Harmless creatures, no less than harmful, survive through favorable characters—some by the wit with which they elude the pursuer, some by flight, some by protective coloration; like the hare whose summer coat resembles the ground and whose winter garment rivals the whiteness of the snow, and the gorgeous butterfly, whose folded wings are a perfect simulation of the form and color of a dead leaf. Where the herbivorous food was insufficient for the mouths that would feed upon it, and where animals devoured one another, it was inevitable that the weak, the stupid, the ill-favored, the incompetent, should fail in the struggle for existence, and that the strong, the cunning, the efficient should survive and hand their superior qualities to their offspring. And this survival of the fittest by right of might that has been the supreme law of the animal world since life appeared upon this globe, was bound to result in the accumulation of favorable variations, in the consequent production of an ever increasing wealth of species, and in the development of ever higher forms of life. [Illustration: 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.