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 natural selection 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. [Illustration: 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.