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.
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 been exterminated 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 in [Fig. 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. 46] partially illustrates the diversity of dogs. The dog is a civilized wolf. And the pugs and poodles, the spaniels and setters, the hairless 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.