Part 8

It will be exceedingly instructive to trace briefly the progressive specialization of the horse family through the Tertiary period. A wonderful series representing this family has been found in the American Tertiaries. First of all, the =Eohippus= is found in the earliest Eocene rocks. This little animal was about the size of a fox and had four perfect toes (hoofed) and a fifth, much smaller, imperfect one (splint) on the fore-feet; the hind-feet had three perfect hoofed toes. Later on, in the middle Eocene rocks, was found the =Orohippus= (Fig. 14), with three toes behind (b), and four in front (a)—the fifth imperfect toe (splint) being lost. This animal was also about the size of a fox. Still later on were found, in the Miocene rocks, the =Mesohippus= (c, d) and the =Miohippus= (e, f), with three toes behind (d, f) and three toes and a splint in front (c, e). The splint in the Miohippus is much smaller than in the Mesohippus, and also in the former the two side toes have become smaller and farther removed from the ground. Both of these animals were about the size of sheep. Further on, in the early Pliocene rocks, appears the =Protohippus= (g, h), with three toes on all the feet, but the middle toe considerably larger and longer than the side toes. This animal was about the size of an ass. Then, finally, came the =Pliohippus= (k, l) and =Equus= (m, n) in the latest Pliocene rocks. Here the middle toe is greatly enlarged and the side toes are reduced to useless splints. Thus by degrees, through slight modifications not only of the feet, but also of the skull, teeth, brain, etc., and by adaptations from age to age, was formed the modern horse. In like manner, by gradual changes and adaptation to surroundings, all the early generalized mammals pass into the higher specialized ones.

=Quaternary Period.= The mammals of this age differ from those of the Tertiary period and from living species. Never before this period, and never since, have mammals been so large. They culminate in this period and then decline. The great mastodon, the great cave-bear, and saber-toothed tiger, the Irish elk, and gigantic sloths and armadillos lived in this age.

Most interesting of all, the remains of _man_ now first appear associated with these extinct animals. Man must have been at first an apparently insignificant creature among the mighty mammals that surrounded him, and must for a time have contended more or less doubtfully with them for mastery. But as he increased in numbers and intelligence he became more and more the ruler of the brute creation; so that this age in which he rules may fittingly be spoken of as the Psychozoic Era, or =Age of Man=.

[Illustration: FIG. 15.—A snake and two lizards; a, snake; c, Bipes (lizard); b, Cheirotes (lizard).

From Shaler’s First Book in Geology. By courtesy of the publishers, D. C. Heath & Co.]

Fig. 15 is introduced to show how easy may be the transitions from one order of animals to another. Two long, slender Lizards are shown and also a Snake. Bipes has four limbs, but they are very small and weak. Cheirotes has lost, probably through disuse, the hind-limbs and the front ones are small and weak, almost useless. The snake has no limbs at all. The two lizards are quite snake-like in external forms. Just as changing environment, use and disuse, etc., can effect transitions in external forms by slight gradations, so also may the whole internal structures undergo marked alterations by slight modifications through long ages.

The brief references we have made to the changing life-forms during the geologic ages, should be very instructive even to a reader not familiar with zoölogy. They teach the important lesson that life, in the main, has ever advanced to higher and higher levels. At one time in the history of the earth no animals were in existence higher than the Invertebrates; these lived through the ages with ever-changing forms. Later, in the course of geologic history, some of the primitive Invertebrates evolved into the lowest Vertebrates, and Fishes finally appeared, at first very primitive and generalized. For long ages there were no creatures on the globe higher than these. Then as the æons slowly rolled on some of the early fishes evolved finally into a higher group of animals, the Amphibians, and these for long ages were the highest creatures in existence. As time slowly passed, some of the primitive amphibians evolved into primitive Reptiles, and these creatures for long ages were the highest in existence and monarchs of the world. Some of the primitive reptiles branched off in one direction leading to modern Birds, and some advanced in another direction that led through primitive mammals to man and other modern mammals. And now in modern times man is monarch among the animals.

We may now begin to realize that truly with the changing and growing continents during the geologic ages, many life-forms have varied and advanced to higher levels, culminating in the final appearance of _man_ as well as other living creatures.

SECTION V.

NATURAL SELECTION.

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.

[Illustration: FIG. 16.—Wild Boar contrasted with a modern Domesticated Pig. Reproduced from Romanes’ “Darwin, and After Darwin.” By courtesy of The Open Court Publishing Company.]

=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.