With the evolution of sexual reproduction is introduced the opportunity for a great step forward, since an incomparably greater sphere of action is opened to the differentiation and variability of specific forms than was possible in the case of species reproduced asexually (Kerner von Marilaun, R. Martin). By means of the sexual union of two =differing= independent individuals, each of which, again, has been brought into the world by the sexual union of two differing individuals, the way is freely opened for a progressive differentiation of the individuals of this species. No one of them is exactly similar to any other. Each one exhibits new peculiarities, new capabilities, and all of these play their part in the struggle for existence. This gradually results in a progress towards higher, better, more perfect forms. The persistence of specific type, due to inheritance, is largely counteracted by sexual reproduction, inasmuch as the conjugation of reproductive cells derived from two different individuals induces a tendency to progressive variation and improvement. Moreover, by this sexual mode of reproduction the preservation of the species is rendered much more secure than by asexual reproduction, whilst at the same time the possibility of differentiation or variation is indubitably increased. We have already insisted on the fact that in the striking difference between the sperm cell of the male and the germ cell of the female we must seek for the ultimate cause of the profound difference between the sexes. Those who maintain the theory of the absolute identity of man and woman must continually be reminded of this fact. Unquestionably the greater motility of the male reproductive cell as compared with the more passive quality of the female cell implies the existence of deeply founded psychical differences; and the existence of these may be assumed with more confidence since we know from experience to what a high degree the finest psychical peculiarities of father and mother can be transmitted by inheritance to the child.