here is complete agreement among
investigators as to its details. When some irregularity or arrest of
development occurs in the process we have one or other of the numerous
malformations which may affect this region. If the arrest occurs at a very
early stage we may even find a condition of things which seems to
approximate to that which normally exists in the adult reptilia.[75] Owing
to the fact that both male and female organs develop from more primitive
structures which were sexually undifferentiated, a fundamental analogy in
the sexual organs of the sexes always remains; the developed organs of one
sex exist as rudiments in the other sex; the testicles correspond to the
ovaries; the female clitoris is the homologue of the male penis; the
scrotum of one sex is the labia majora in the other sex, and so
throughout, although it is not always possible at present to be quite
certain in regard to these homologics.

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 p