Part 11

=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).

The splendor of plumage which characterizes the male pheasants is entirely wanting in the females. The intense crimsons and pure whites, the gorgeous purples and blues of the male chatterers contrast strikingly with the dull browns or olive greens of the females. The sober hues of the females have been accentuated by Natural Selection.[12] When the females were brooding on the eggs in their nests, those of them that had varied in the direction of conspicuousness would most readily be detected by their enemies, the hawks, and would be exterminated; but those that had quiet and dull ornamentation would most frequently escape discovery, and would pass successfully through the brooding season; thus living and transmitting their color patterns to posterity,—Natural Selection ever eliminating the conspicuous and preserving the sober-hued,—in the course of time the dull ornamentation of the females would become more and more pronounced.

But another factor has been at work in accentuating the marked differences in the ornamentation of the sexes. This factor is Sexual Selection.[13] In the brooding season there is an intense rivalry among the males for the possession of the females. Among them the art of courting has become, indeed, one of the fine arts. The male birds, like the males of almost all animals, have stronger passions than the females, and with rarest exceptions are much more eager than the females. In courtship they display their adornments and accomplishments most zealously before the females; they strut around them in most eager courtship and pompous vanity, displaying the utmost rivalry. The males charm the females in various ways, such as dancing, or performing fantastic antics either in the air or on the ground; and then again by most melodious song. After man, the female birds appear to be the most æsthetic of all animals; therefore, those male birds that are the most pleasing songsters, or the most attractive in their dances and fantastic performances, are the ones selected by the females for mating. In this selection the female birds have paid the minutest attention to fleeting fashions in strut and dance, in form and color,—the progeny of those males that have been selected by the females for mating, transmitting the inherited peculiarities of the parents, have tended more and more to stamp as fixtures these fleeting fashions, and in this way the males have become endowed with all sorts of decorations and accomplishments.[14] Thus have been produced in them the many forms of topknots, wattles, combs, plumes, and feathers elongated and springing gracefully from many portions of the body; also the naked skin of the head and the beak, frequently colored gorgeously. The feathers, through this means, are often most beautifully tinted in charming patterns. As bearing upon this theory of Sexual Selection, it can be observed that birds pay the closest attention to the songs of each other. A bullfinch, for instance, had been taught to pipe a German waltz, and in doing so was a most excellent performer. He was placed in a room where there were kept some eighteen canaries and linnets, and immediately commenced producing his melodies. The birds all ranged themselves on the sides of their cages nearest the performer, listening to his singing with the greatest interest. Undoubtedly this singing is most often a matter of courtship; the female finch selects that one out of a hundred males whose notes charm her the most; the female canary always chooses the best singer. The soft cooing of pigeons and of turtledoves is a matter of courtship. In the breeding season there is the most intense rivalry between the males in singing; a bird will sometimes sing until he drops down almost dead.

That female birds exercise choice in mating was believed by Audubon. He describes how a woodpecker hen was followed by six gay suitors who continued to perform strange antics until a marked preference was shown for one of them. A study of the Australian bower-birds illustrates both the courting antics of the males and the exercise of choice by the females. These birds build bowers which are sometimes quite large. That of one species is raised on a thick platform of sticks and is nearly four feet in length and eighteen inches in height. These bowers are built on the ground, and are for the sole purpose of courtship, since the nests are formed in the trees. They are highly decorated with leaves, berries, feathers, shells, and kindred objects. Both sexes assist in the erection of the bower, although the male bird is the principal worker. The bower-constructing instinct is so strong that it is practiced even in confinement. A naturalist in describing the habits of some satin bower-birds kept in an aviary says that at times the male will chase the female all over the aviary, then go to the bower, pick up a gay feather or a large leaf, utter a curious kind of note, set all his feathers erect, run round the bower, and become so excited that his eyes appear to start from his head. He continues opening first one wing and then the other, uttering a low, whistling note, and, like the domestic cock, seems to be picking up something from the ground, until at last the female goes gently toward him and the wooing is completed.

Instances of love dances may be taken from all classes of the animal kingdom. “Mr. Peckham has described a very interesting love-dance by a certain species of spider (_Saitis pulex_). He placed a male in a box with a female. As soon as the former saw the latter, about twelve inches away, he became excited and at once moved towards her; when some four inches from her, he stood still, and then began the most remarkable performances that an amorous male could offer to an admiring female. She eyed him eagerly, changing her position from time to time so that he might always be in view. He, raising his whole body on one side by straightening out the legs, and lowering it on the other by folding the first two pairs of legs up and under, leaned so far over as to be in danger of losing his balance, which he maintained only by sidling rapidly towards the lowered side. The palpus, too, on this side, was turned back to correspond to the direction of the legs nearest it. He moved in a semicircle of about two inches, and then instantly reversed the position of the legs, and circled in the opposite direction, gradually approaching nearer and nearer the female. Now she dashes towards him, while he, raising his first pair of legs, extends them upward and forward as if to hold her off, but withal slowly retreats. Again and again he circles from side to side, she gazing towards him in a softer mood, evidently admiring the grace of his antics. This is repeated until are counted one hundred and eleven circles made by the ardent little male. Now he approaches nearer and nearer, and when almost within reach whirls madly around and around her, she joining and whirling with him in a giddy maze. Again he falls back and resumes his semicircular motions, with his body tilted over; she, all excitement, lowers her head and raises her body, so that it is almost vertical; both draw nearer; she moves slowly under him, he crawling over her head, and the mating is accomplished.”[15]

In addition to that form of Sexual Selection where the female chooses a mate from among a number of competing males, and which may be designated _preferential mating_, there is another form of selection in which the males fight with one another for the mastery and the possession of the females. Among the higher mammals it is a very general fact that the males fight together for the possession of the females. This leads, especially in polygamous animals, to the better armed or stronger males becoming the parents of the next generation, which inherits the peculiarities of the parents. Thus the offensive weapons and the vigor of the males are continually increased, resulting in the antlers of the stag, the tusks of the boar, the fighting instinct and spurs of the gamecock, and the horns and strength of the bull. Even mammals that are not specially armed fight to the death for the possession of the females, such as beavers, moles, squirrels, and hares. Almost all male birds are especially pugnacious during the breeding season. Battles have been observed in such different groups as ducks, finches, woodpeckers, humming birds, and waders. Among fishes deadly battles occur between the males of sticklebacks. Also the males of salmons engage in deadly contests; among reptiles fighting occurs among the male tortoises, crocodiles, and lizards. Spiders and many butterflies often fight for the females. Thus Sexual Selection through the _law of battle_ occurs widely throughout the animal kingdom. This form of Natural Selection greatly increases the vigor and fighting power of male animals; for, in every case, the weaker males are either driven away, killed or wounded, and the field is left to the most vigorous for procreating their kind.

The male stickleback is a little fish that builds its nest among the weeds, weaving the material together by a secretion from its kidneys. It is a very passionate little animal, and is exceedingly pugnacious in relation to its male rivals. The battles of the males are often very desperate. The combatants fasten tight to each other for a time, tumbling over and over again, until they appear to be completely exhausted. The males of the rough-tailed stickleback, when fighting, swim round and round one another, endeavoring to pierce each other with their raised lateral spines. In fighting they are perfect little furies, and their bite is very severe. Their lateral spines are used with such fatal effect during a battle that a male has been observed to rip open his opponent, so that the latter sank to the bottom in a dying condition. The females are very peaceful. When they come out of their hiding-place and view the nest that the male has made he is mad with delight.

The male salmon is as pugnacious as the little stickleback. Two males have been known to virtually battle with each other all day long. In breeding ponds the males can be seen constantly fighting and tearing one another on the spawning beds, and so many are injured in consequence that they may be seen swimming near the banks in a state of exhaustion, many of them apparently dying.

Among birds the law of battle holds as well as the law of preferential mating. During the breeding season they are exceedingly pugnacious. The humming birds, the smallest of any, are among the most quarrelsome. Two males rarely meet without a fierce fight on the wing.

Darwin gives the following illustration of the invincible courage and fighting instinct of the gamecock. One of these birds had both of its legs broken in a cockpit, and its owner made a wager that if his legs could be supported with splints and bandages so that he could stand upright he would keep on fighting. This was accomplished, and the bird fought on with dauntless courage until he received a death stroke.

It is probable that even with the most pugnacious species of birds the pairing does not depend alone on the courage and strength of the male, for these males are usually decorated with various ornaments. Furthermore, these decorations during the breeding season often become more brilliant, and are eagerly displayed before the females. Darwin states that twenty or more males of the _Tetrao cupido_ (species of grouse) will assemble at a particular spot, keeping up a tremendous chattering and strutting about. At the first response from a female the males take to fighting furiously, and the weaker are vanquished. Both the victors and the vanquished pay court to the female, so that the latter must make a choice or the battle is renewed. Here we have the combined action of selection through battle and by preferential mating.

Among mammals the males win the females much more through fighting than through the display of charms. In the breeding season the most timid male animals, which are not even supplied with any special weapons for fighting, engage in the most desperate conflicts. Two male hares have been seen to fight until one was killed. Those male mammals which are provided with special weapons for fighting enter into the fiercest and most deadly conflicts. The wild male elephant, during the period of love, is one of the fiercest fighters in the world. Lions engage in terrible battles, and a young lion dare not approach an old one.

Male seals fight most desperately during the breeding season, using both their claws and teeth. The conquerors appropriate the females and transmit their qualities to their offspring.

These are all cases of the survival of the fittest; of the survival of the males best adapted to the exigencies of their surroundings—in short, additional illustrations of Natural Selection.

=Insect Selection.= Flowers usually consist of several parts, such as the stem, the calyx composed of green sepals, the corolla formed of attractively colored petals, the stamens, the pistil, and finally of certain nectar-forming organs situated at the base of the last-named. The upper portions of the stamens are known as antheridia. The free extremity of each pistil is called a stigma, the intermediate stalk is the style, and the base is the ovary. The flowers constitute the sexual parts of the plant. The stamens are the male elements and the pistils are the female structures. In some species of plants the flowers possess the stamens but not the pistils,—they are male flowers; in others the flowers have pistils but not stamens,—they are female flowers. In still other plants the flowers possess both the male and the female structures, and are therefore bisexual or hermaphroditic flowers. The ovary of a pistil has a number of cells in it called ovules (female germ cells); the antheridia on the stamens have cells in them called pollen (male germ cells). When a pollen cell is carried by any means to the stigma of a pistil, it sends down through the latter a tubular prolongation by which the nucleus and protoplasm of the pollen cell unite with the nucleus and protoplasm of the ovule; so that we now have a fertilized egg,—the germ of a new plant. This fertilized ovule by repeated cell multiplication can grow into an adult plant.