In analyzing the sexual impulse we have seen that the process whereby the conjunction of the sexes is achieved falls naturally into two phases: the first phase, of tumescence, during which force is generated in the organism, and the second phase, of detumescence, in which that force is discharged during conjugation.[72] Hitherto we have been occupied mainly with the first phase, that of tumescence, and with its associated psychic phenomena. It was inevitable that this should be so, for it is during the slow process of tumescence that sexual selection is decided, the crystallizations of love elaborated, and, to a large extent, the individual erotic symbols determined. But we can by no means altogether pass over the final phase of detumescence. Its consideration, it is true, brings us directly into the field of anatomy and physiology; while tumescence is largely under control of the will, when the moment of detumescence arrives the reins slip from the control of the will; the more fundamental and uncontrollable impulses of the organism gallop on unchecked; the chariot of Phaëthon dashes blindly down into a sea of emotion. 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.