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
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