Yet detumescence is the end and climax of the whole drama; it is an anatomico-physiological process, certainly, but one that inevitably touches psychology at every point.[73] It is, indeed, the very key to the process of tumescence, and unless we understand and realize very precisely what it is that happens during detumescence, our psychological analysis of the sexual impulse must remain vague and inadequate.

From the point of view we now occupy, a man and a woman are no longer two highly sensitive organisms vibrating, voluptuously it may indeed be, but vaguely and indefinitely, to all kinds of influences and with fluctuating impulses capable of being directed into any channel, even in the highest degree divergent from the proper ends of procreation.  They are now two genital organisms who exist to propagate the race, and whatever else they may be, they must be adequately constituted to effect the act by which the future of the race is ensured.  We have to consider what are the material conditions which ensure the most satisfactory and complete fulfillment of this act, and how those conditions may be correlated with other circumstances in the organism.  In thus approaching the subject we shall find that we have not really abandoned the study of the psychic aspects of sex.

The two most primary sexual organs are the testis and the ovary; it is the object of conjugation to bring into contact the sperm from the testis with the germ from the ovary.  There is no reason to suppose that the germ-cell and the sperm-cell are essentially different from each other.  Sexual conjugation thus remains a process which is radically the same as the non-sexual mode of propagation which preceded it.  The fusion of the nuclei of the two cells was regarded by Van Beneden, who in 1875 first accurately described it, as a process of conjugation comparable to that of the protozoa and the protophyta.  Boveri, who has further extended our knowledge of the process, considers that the spermatozoon removes an inhibitory influence preventing the commencement of development in the ovum; the spermatozoon replaces a portion of the ovum which has already undergone degeneration, so that the object of conjugation is chiefly to effect the union of the properties of two cells in one, sexual fertilization achieving a division of labor with reciprocal inhibition; the two cells have renounced their original faculty of separate development in order to attain a fusion of qualities and thus render possible that production of new forms and qualities which has involved the progress of the organized world.[74]