Fig. 44.—The Genealogical Tree of Humanity.
Fig. 44.—The Genealogical Tree of Humanity.
The fitness of the present forms has been moulded by the death dealing teeth of the past.
Though not by this means alone. Another potent factor that has wrought tremendously for evolution has been the changing environment. We may briefly consider the character of its work. For millions of years before the close of the Carboniferous period, some ten million years ago, the whole earth enjoyed a perpetual summer. During the last million years or more of that vast era, while the dense Carboniferous forests, overwhelmed again and again by the growing soil, were forming the coal deposits of the earth, many kinds of amphibians and low reptiles wandered over the marshes and among the trees.
Then in the succeeding period—the Permian—the climate began to be colder. For a hundred thousand years or thereby the cold increased. At last the summit of an ice age was reached, and some four million square miles of the earth’s surface lay beneath a thick mantle of ice and snow. That ice sheet covered a continent that then extended from India to Australia, on the one hand, and to Africa on the other. The ice age, which was due, probably, to the gradual elevation of the land, wrought fearful havoc among the land animals, and with a ruthless hand pushed evolution forward. It compelled animals—and plants also—to change or die.
In the warm climate that preceded the ice age, all animals were cold-blooded,—warm blood was not needed—and amphibians and reptiles left their eggs on the earth to be incubated by the genial air. But in the awful cold that followed warm blood was required, and, as the cold destroyed their eggs, it was necessary for animals to develop means of wooing their young to life within their bodies. Another required change was that the cold reptile covering be abandoned for a warm coat of hair.
Only a few animals could make these adjustments. Accordingly, as calculations based upon the fossils prevalent before and after the ice age have shown, “thirty-nine out of every forty of all the species of animals and plants on the earth during the coal forest age were destroyed.” Of the animals that survived, some were saved by migrating to favorable regions, and some by adapting themselves to the rigors of the environment. These developed four-chambered hearts to keep their blood warm; they developed coats of hair or fur to retain the heat of their bodies; they made such changes in their internal economy as enabled the young to develop in the body of the mother.
To produce these changes, Nature preserved the favorable variations that appeared in different animals. From age to age these variations along advantageous lines were accumulated, and so, in time, the new creatures were evolved. The result was obtained through naturalselection of the fittest. In this manner arose the mammals, and, through a variant line of development, the birds; for birds, like mammals, have four-chambered hearts, and feathers, like hair, are but a modification of primitive scales; while the bird hatches her eggs with the warmth of her body. So we may thank that early ice age for the gift of all mammalian and bird life. The originals of those animals whose development culminated in man, and of our feathered friends of the air, were fashioned by that incomparable sculptor—environment.
With the gradual passing of the Permian ice age, through the lowering of the land, the long backward spring slowly merged into another universal summer that continued for several million years. In this mild period, when Europe was, for the most part, a group of scattered islands, when a continent extended from America to Scotland, a new garment of multiformed vegetation appeared upon the low-lying earth; huge sharks and reptiles dominated the sea; while on the land, weird and prodigious monsters basked in the brilliant sun. It was the age when the giant reptiles,—some with sail-like wings, and all with frightful teeth—and ponderous vegetarians held the earth their own and led in the struggle for existence.
Fig. 45.—Two Pigs.
Fig. 45.—Two Pigs.
Then came another cold period, less drastic than the former, but severe enough to give a signal forward urge to the work of evolution. The great reptiles disappeared forever. And now the primitive birds and mammals whose evolution had been retarded by their numerous enemies and by the climate in the warm reptilian age, entered upon a period of more rapid development. The bracing cold that required movement served as a stimulant to accelerate their evolution into various forms.
Since that remote time there have been several glacial periods—periods when great ice caps covered vast areas of the earth—periods whose tremendous cold was followed by ages when the climate was temperate and benign. These major changes in the environment, and the numberless other changes of less striking character, have been among the leading factors in the work of evolution.
Among the lesser, though important, environmental changes, may be mentioned the gradual submergence of land beneath the water, occasioning animal migrations which resulted in the mixing of stocks; the drying up of swampy regions bringing new plant life; changes in the food supply transforming animals into vegetarians or carnivores, or otherwise modifying their habits, and in consequence, their structure.
In these changes that have succeeded one another in the lapse of the innumerable ages, countless species of animals and plants have beenexterminated altogether; of others, only a few hardy or otherwise adapted specimens have survived; from these have been bred the animals and plants of the succeeding ages, which, in turn, have had to adapt themselves to their environment or disappear; and everywhere the struggle, whether with the environment or with other living things, has resulted in the survival of the fittest and the slow improvement of the fauna and the flora of the world.
The process of selecting the fittest to breed from, as it is carried on by Nature in her blind way, is called natural selection; and when men, in breeding stock, select the specimens possessing the most desirable variations to breed from, the process is named artificial selection.
The great variety of our domestic animals and plants we owe to artificial selection. Note, for example, the two pigs shown inFig. 45. The grizzly one, the wild boar, is the remote ancestor of the other, the modern, favorite domestic porker. By breeding for many generations from pigs possessing the most desirable details of form, the legs and snout of the animal have been shortened, his body has been made more shapely and greatly enlarged, and his commercial value enormously increased.
Fig. 46partially illustrates the diversity of dogs. The dog is a civilized wolf. And the pugs and poodles, the spaniels and setters, thehairless dogs and hounds, the terriers and mastiffs, the faithful collies, the noble St. Bernards, and all the other dogs of every size and form, have been developed by selecting parents with approved variations and breeding for desirable results along chosen lines.
In like manner, the barnyard fowls have been developed by selective breeding. The Leghorns and Minorcas, with their beautiful combs and wattles, the Golden Spangled and the Silver Spangled Crested Hamburgs, with their crowning hoods, the noble Plymouth Rocks, with their barred bodies, the magnificent Light Brahmas and Buff Cochins, arrayed with wing-like feathery growths to their very toes, the game fowls of the various varieties—those sleek, trim birds that would rather fight than eat—and the little Bantams of every sort, proud and beautiful in their bearing—all these are the children of the rude jungle fowls whose male birds still shriek a welcome to the morning sun from the wilds of India and the Malay Islands.
Likewise, the hundred and fifty varieties of our pigeons—the graceful Modena, the large Runt, the tall Carrier, the Trumpeter with his strange coo, his hooded head, his winged legs, the Frill-back with his feathers curling towards his head, the Pouter, with his tall, slender body, and throat inflated as though a feathered football were pinched between his beak and breast, the compact Fantail, leaning back with pride in his beautiful tail that rises above his head like a screen, the Tumbler, that from a lofty height tumbles with utter abandon through the air—all these pigeons, whatever their size, their form, their markings, their habits, have been developed during the last few centuries from the modest blue rock pigeons of the European coast (Fig. 47).
Fig. 46.—Dogs.Reading from the top downwards: St. Bernard, Bloodhound, Greyhound, English Setter, Irish Spaniel, Dalmatian, Pug, Skye Terrier.
Fig. 46.—Dogs.
Reading from the top downwards: St. Bernard, Bloodhound, Greyhound, English Setter, Irish Spaniel, Dalmatian, Pug, Skye Terrier.
Such is the story that rounds the vast circumference of life. All life is one, and by the same means its varied ends have been attained. By breeding from those creatures whose qualifications enabled them to survive in their environment, Nature, without knowing why, selected the fittest to become the parents of the creatures that were to follow. Thus, of necessity, the life of the world, ever branching into an increasing diversity of species, gradually improved in form, function and intelligence. And with millions of ages in which to try every experiment, to test every detail, to destroy her failures and seek success along other lines, Nature was bound to reach our day with an array of living creatures of finer development than those that perished in the struggles of the vanished years.
The laws in accordance with which the humbler creatures have been developed from still lower forms have presided over the evolution of man from the beast; and, never pausing in their action, these forces have slowly fashioned the glories of civilization from the dark crudity of savage thought. The story of man but continues in another form the story of life before man appeared.
Fig. 47.—Pigeons.Modena 10, Runt 19, Carrier 1, Trumpeter 4, Frill-back 37, Pouter 2, Fantail 6, Tumbler 15. Rock Doves in Centre.
Fig. 47.—Pigeons.
Modena 10, Runt 19, Carrier 1, Trumpeter 4, Frill-back 37, Pouter 2, Fantail 6, Tumbler 15. Rock Doves in Centre.
The plant creations of Luther Burbank are renowned throughout the world. The Burbank potato, the Burbank rose, the new walnut trees, the varieties of plums and prunes, the pineapple quince, the spineless cactus, the everlasting flower, the beautiful lilies—the many new species of vegetables, plants, fruits and flowers which Burbank has created by crossing inferior forms and breeding from the best resulting specimens—these experimental triumphs in the plant world are indubitable proofs of the truth of evolution. Mr. Burbank has shown that plant life is not fixed in form, but plastic and ever-changing, and that by breeding and selecting in accordance with Nature’s laws, man can cover the earth with new vegetables, fruits and flowers and vines and trees and grasses, more luxuriant and more wonderful than any found in an uncultivated condition.
Life, like clay, is malleable. Slowly, under the urge of Nature, rapidly in the hands of man, it shapes itself into new moulds. A problem in mathematics is known to be solved when it can be proved. In like manner, evolution is known to be certainly true, since, by following its laws, man now brings into the world new species of animals and plants that did not exist before.
A wonderful panorama is the story of evolution. From nebula to crusted earth, to life’s first spark in the primal sea, to the myriad forms that fought with teeth and claws in forest wilds, to the fierce-browed creature that earth first knew as human, to the warmth of love and the glow of thought in the heart and brain of finished man, the forces of Nature have shaped existence and crowned it with the power of the human brain. Man the builder, so long the victim of ignorance and fear, may now shape his destinies in a world fashioned to his choice. The desert has been made to blossom like the rose; cities of solid masonry have arisen from the noisome swamp; industries supplying a thousand human wants now occupy the sites where wild beasts once filled the jungle with their savage roars; the ocean, earth and air have yielded to the conquering power of thought; invention, art, discovery, have tapped exhaustless stores of wealth and culture; literature, the press, schools, disseminate intelligence; and, rising above earth, man now solves the secrets of the stars and calculates with precision their changing movements. Man, Nature’s gifted son, is Nature’s conqueror! Let him then learn the means by which to further improve his life. Let him equip his mind with truth. Let him solve his problems in the light of knowledge and enjoy a rational existence in a well ordered world.