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 another argument for the belief in common descent. In man, for example, there exist over one hundred of these vestigial or rudimentary organs, as the vermiform appendix, the pineal gland, and the like. Many of these vestigal organs, which are now functionless in man, perform functions in lower animals, and this is held to show that at some remote period in the past they also functioned in man's ancestors. (3) The facts of embryology seem to point to the descent of the higher types of animals from the lower types. The embryo or fetus in its development seems to recapitulate the various stages through which the species has passed. Thus the human embryo at one stage of its development resembles the fish; at another stage, the embryo of a dog; and for a long time it is impossible to distinguish between the human embryo and that of one of the larger apes. These embryological facts, biologists say, indicate genetic relation between the various animal forms which the embryo in its different stages simulates.