Camels, dromedaries and cats have the end of the penis bent backward (this explains the tom-cat's manner of urination), the tip does not straighten itself or point forward save in erection. Not only the prong but the sheath of rodents points backward and ends near the anus, and in front of it. The penis is slender in ruminants, and in wild boar; thick and round in solipedes, elephant, lamentin (sea-cow, manatee); thick and conic in the dolphin, cylindrical in rodents and primates. The gland, which takes all intermediary forms between ball and point, has in the rhinoceros the shape of a gross fleur-de-lys. In the cats small spikes rise and point toward the base, and in agouti and gerboa there are holding flanges which grip the organs of the female.
The prong of many mammifers, a real member, is held up by an interior bone, formed at the cost of the conjunctive partition which separates the two hollow chambers. This penial bone is found in many quadrumanes, chimpanzees, orang-outangs, most carnivora, dogs, wolves, felines, martin, otter, badger, among rodents, beaver, seal, and cetaceous animals; it is lacking in ruminants, pachyderms, insectivora, toothless animals. In man one sometimes finds a trace of it in the form of a slender prismatic cartilage. In the enormous penis of the whale it resembles a bell-clapper. The penial bone diminishes the erectile capacity of the prong in stopping the development of the hollow chambers, but it assures the rigidity of the member, obtained in the other penial type by the inflow of blood which causes the swelling. Man ought to have the penial bone; he has lost it in the course of ages, and this is doubtless fortunate, for a permanent rigidity, or one too easily obtained would have increased, to madness, the salacity of his species. It is perhaps for this reason that great apes are rare, although they are strong and agile. This view would be confirmed if the penial cartilage were found regularly in very lustful men or with a certain frequency among human races most addicted to eroticism.
The penis is found in woman in the form of clitoris. This is almost as voluminous as a true penis in quadrumanes; it is atrophied in other species. It varies individually in women, certain of them being in this respect quadrumanes. Sometimes the clitoris is pierced for the passage of the urethra (certain apes and the mole); a slight trace of this meatus is seen at the head of the woman's clitoris. In species whose males possess a penial bone the female has often a clitoridian bone; nothing more clearly affirms the parallelism of these two organs, whereof one serves only for pleasure, after having been, perhaps in a long distant era, when man romped among marine invertebrates, a real instrument of fecundation. The greater labia, limiting the general orifice of the vulva, exist only in woman and, less markedly, in the female orang-outang. Circular in rodents, transversal in the unique case of the hyena, a heteroclite animal, the vulva is longitudinal in all other mammifers. Completely imperforate in the mole the vagina is more or less closed by a membrane, which the male penis tears in first encounter, in women, and several quadrumanes, certain small monkeys, the marmoset, certain carnivora, the bear, hyena, white-bellied seal, the daman (nailed); it is replaced in dog, cat, ruminants by an annular gripping between the vagina and the vestibule. The maidenhead is, therefore, not peculiar to human virgins, and there is no glory in a privilege which one shares with the marmoset.