Part 11
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.
There are two agencies by which the pollen is carried to the pistil; first, by the wind, and, secondly, by insects (or, occasionally, by humming birds). Flowers that are fertilized through the agency of insects are the most beautiful in existence, displaying all the varied hues and gorgeous patterns that are found in the organic world. On the other hand, the flowers which are fertilized through the agency of the wind are incomparably less beautiful than those of insect production, and they do not secrete sweet juices or nectar. The earliest flowers in geologic time consisted only of those essential portions, the stamens and pistils, and had no colored whorl of petals within another colored whorl of sepals. The poorly developed nectaries secreted only small quantities of honey. The food-seeking insects visited these primitive flowers for the pollen and nectar, even as they do now. The nectaries in the plants were so situated that the insects could not get at the honey without rubbing off some of the pollen and carrying it to other plants which it visited for honey. In this way the nectar-seeking insects would often carry pollen from the flower of one plant to the stigma of another plant’s flower, and thus secure cross-fertilization. Undoubtedly often the stigma of a plant’s flower was fertilized by pollen from the same flower; but it is known that seeds produced by cross-fertilization are much more vigorous and hardy than those produced by self-fertilization in the same flower; therefore, those plants that varied most in the direction best adapted for securing cross-fertilization would have decided advantages in the struggle for existence. Their fertilized eggs (seeds) would be most vigorous, and would most readily secure nutriment and withstand adverse circumstances. The variations of the primitive flowers most calculated to secure cross-fertilization would be those that made the flower more conspicuous by the appearance of bright colorations and delightful odors, such as would attract the attention and visits of nectar-seeking insects. These variations being transmitted, by heredity, to the descendants and accentuated as the ages passed would ultimately lead to all the wonderful adaptations of flower and insect structure to one another that are found in nature.
[Illustration: PLATE IX.—Two Primrose flowers (_Primula elatior_). A, with a long style; B, with a short one. Vertical sections have been made through the flowers.]
Natural Selection has acted in a double manner here, preserving those flowers that had the most delightful odors or the most conspicuous and enticing colorations and patterns; and at the same time selecting those insects that varied most in the direction of keenness of scent, acuteness of vision, and color perceptions. It will be instructive to briefly describe the fertilization of the primrose and an orchis by bees. The _primrose_ (Plate IX) has two sorts of flowers that are never found on the same plant; one has stamens high up the corolla tube and a short style with its stigma below the anthers; the other has the stamens far down the tube and a long style with the stigma above the anthers. In both cases the nectar is at the very bottom of the corolla tube. When a bee visits a short-styled flower, it extends its proboscis to the bottom of the tube and in withdrawing it brings away some pollen cells at its base. If it should next visit another short-styled primrose, it would not fertilize it because the pollen on the base of the proboscis could not reach the stigma; it would only gather still more pollen. But when it visits a long-styled primrose, it will necessarily deposit some pollen cells on the stigma, inasmuch as that is at the commencement of the corolla tube. If the bee should first visit a long-styled form of flower, it will carry off pollen on the tip of its proboscis, and when it visits a short-styled primrose will deposit the pollen on the stigma.
In the _orchis_ the stigma is placed below the anther. The stigma is in the front part of the flower and at the base of the lip, the latter being prolonged into a long tube full of nectar. The stigmatic surface is composed of very viscid matter. A bee when seeking the nectar pushes its head against the anther, and in so doing detaches the two sticky glands to which the club-shaped masses of pollen cells are attached (Plate X). It carries these away on the front part of its head. So long as the masses of pollen cells remain erect on the bee’s head, they cannot reach the stigma of any other orchis that it visits. As the sticky glands on the head contract, the pollen masses incline forward and become horizontal, so that they touch the next sticky stigma that is visited. The greater stickiness of the stigma detaches the pollen masses from the bee’s head, so that the flower is fertilized. It takes so long for the pollen masses on the bee’s head to assume the horizontal position that the insect has visited all the flowers on one orchis and then visits another plant. By this time the pollen masses are in the proper position (horizontal) to fertilize the flowers of another plant. In this way cross-fertilization is secured and the vigor of the plant maintained.
It may now perhaps be appreciated how intimate are the relations between the form and habits of insects and the structure and coloration of flowers. Our standards of beauty have largely been created for us through Insect and Sexual Selection. If insects had not been developed on the earth, plants would not be ornamented by beautiful flowers, but would show only such poor and inconspicuous ones as are found on grasses, on oak, ash, and fir trees. Grant Allen has beautifully written that, “while man has tilled only a few level plains, a few great river-valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread itself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. His buttercup, his dandelion, and his meadowsweet grow thick in every English field. His thyme clothes the hillside; his heather purples the bleak grey moorlands. High up among the Alpine heights his gentian spreads its lakes of blue; amid the snows of the Himalayas his rhododendrons gleam with crimson light. Even the wayside pond yields him the white crowfoot and the arrowhead, while the broad expanses of Brazilian streams are beautified by his gorgeous water lilies. The insect has turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers to his allurement.”
[Illustration: PLATE X.—Illustrating the fertilization of an orchis by an insect. A, represents a section of the flower and shows a bee standing on the flower’s lip with its head touching the sticky portion of the pollen masses; C, shows the pollen masses stuck to the bee’s head and erect; B, shows the pollen masses horizontal, the proper position to leave them detached from the bee by the sticky stigma.]
Natural Selection is the great agency that accounts not only for the color patterns and forms of living creatures, but also for the great majority, if not all, of the useful characteristics of organic creatures, including their internal organization.
One more illustration of the power of Natural Selection may be given, outside of the subject of color patterns. It may be interesting to readers who are not familiar with the elements of zoölogy to know that whales are not fishes but mammals. They belong to the same class that man does. The embryology of a whale reveals that it is descended from ancestors that were _land_ mammals, and that these mammals had a scanty covering of hair, teeth of different shape, broad tails like beavers, short fore and hind legs, and well developed sense organs. The olfactory organ was especially well developed. It is probable that these ancestors of the whale lived in marshy districts and were omnivorous, eating both plant and animal food. They sought their food in both shallow water and in swamps. As the conditions of life became more and more unpropitious on land, they were slowly modified through the ages under the action of Natural Selection into creatures somewhat like dolphins. At first they lived in fresh water, but finally they found their way into the sea and became the rulers of the ocean, from which the giant sea reptiles of earlier epochs had vanished. Hence are explained the adaptive changes of structure: the fore-limbs were modified into flippers enclosed in a fin-like sac, but retaining the bones corresponding to like structures in other _mammals_, as in the arm of man, the wing of the bat, and the fore-leg of the horse. Traces of the hind legs may be detected in a few species; the tail, which acted as a powerful swimming organ, became divided into two lobes; the head became fish-like in shape; the seven bones of the neck, common to most mammals, grew together; the skin became hairless; and the teeth, which appear in the young of the true whale, but are never cut, gave place to hanging fringes of whalebone, in the meshes of which the animal entangles the minute organisms it feeds upon.[16]
ISOLATION OF VARIETIES IN NATURE.
The following analysis of Isolation will be useful:
{ Preferential Mating. { Sexual. { Isolation { { Cross-Sterility. { Geographical.
It has been stated on an earlier page that the commingling of diverse hereditary units accomplished through the fertilization of an ovum by a spermatozooid is the source of many variations in the offspring. In this fertilized ovum the complexity of chemical substance, and, therefore, the complexity of inheritance, gives instability to the embryo, and thus produces variations in the offspring. In this embryo there is a struggle among the hereditary units,—a struggle among the various qualities inherited from both sides,—and a survival of the fittest, a veritable intra-cellular Natural Selection. It is a well-established law in biology that the union of germ cells of very closely related individuals, that is, of consanguineous individuals (in and in breeding) leads to less vigorous and variable offspring, and the parents are less prolific; while the commingling of diverse heritages by the union of germ cells from individuals belonging to strong but different varieties leads to vigorous and quite variable offspring. The union of such individuals is also most prolific. On the other hand the union of individuals belonging to very diverse varieties becomes less and less prolific until the cross-sterility of species is reached, although there are many exceptions to this rule of cross-sterility. The individuals of a species living in a state of nature are constantly varying. With every generation trivial variations take place in all directions and of all kinds. But these variations are all funded in the common stock, for the varieties freely mingle among one another and cross-breeding is constant not only between them, but with the parent stock. Of the variations that are constantly taking place some are advantageous to the creatures, some are disadvantageous, while many are neutral, being neither useful nor harmful. Natural Selection is ever alert, selecting the advantageous variations and eliminating the disadvantageous ones. The neutral variations are not touched by Natural Selection; among these intercrossing of varieties probably affords Nature an opportunity to make almost endless combinations, some of which might be useful to the animals, and others harmful, and in either case would come under the influence of Natural Selection. The commingling of diverse heritages due to the union of more or less pronounced varieties of the species in nature not only leads to a funding of varietal characteristics, but also increases the instability of the offspring, augmenting their plasticity, so that more numerous and diverse variations take place. According to Romanes the reproductive organs are among the most variable in the body. Of the numerous variations taking place in the individuals of a species under nature, some, therefore, affect the reproductive organs in such a way that certain of these individuals are cross-fertile with one another, but _cross-sterile with other varieties and with the parent stock_. This interesting and very important kind of variation is known to occur in some individuals of the human species. It is well known that a man and woman have been cross-sterile with one another, being unable to have children; yet when separating and mating with others they have both been cross-fertile, families being reared by both of them.
Variations are commencing species; isolated variations diverge more and more into distinct species. This fact, then, of the occurrence in nature of variations in some of the individuals of a species by which they are cross-fertile with one another, but cross-sterile with other varieties and with the parental stock, shows that Nature has a most effectual means by which varieties may be isolated from one another,—just as effective means as man, the breeder of varieties of domestic animals, possesses in isolating these domestic varieties by physical barriers, as fences, etc. Cross-sterility, therefore, in nature, is a most effective sexual barrier. The special form of it under consideration is what Romanes has called Physiological Selection. The varieties that are isolated by this sexual barrier have got to run the risks of in and in breeding, which Darwin has shown occur in domesticated animals, but which Wallace thinks are much less in a state of nature.
Another very important mode of Nature for isolating varieties is that which arises out of the instinctive preferences of animals. There is a tendency for like to breed with like where varieties are formed. The pale and dark colored herds of fallow deer in the Forest of Dean have never been known to interbreed. In the Falkland Islands all the cattle are known to have descended from the same stock. Here there are differently colored herds of cattle, and those cattle of the same color will interbreed with each other, but not with individuals outside their own color-caste, as Morgan expresses it. When two flocks of heath sheep and merino sheep are mixed together, they do not interbreed. This isolation of varieties by instinctive preferences for those individuals with like color patterns may be spoken of as _preferential mating_ through _recognition marks_. A very obvious mode of isolation in nature is by geographical barriers (including migration). In treating of environment we learned that during the geologic ages of the globe, the physical geography and climate have repeatedly changed. A very cold temperature, a mountain chain, a body of water, a stretch of desert land, may completely prevent interbreeding, on either side of the barrier, between the individuals of a species.
It should now be understood that Nature, like man, produces divergences from the parental stock, and isolates them by various effective agencies. Natural Selection ever carefully watches over these processes, eliminating the unfit variations and selecting the useful ones. Variation, Heredity, Environment, Isolation, and Natural Selection, having been acting and reacting through the ages, have produced, from a common parental stock, all the innumerable divergent and adaptive forms of living creatures that can be traced through the geologic strata to those on the surface of the earth to-day; have produced, therefore, _man_ also as the inflorescence of the topmost branch of the tree of life.
SECTION VI.
EVOLUTION OF MAN.
EVOLUTION OF MAN.
The detailed study of the development (ontogenetic and phylogenetic) of man is so vastly intricate and extensive a subject that it will be impossible in a work of this character to do more than refer to it in brief outline. One of the best and simplest methods to approach the study of such a subject is to acquire some idea of the _development of the frog_.
The fertilized frog’s egg, which is the starting point in the life of a new frog, is deposited in water and hatched by the warmth of spring. After fertilization the egg or cell divides into two cells, these two into four, the four into eight, the eight into sixteen, and so on till a large number of small cells, associated together, is formed (Fig. 10). These cell phases represent different stages in the life of the growing frog. It is thus seen that the frog, in its earliest stages, consists of nothing but many small cells. These cells, through the mysterious powers of heredity, are going to differentiate as development proceeds into the various organs (groups of various kinds of cells) that form the tadpole and finally the frog. For further stages in the development of the egg (oösperm) towards the tadpole, works upon Comparative Embryology should be consulted.