We are now, however, at all events, in a better position to define the contents of the sexual impulse. We see that there are certainly, as Moll has indicated, two constituents in that impulse; but, instead of being unrelated, or only distantly related, we see that they are really so intimately connected as to form two distinct stages in the same process: a first stage, in which—usually under the parallel influence of internal and external stimuli—images, desires, and ideals grow up within the mind, while the organism generally is charged with energy and the sexual apparatus congested with blood; and a second stage, in which the sexual apparatus is discharged amid profound sexual excitement, followed by deep organic relief. By the first process is constituted the tension which the second process relieves. It seems best to call the first impulse the process of tumescence; the second the process of detumescence. The first, taking on usually a more active form in the male, has the double object of bringing the male himself into the condition in which discharge becomes imperative, and at the same time arousing in the female a similar ardent state of emotional excitement and sexual turgescence. The second process has the object, directly, of discharging the tension thus produced and, indirectly, of effecting the act by which the race is propagated.
It seems to me that this is at present the most satisfactory way in which we can attempt to define the sexual impulse.
 C. Lloyd Morgan, “Instinct and Intelligence in Animals,” Nature, February 3, 1898.
 Essais, livre iii, ch. v.
 Fere, “La Predisposition dans l’etiologie des perversions sexuelles,” Revue de medecine, 1898. In his more recent work on the evolution and dissolution of the sexual instinct Fere perhaps slightly modified his position by stating that “the sexual appetite is, above all, a general need of the organism based on a sensation of fullness, a sort of need of evacuation,” L’Instinct sexuel, 1899, p. 6. Loewenfeld (Ueber die Sexuelle Konstitution, p. 30) gives a qualified acceptance to the excretory theory, as also Rohleder (Die Zeugung beim Menschen, p. 25).