What would appear to be the essential of pleasure is unknown to them. The males do not know possession nor the females surrender, no touch, no rubbings, no caress. The object of male desire is not the female but the eggs, he watches for those she is about to lay, he searches for those she has laid, an excitement quite like those produced by onanism, or which are engendered by fetishism in certain distorted minds and which operate at the sight of a slipper or ribbon, and die down, even to frigidity in the presence of the woman herself. The fish spends his semen on eggs which he finds floating and whose mother he has never seen. Often both eggs and male milt are left floating and meet only in the chance of current and wave. Sometimes fish form a separate couple. The female swims up stream, stops over a grass or sand bottom, the male follows, obeying her gesture. Such habits have permitted people to breed fish with as great a certainty as they breed mushrooms, or more so. One takes a female swelled with eggs, squeezes her like an orange, then one empties a male of his milt, and nature takes charge of the rest. This procedure is not possible with certain species which act in concert, the male tilted onto his back, his genital orifice beneath that of the female, and ejaculating in time with her. One knows that salmon swim up rivers in troops, often very dense, and into the branch streams and creeks, to lay their spawn in quiet, favorable nooks. Then they go down stream worn out by the dams and waterfalls which they have mounted by tail-swishing, and tired by their genital exercises. The column is often led by a female, the other females follow. Then swim the old males and lastly the young males. When the leaderess has found a suitable place, one of the roes stops, hollows the sand with her belly, leaves a packet of eggs in the hole, an old male drenches them at once, but the patriarch has been followed by young bucks who imitate him and fecundate the same eggs. Thus, with these fish there is a sort of school where the experienced teach the newcomers the procedure of fecundation. This mixture of eggs and semen from fish of all ages should be very favourable to the maintenance of a specific type, if the instability of milieu did not bring about the encounter of elements belonging to different neighbouring varieties: despite the good will of naturalists, salmon and trout form practically only one family, and nothing is more difficult, for example, than to determine the specie of a young salmon, or to state the difference between a salmon and a sea trout.