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 applied to a very different purpose, as that change of the fish's air-bladder into a lung which accompanied the first development of land vertebrates. But still greater becomes the revolution when a certain process goes on Until it sets going a number of Other processes, unlocking series after series of causal agencies until a vast and complicated result is reached, such as could by no possibility have been foreseen. The creation of Man was one of these vast and complicated results due to the unlocking of various series of causal agencies; and it was the beginning of a deeper and mightier difference in kind than any that slowly evolving Nature had yet witnessed.