Thomisus citreus_) of a creamy-white color whose abdomen completely resembles in color and contour the unopened buds of the flowers among which it rests. It has been seen to capture flies that were attracted to the flowers. There is another species of spider that looks exactly like the excreta of birds, and through this alluring resemblance captures certain butterflies. A naturalist has related how, in pursuing a butterfly through a jungle in Java, he was stopped by a dense growth of bushes. Here he observed a leaf with a bird’s dropping upon it, and sitting on this dropping was a beautiful butterfly. Surprised at such a usually dainty and pretty butterfly seeking such inappropriate food, he carefully approached to study the actions of the insect. The insect permitted him to get so close that he seized it by the wings, and to his astonishment a part of the body remained behind as if the bird’s dropping was very adhesive. He touched the dropping to see if it really was sticky, and found that his eyes had been deceived and that what he took for the excreta of a bird was a most artfully colored spider, lying on its back with its feet crossed and depressed closely to the body. The spider had been firmly holding the butterfly.

A very pretty illustration of alluring coloration is furnished by the common spider (_Misumena vatia_), which is often found spread out upon the yellow heart of an ox-eyed daisy and in like position upon Coreopsis (Plate V). It so closely resembles the flower upon which it lurks that the ordinary observer might well fail to notice its presence. The coloration facilitates the taking of prey and protects the creature from the assaults of enemies (Rev. H. C. McCook). It will be observed from the illustration that the flower is mostly yellow with some red; the same is true of the spider. This case is a very instructive illustration of alluring coloration by which the insect has become a living bait for entrapping butterflies and other insects. Nature has brought about this complete color adaptation, from the variations of myriads of spiders. For those ancestral spiders that varied most in the direction of color adaptation to their environment were less often shunned by shy insects which could serve as prey. The spiders with useful color variations would thus most likely secure an abundance of food, and, thus living, transmit to many of their offspring their useful variations; while those with inharmonious, and therefore harmful, variations would not be so able to deceive their food insects, and would thrive poorly or starve altogether. This is another illustration of the survival of the fittest, the selection by nature of those best adapted to or in harmony with the environment; in short, it is Natural Selection.

Wallace, from whom many of these illustrations are taken, says that to many persons it will seem impossible that such beautiful and detailed adaptations and resemblances—and these are only samples of thousands that occur in all parts of the world—can have been brought about by the preservation of fortuitous useful variations. Yet this will not seem so surprising, continues Wallace, if we keep in mind the facts of the rapid multiplication of animals, the severe struggle for existence, and the constant variability of these and all other organisms; and, further, that we must remember that these delicate adjustments are the result of a process, Natural Selection, which has been going on for millions of years, and that we now see the small percentage of successes among the myriads of failures. “From the very first appearance of insects, for instance, and their various kinds of enemies, the need for protection arose and was usually most easily met by modifications of color. Hence we may be sure that the earliest leaf-eating insects acquired a green color as one of the necessities of their existence; and, as the species became modified and specialized, those feeding on particular species of plants would rapidly acquire the peculiar tints and markings best adapted to conceal them upon those plants. Then, every little variation that, once in a hundred years perhaps, led to the preservation of some insect which was thereby rather better concealed than its fellows, would form the starting-point of a further development, leading ultimately to that perfection of imitation in details which now astonishes us.” So it is with the beautiful color adaptation of birds, mammals, lizards, and other animals. There is a lizard (_Phrynocephalus mystaceus_) inhabiting certain sandy districts in Asia, whose body is protectively colored and some of whose mouth-parts have alluring coloration and form. The general surface harmonizes with the sand in which it is found, while the skin at each angle of its mouth is of red color and so folded as to closely resemble a little red flower which grows in the sand. The lizard, being thus in harmony with its surroundings, resembling the sand and the flowers, is hidden from its enemies, the reptile-eating creatures. But at the same time insects, being attracted by what they take to be flowers, approach the lizards and are thus captured, being allured to their destruction.

=Warning Coloration.= Many animals possess color patterns that render them very _conspicuous_ in their environment. It is a very interesting fact that most of these creatures are the possessors of some deadly weapons, as poison-fangs or stings, or that they are very disagreeable and unpalatable food for other animals. Warning colors are most abundant and best developed among insects. A family of butterflies (Heliconidæ), in tropical South America, possesses very pronounced and conspicuous color patterns, so that they are easily seen in their native haunts. Many of them have deep blue-black with vivid red, white, and yellow spots and bands, totally unlike those butterflies in the same locality that are protectively colored and palatable. Their bodies have juices that exhale a powerful odor. If one kill them by pinching the body, a liquid exudes that stains the fingers yellow and leaves an odor on them that can be removed only by repeated washing of the hands. There is a great deal of evidence to show that this odor is very offensive to insect-loving animals. Protectively-colored butterflies fly with great rapidity and are very wary and seek concealment; while the butterflies with conspicuous colors fly slowly, and do not conceal themselves, as if conscious that they have no enemies.

Many caterpillars have gay and conspicuous colors and do not conceal themselves. Bates noticed one in South America four inches long, striped across the body with yellow and black bands, and with bright red head, tail, and legs. It could be seen by any one who passed by, even at a distance of many meters. All of these conspicuous and brightly-colored caterpillars are unpalatable, and are refused as food by insect-eating creatures.

Grasshoppers and locusts generally possess green protective tints and are very palatable, but in tropical regions there are many species most gaudily decorated with blue, red, and black colors. They are inedible and are invariably rejected as food by lizards and birds.

A spider whose bite is exceedingly poisonous is found in Queensland. Its bite will kill a dog, and produces serious illness in man, with agonizing pain. It is black with a bright red patch on the middle of its body. This warning coloration is so conspicuous that even the spider-hunting wasp avoids it.

In all parts of the world frogs are usually protectively colored with browns or greens; and little tree-frogs are either curiously mottled to imitate dead leaves or bark, or they are green like the leaves they rest upon. These protectively colored frogs are always eagerly sought after by snakes and other enemies. But there are some frogs that are very conspicuously colored and that hop about with impunity, being avoided by the snakes and birds of prey. Such is a little frog in Nicaragua which with its “scarlet vest and stockings of blue” is very conspicuous in its native haunts.[9] Such, also, is a small toad found in South America, which is colored a bright vermilion and an intense black, which crawls about in the sunshine over the sands of arid places. Both of these animals are altogether avoided by the frog-eating creatures, because they have disagreeable properties that make them inedible.

In tropical America the very poisonous snake Elaps (Plate VI) is found abundantly. Its style of coloration is very conspicuous and one that does not occur in any other group of snakes, consisting alternately of rings of red, black, and yellow, or red and black of varying width and arranged in different patterns. Snake-eating birds and mammals have learned, through hereditary experiences, to avoid these snakes with gay livery because they are poisonous and therefore dangerous. In Plate VII the conspicuously-colored black and yellow salamander is an animal with warning coloration. It is inedible and avoided by carnivorous birds. These warning colorations have been evolved through the Natural Selection of fortuitous useful color variations in the ancestors during the Geologic Ages.

=Mimicry.= Protective resemblance of a harmless animal to another of a different species that is harmful is known as mimicry. Mimicry is bound up with and altogether dependent upon warning coloration. Some beetles are protected by having integuments, etc., of very great hardness. Several genera of weevils are in this way saved from attack by insect-loving birds. These weevils are often closely imitated in appearance by softer and more eatable species of different genera from the weevils. Wasps and bees are often mimicked by insects of other orders.

Insectivorous birds are very active in hunting out the edible beetles (Longicornia), and everywhere in tropical regions these beetles so closely resemble other insects which are avoided by the birds that the longicorns are very frequently avoided and thus protected.

In tropical America many butterflies (Heliconidæ) are found that possess warning coloration. They possess an offensive taste and odor which almost entirely exempt them from the attacks of insect-eating animals. The insectivorous birds have learned, by transmitted experiences (heredity) to avoid the Heliconidæ. It is an interesting fact that in the same locality with these distasteful butterflies are other species that are very palatable to insectivorous creatures; but they so closely resemble the non-edible species that the birds pass them by, not recognizing their character.

There are some cases of mimicry among birds. There is a genus of large honey-suckers known as friar birds found in the Malay Archipelago. They are noisy and powerful birds which go in small flocks. They have sharp beaks which are long and curved, and also powerful grasping claws. They are perfectly able to defend themselves, often driving away such birds of prey as hawks and crows when they approach them too closely. In the same environment are weak and timid birds known as orioles, which trust chiefly to their retiring habits and concealment for protection. The orioles, although an entirely distinct species from the friar birds, very closely resemble the latter. In each of the great islands of the Malayan Archipelago there is a distinct species of friar birds, and always in the same locality is a species of oriole that exactly mimics it. The separate species often look so thoroughly alike that competent naturalists, prior to a very close examination, have considered them as belonging to the same species.[10]

The most remarkable cases of mimicry are those in which poisonous snakes are mimicked by harmless ones. There is an egg-eating snake in South Africa that possesses neither teeth nor fangs and is not poisonous. It very closely resembles the poisonous Berg adder. When alarmed it still more closely resembles the adder by the habit of flattening its head and darting forward as if to strike an enemy, hissing at the same time.

In tropical America, in the localities where the poisonous genus Elaps is found so abundantly, are several genera of harmless snakes of other families, some species of which so closely resemble or mimic the poisonous species that they are distinguished from them with difficulty. The peculiar color patterns of the poisonous snakes serve as warning colors to snake-eating mammals and birds. The mimicking snakes by flying these danger-flags are protected.

In Plate VI Elaps is the venomous snake, and illustrates warning coloration; Erythrolamprus is the edible, non-venomous reptile that has acquired, through Natural Selection, a protective resemblance to Elaps, and illustrates mimicry. At the first glance these two snakes look very much alike; but a closer inspection will show that the detailed color patterns differ in the two cases. Elaps, though a very poisonous snake, has the reputation of not being venomous. This error has probably originated from the fact that it has a gentle disposition and mild temper, and also from the fact that no doubt it is frequently mistaken for the mimicking non-venomous species.

Both warning coloration and mimicry are interesting illustrations of adaptation to environment through Natural Selection; for the myriads of ancestral forms were continually giving slight variations in color patterns, some of which were useful to the creatures and others harmful. Those that harmonized mostly with the environment gave their possessors an advantage in the fierce struggle for life,—the struggle for food and place and safety; and procreating their kind age after age, led to the perfection of mimicry as we behold it to-day.

=Coloration as Recognition Marks.= In gregarious animals, whether herbivora or carnivora, and whether mammals or birds, a ready recognition of their own kind at a distance, in the dim twilight, or during rapid motion, is of the greatest use, and probably often leads to the preservation of life. Gregarious animals will not usually permit a stranger in their midst. So long as these animals keep together they are generally safe from enemies; but a single animal straggling off by itself may become an easy prey to enemies. In such cases it is of the highest importance to an animal that it should have every facility for quickly discovering its companions at any distance within the field of vision. Also to the young and inexperienced of each herd some means of easy recognition is of vital importance. Recognition marks also enable the sexes to identify their kind readily. The necessity for easy recognition probably is at the basis of the bilateral symmetry in the coloration of animals.[11] In the struggle for existence those gregarious creatures that have the best color patterns for recognition marks are the most likely to get the scattered members of a herd together with the greatest rapidity, and thus to save them from their enemies. The surviving members, transmitting their useful variations in recognition marks to their offspring, are thus able in the course of ages to bring into existence pronounced color patterns. Thus Natural Selection can account for much of the coloration in animals known as recognition marks.

One or two illustrations of recognition marks in creatures will be sufficient. A rabbit when alarmed and fleeing to its burrow displays a conspicuous, upturned white tail. The rest of the body is protectively colored. This conspicuous white surface of the upturned tail is a signal flag of danger. The rabbit mostly feeds during moonlight nights, or soon after sunset. The white upturned tails of alarmed rabbits serve as signals and guides to the feeble and young, and also to those at a more remote distance. Thus a number of rabbits, each following the one or two in front, are all able in the quickest manner to reach a place of safety.

The spring-bok has a white patch on the face and one on the sides. It also has a curiously well-marked white stripe above the tail. When the animal is at rest this last-mentioned white stripe is very nearly concealed by a fold of skin, but when it is in motion it comes into full view, like the upturned tail of the rabbit, and serves as a guide to friends. There are some animals inhabiting the Arctic regions that are not white,—they are not protectively colored. Such is the musk-sheep. This animal, though living in Arctic regions, is yet brown and conspicuous. Its safety depends upon its association in small herds. It is gregarious. Therefore it is of much more importance to this animal that it should be able to quickly recognize its companions at a distance than that it should be protectively colored and so concealed from its enemies. So long as they keep together in herds they are abundantly able to protect themselves. This is an exception to the rule of local color adaptation that proves the rule.

=Sexual Selection.= Among most backboned animals it is the rule that both sexes should be alike in color. This is especially true among the fishes, reptiles, and mammals. But in birds the diversity of sexual coloring is very frequent. It is among this class, therefore, that Sexual Selection can best be studied. One of the most fundamental characteristics of birds is the greater conspicuousness of coloration in the males. In the tropical regions especially are found the most striking examples of divergence in sexual color patterns. In humming birds, the pheasants, the peacocks, chatterers, tanagers, and birds of paradise, the females are exceptionally dull-colored and plain, while the male birds are gorgeously colored and conspicuously attractive. The male birds of paradise, for instance, are not only brilliantly colored, but also have remarkable gorgets, plumes, and crests; whereas the female paradise birds are without these decorations and as plain as our thrushes in their ornamental plumage (Plate VIII).