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 ; yet a hoof is made up of five claws grown together and furnished with a nail in common.