The “aperture-problem” itself leads us, in the course of progressive development, to the “member-problem”--that is to say, to the problem of the more accurate apposition of the two reproductive apertures. The penis, by its introduction into the body of a member of the opposite sex, acts as a means for the shortening of distance-love; it serves for the fixation, for the clamping together, of the copulating pair, which in earlier stages of animal life was effected by sucking and biting; for example, in birds, who for the most part lack an actual penis, the cock holds the hen firmly with his beak during intercourse, and the sucking and biting which often occur in human beings in the sexual act persist as a reminiscence of these relations. In various vertebrates other means of fixation are employed: by the shape of fins, of arms, or of legs, a close “embrace” is rendered possible; finally, the evolution of a special member for sexual purposes closed the long series of means of ensuring union. Originally no more than a peg or a spine, in man the penis is first developed into the form of an absolutely free limb. Dogs, beasts of prey, rodents, bats, and apes, have a strong bone in the organ, the so-called “penis-bone.” In man this bone is lacking; the penis has become entirely free. W. Bölsche writes: “In relation to the large, heavy, massive trunk and thighs, the sharply individualized, independent, mobile penis appears as a kind of spiritualized central point; as it were, a finger or a small third hand to the trunk, appearing to the eye to stand in rhythmical relation with the hands, right and left.” In phylogenetic parallelism with the development of the penis, proceeds (from the marsupials upwards) the _descensus testiculorum_, the descent of the male reproductive glands, the testicles, until they attain their final position in the scrotum, beneath the penis. Here also we can recognize the principle of “limb-mobility,” mentally refined mobility. In the clitoris woman also possesses a rudiment of a primitive penis. By the apposition of the two limbs, a more complete and rapid conjunction of the reciprocal sexual products must have been effected. But the further development of the large sexual aperture of the female checked the progressive development of this primitive penis, made it to some extent superfluous, since now, by the adaptation of the male penis to the female sexual aperture, a sufficient internal fixation in the act of copulation was rendered possible. Thus the female penis came to subserve other purposes: a portion of it formed the labia minora; another portion, the upper, the clitoris, the name of which sufficiently indicates the fact that, like the penis of the male, its function is connected with the voluptuous sense.