This is only one of the many bearings which Darwin's theory has upon the
study of social problems; but it is evident even from this that it
revolutionizes sociology. So long as it was possible to look upon human
society as a distinct creation, as something isolated, by itself in
nature, it was possible to hold to intellectualistic views of the origin
of human institutions.

But some one may ask: Why should the sociologist accept Darwin's theory?
What proofs does it rest upon? What warrant has a student of sociology
for accepting a doctrine of such far-reaching consequences? The reply
is, that biologists, generally, during the last fifty years, after a
careful study of Darwin's arguments and after a careful examination of
all other evidence, have come substantially to agree with him. There is
no great biologist now living who does not accept the essentials of the
doctrine of descent. Five lines of proof may be offered in support of
Darwin's theories, and it may be well for us, as students of sociology,
briefly to review these.

(1) The homologies or similarities of structure of different animals.
There are very striking similarities of structure between all the higher
animals. Between the ape and man, for example, there are over one
hundred and fifty such anatomical homologies; that is, in the ape we
find bone for bone, and muscle for muscle, corresponding to the
structure of the human body. Even an animal so remotely related to man
as the cat has many more resemblances to man in anatomical structure
than dissimilarities. Now, the meaning of these anatomical homologies,
biologists say, is that these animals are genetically related, that is,
they had a common ancestry at some remote period in the past.

(2) The presence of vestigial organs in the higher animals supplies