We have observed that, according to naturalists, the earliest forms of life which appeared on the earth were androgynous or hermaphrodite, that the two elements necessary for reproduction were originally confined within one and the same individual within which were carried on all the functions of reproduction. Later, however, a division of labour arose, and these two original functions became detached, after which time the reproductive processes were carried on only through the commingling of elements prepared by, or developed within, two separate and distinct individuals. As the belief is entertained by our guides in this matter that greater differentiation, or specialization of parts, denotes higher organization, it is believed that the division of labour by which the germ is prepared by one individual and the sperm by another individual, as is the case at the present time with all the higher orders of life, constitutes an important step in the line of progress. Here this line of argument ceases, and, until very recent times, concerning the course of development followed by each sex little has been heard. This silence on a subject of such vital importance to the student of biology is not perhaps difficult to understand; the conclusion, however, is unavoidable that the individual which must nourish and protect the germ, and by processes carried on within her own body provide nourishment for the young during its prenatal existence, and sometimes for years after birth, must have the more highly specialized organism, and must, therefore, represent the higher stage of development. Indeed, it is admitted by scientists that the advance from the egg-layers to the milk-givers indicates one of the most important steps in the entire line of development; and yet the peculiar specialization of structure necessary for its accomplishment was for the most part carried on within the female organism. Concerning the origin of sex in the individual organism little seems to be known; as a result, however, of observations on the development of the reproductive organs in the higher vertebrates, and especially in birds, it is believed that there exists a “strict parallelism between the individual and the racial history,”—that the three main stages in the development of the chick, viz.: (1) germi-parity, (2) hermaphroditism, and (3) differentiated unisexuality, correspond to the three great steps of historic evolution. By a careful investigation of the facts connected with the development of unisexual forms, we are enabled to discover the early beginnings of the characteristics which distinguish the two sexes throughout their entire course. We are told that with animals which have their sexes separate, in addition to strictly sexual difference