In man and other placentaires, the forked prong is a teratological fact only encountered in incomplete double monsters. It is, on the contrary, the most general form among marsupials. A double vagina corresponds to this penis, double at least from the gland, thus in kangaroo and opossum. This original biparity is found regularly in the uterus of certain placentaires, hares, rats, bats, carnivora. The uterus of marsupials is simple without narrowing of the throat. One knows that their young stay there but a short time, that they are born not as foetus but as germs, and complete their development in the marsupial pouch. An opossum, destined to attain about the size of a cat, is at birth about bean-size. These animals, therefore, differ profoundly from other mammifera. Some reptiles, like crocodiles and most chelonians, have only a simple prong; some tortoises have a forked tip to the penis, it is many-branched in the trionix, carnivorous tortoise rightly called ferocious. The saurians and ophidians can deploy outside the cloaca two erectile prongs; in saurians, lizards, they are short, round and bristle with prickles. The females have no clitoris save when the male has a single prong; at least the clitoris is only well constituted in crocodilians and chelonians. Copulation is unknown to batrachians, whose contact is nevertheless very dose; it is unknown to most fish, whose amours are without even contact. Certain selacians however (dogfish, skates), and perhaps also one or two teleostians (bony fish), and the lamprey, have a copulating organ which really enters the organ of the female. The birds which have a penis or an erectile and retractile tubercle which serves, are the ostrich, the cassowary, the duck, the swan, the goose, the bustard, the mandou and certain neighbouring species; their hens have a ditoridian organ. The ostrich has a true prong, five or six inches in length, cut by a groove which serves as conduit for the seminal liquor; it is enormous in erection and tongue-shaped. The ostrich hen has a clitoris and coition occurs exactly as among mammals. The swan and duck are also very well provided with an erectile tubercle suited for copulation, and this explains at once the story of Leda, the libidinous reputation of the duck, and his exploits in the barn-yards, veritable abbeys of Thélème. One can not here describe the copulative organs of arthropodes, comprising insects properly so-called. Enough to note that, however varied their forms, they behave very much as those of superior mammifers and are composed of two essential parts, the penis, sheathed in a penial scabbard, and the vagina, prolonged by the copulative pouch which receives the penis. Fish and birds, lacking external apparatus are reduced to methods which will be later examined. Hermaphrodite mollusks, with a marvellously complicated sexual apparatus, ought also to be studied separately. Finally, the amorous habits of insects form a series of illustrative chapters.