Since the object to be attained by the sexual organs in the human species is identical with that which they subserve in their pre-human ancestors, it is not surprising to find that these structures have a clear resemblance to the corresponding structures in the apes, although on the whole there would appear to be in man a higher degree of sexual differentiation.  Thus the uterus of various species of semnopithecus seems to show a noteworthy correspondence with the same organ in woman.[76] The somewhat less degree of sexual differentiation is well shown in the gorilla; in the male the external organs are in the passive state covered by the wrinkled skin of the abdomen, while in the female, on the contrary, they are very apparent, and in sexual excitement the large clitoris and nymphae become markedly prominent.  The penis of the gorilla, however, more nearly resembles that of man, according to Hartmann, than does that of the other anthropoid apes, which diverge from the human type in this respect more than do the cynocephalic apes and some species of baboon.

From the psychological point of view we are less interested in the internal sexual organs, which are most fundamentally concerned with the production and reception of the sexual elements, than with the more external parts of the genital apparatus which serve as the instruments of sexual excitation, and the channels for the intromission and passage of the seminal fluid.  It is these only which can play any part at all in sexual selection; they are the only part of the sexual apparatus which can enter into the formation of either normal or abnormal erotic conceptions; they are the organs most prominently concerned with detumescence; they alone enter normally into the conscious process of sex at any time.  It seems desirable, therefore, to discuss them briefly at this point.

Our knowledge of the individual and racial variations of the external sexual organs is still extremely imperfect.  A few monographs and collections of data on isolated points may be found in more or less inaccessible publications.  As regards women, Ploss and Bartels have devoted a chapter to the sexual organs of women which extends to a hundred pages, but remains scanty and fragmentary. (Das Weib, vol. i, Chapter VI.) The most systematic series of observations have been made in the case of the various kinds of degenerates—­idiots, the insane, criminals, etc.—­but it would be obviously unsafe to rely too absolutely on such investigations for our knowledge of the sexual organs of the ordinary population.

There can be no doubt, however, that the external sexual organs in normal men and women exhibit a peculiarly wide range of variation.  This is indicated not only by the unsystematic results attained by experienced observers, but also by more systematic studies.  Thus Herman has shown by detailed measurements that there are great normal variations in the conformation of the