Mate-hunger among the fishes seems generally to find peaceable modes of expression, either in “display” or in consorting in vast shoals, though, so far, the factors which govern their conduct in this matter are as yet unknown. But here, as with the higher vertebrates, there are some species which adopt more violent methods. A good illustration of such conduct is furnished by the Salmon, which, during the period of sexual activity, develops a curious modification of the lower jaw, which is produced forwards and upwards to form a hook-shaped projection of fibrous tissue. When the mouth is closed this hook is received into a cavity formed within the fore-part of the roof of the mouth. It has been described as a weapon of offence. But this it can hardly be. On the other hand it has been suggested that it serves to protect the jaws when charging a rival, for the shock on such occasions is considerable. It answers, in short, like to the fibrous mass of tissue which protects the fore-part of the head in Whales like the Black Whale (_Globicephalus_) and the Bottle-nose Whale (_Hyperoodon_), serving as a battering-ram. In the Pacific Salmon (_Onchorhynchus_) both jaws are hooked, so that when the mouth is closed the hooks cross one another as in the beak of the Crossbill. In this Salmon, too, the front teeth attain a considerable length, while the body becomes laterally compressed and a hump forms at the shoulders. Little, however, seems to be known as to the nature of their battles. The combats of the Salmon of our own islands, however, are evidently severe, and this has long been known, for Darwin speaks of as many as three hundred, all with one exception males, being found dead in the Tyne during the month of June, killed by fighting. Such battles are fought, it is to be noticed, not so much for the possession of females—for it is a polygamous fish—as for the privilege of fertilizing the eggs as they are shed. The absence of a “display” here is a noticeable feature, and it is on this account, probably, that the reproductive period is not associated with the appearance of any form of resplendent livery. On the contrary, the marvellous silvery sheen which adorned both sexes on their arrival at the spawning ground from the sea has entirely vanished by the time that the consummation of the journey has been attained, and in its place is naught but a slimy, dingy copper-coloured hue. But no sooner has the reproductive period passed than the silver lustre makes its appearance once more.