Germinal Variations—Fishes and mate—hunting—Some remarkable Sexual differences displayed by the Teeth of Rays—The Double-eyed Fish—The Coloration of the Dragonet—Some curious facts about Salmon—The strange use of the kidneys in the Stickle-back—The Stickle-back and parental duties—Siamese Fighting-fish. Mammals, Birds, Reptiles and Amphibia, as has already been shown, all exhibit practically the same line of conduct in regard to their mate-hunting instincts; all use like modes of expression. And this is a very significant fact. It becomes more so when we turn to the fishes, for here again we meet with the same behaviour, and here again we meet with the same rules in “secondary sexual characters.” An instance or two of the latter distinction between the sexes should suffice. As a rule, among fishes, the males are smaller than the females: commonly there is no other external distinguishing feature between them. In many cases, however, the males are more or less strikingly different, thereby showing a departure in the nature of a higher degree of complexity, or “specialization,” just as obtains among the birds. And the same sequence to this also obtains. That is to say, as has already been remarked in the case of the Mammals and the Birds, the new features first appear in the males, leaving the females and young of both sexes unmodified. A singular illustration of this is afforded by some of the Rays, or Skates, as they are often called. In the Thorn-backed Ray (_Raia clavata_) the teeth of the adult male are sharp-pointed and directed backwards, while those of the female are broad and flat, forming a sort of mosaic or pavement. The young male agrees with the female in this respect. In the Common Blue Skate (_Raia batis_) the teeth are pointed in both sexes, though more so in the adult male. In the Spotted Skate (_Raia maculata_) the teeth are fully pointed in both sexes. Here, then, the normal course in the evolution of new characters is followed, but it is remarkable that the teeth, which are so intimately related to the capture of food, should be thus affected. Whether the change of teeth is associated with a change of food, or whether neither pointed nor pavement teeth affect the feeding, is unknown.