LOVE-MAKING AMONG FISHES 179 



have attained their full stature. With the Salmon, for 

 instance, ripe spermatozoa have been found in individuals 

 of not more than a few inches in length, and in this 

 species also the male is the smaller. Ova take longer 

 to attain maturity, for in addition to the germ-plasm 

 they must be provided with a more or less extensive 

 amount of food-material in the shape of yolk. The forma- 

 tion of this is inhibited until the demands on the system 

 for the building up of the body have begun at least to 

 lessen. 



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 

 forepart 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 j aws 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 (Globi- 

 cephalus) 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 



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