A first book in organic evolution

by D. Kerfoot Shute

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.