218 SEXUAL DIMORPHISM 



tected by bony plates, probably representing scales. Yet the 

 spines and armour are not described as confined to, or more 

 developed in the males. The fighting, however, appears cer- 

 tainly not to be confined to adult or breeding males, but to 

 be practised by both sexes at various ages. 



The building of the nest consists merely in gathering 

 together bits of weed and rubbish at the bottom of the 

 water, felting it together to some extent, and arranging it 

 into a round form with a hole in the top. This work the fish 

 performs with his snout, and it is not of a kind likely to lead 

 to any special modifications of structure. The courting of 

 the female consists in the male swimming in little jerks 

 about her, occasionally pushing her with his mouth, until 

 she follows him to the nest, swimming just above him 

 as he leads the way. When the eggs are deposited and 

 fertilised the male takes care of the nest, usually hovering 

 over it, so that the motion of his pectoral fins produces a cur- 

 rent of water over the eggs. In all this there is no definite 

 action or gesture which is exclusively confined to the male. 

 If it is true that the fighting habits are not confined to the 

 male, and that the arms and armour are equally developed in 

 both sexes, then it is not inconsistent with my views that the 

 only constant difference between the sexes is that of the 

 colour, which is chiefly developed in the breeding season, 

 and which is to be explained as in other cases already 

 discussed. On the other hand, Darwin quotes a state- 

 ment by one observer of sticklebacks that " the females 

 are quite pacific," and if this is correct it is difficult to 

 harmonise it with an equal structural development in the 

 sexes, unless it could be shown that the parts used for fight- 

 ing in the male are exercised for some other purpose in 

 the female. 



Of G-asterosteus pungitiits, which has nine or ten short 

 spines in front of the dorsal fin, Day found specimens in 



