This secondary sexuality of mankind is, in correspondence with the differentiation of the various organs of his body, extremely complicated, and it is by no means solely dependent upon the structure of the special =reproductive= or =copulatory= organs; it is also intimately connected with other parts of the body, and more especially with the sense organs and the nervous system. Thus it has accommodated itself to all the external influences to which the species has been subjected in the long course of its development history. We may say that the =criterion, the characteristic mark of distinction between the human body and that of the lower animals, is also the distinctive differential characteristic between human sexuality and that of the lower animals=. And this criterion is the =brain=. The present physical and mental constitution of man is the result of an evolutionary process, of which the most marked characteristic has been a continually more rapid increase in the size and complexity of the brain. Phylogeny and ontogeny clearly demonstrate the evolution of the human body from lower states to higher, the slow but sure improvement in the direction of a continual enlargement and increasing convolution of the brain, which has by no means yet attained finality, but which may be expected to continue into the far-distant future; and associated with this physical development will undoubtedly proceed an equally extensive improvement in the quality of human consciousness. This progressive development of the brain has resulted in a retrogression and arrest of development of other parts and organs, and among these some more or less closely associated with the sexual functions, and originally of considerable importance. Gegenbaur, in his “Anatomy,” and Wiedersheim, in his interesting work on “The Structure of Man as Bearing Witness to his Past,” recognize in the unlimited plasticity of the human brain the sole cause of the arrest of development and retrogressive metamorphosis of many organs and functions which persist in other members of the animal kingdom.