To the creature thus produced through a change in the direction in which natural selection has worked, the earth and most of its living things have become gradually subordinated. In all the classes of the animal and vegetal worlds many ancient species have become extinct, and many modern species have come into being, through the unchecked working of natural selection, since Man became distinctively human. But in this respect a change has long been coming over the face of nature. The destinies of all other living things are more and more dependent upon the will of Man. It rests with him to determine, to a great degree, what plants and animals shall remain upon the earth and what shall be swept from its surface. By unconsciously imitating the selective processes of Nature, he long ago wrought many wild species into forms subservient to his needs. He has created new varieties of fruit and flower and cereal grass, and has reared new breeds of animals to aid him in the work of civilization; until at length he is beginning to acquire a mastery over mechanical and molecular and chemical forces which is doubtless destined in the future to achieve marvellous results whereof today we little dream. Natural selection itself will by and by occupy a subordinate place in comparison with selection by Man, whose appearance on the earth is thus seen more clearly than ever to have opened an entirely new chapter in the mysterious history of creation. IV. The Origin of Infancy. But before we can fully understand the exalted position which the Darwinian theory assigns to man, another point demands consideration. The natural selection of psychical peculiarities does not alone account for the origin of Man, or explain his most signal difference from all other animals. That difference is unquestionably a difference in kind, but in saying this one must guard against misunderstanding. Not only in the world of organic life, but throughout the known universe, the doctrine of evolution regards differences in kind as due to the gradual accumulation of differences in degree. To cite a very simple case, what differences of kind can be more striking than the differences between a nebula, a sun, a planet like the earth, and a planet like our moon? Yet these things are simply examples of cosmical matter at four different stages of cooling. The physical differences between steam, water, and ice afford a more familiar example. In the organic world the perpetual modification of structures that has been effected through natural selection exhibits countless instances of differences in kind which have risen from the accumulation of differences in degree. No one would hesitate to call a horse's hoof different in kind from a cat's paw; and yet the horse's lower leg and hoof are undoubtedly developed from a five-toed paw. The most signal differences in kind are wont to arise when organs originally developed for a certain purpose come to be