PHENOMENA OF SEX 313 



the females the genital aperture is covered by a large scale, called 

 a foricula or little shutter, which is attached on one side and 

 open on the other, the opening being to the right in some 

 specimens, to the left in others. Garman found in a number of 

 Anableps examined the majority of the males were rights and 

 of the females, lefts; the number of lefts was 35 per cent, in 

 the males, 62 per cent, in the females. We have here, therefore, 

 a case of asymmetry closely similar to that of the flat-fishes : 

 in the one case the asymmetry is related to different relations 

 of the two sides of the body to the external world, in the other 

 to reciprocal relations between the sexes. In the flounder, as 

 in the Cyprinodonts, we find lefts and rights in the same 

 species. 



In many fishes, as already mentioned, the males are smaller 

 than the females. This is the case where the male guards the 

 spawn or the nest as in the Lump-sucker so common on the 

 coast of Scotland, where the two sexes are known as cock- 

 paidle and hen-paidle. In the flat-fishes, e.g. sole and plaice, 

 the males are considerably smaller, and in plaice evidence has 

 been recently obtained to show that this is partly due to the 

 fact that the females live to a greater age. The maximum dif- 

 ference seems to be reached in the eel family as more fully de- 

 scribed below in the life-history of these fishes. The present 

 writer does not believe that these sexual differences are ex- 

 plained by sexual selection ; he has pointed out that the process 

 of selection or survival cannot account for the limitation of the 

 characters to one sex, to the adult age, and often to one period 

 of the year, namely the spawning season, while on the other 

 hand the characters always correspond to external irritations 

 which occur only under the same limitations as the characters. 

 The reasonable conclusion is therefore that the characters have 

 been caused by these irritations, arising from the actions of the 

 males under sexual excitement. The smaller size of the males 

 which guard the spawn or the nest may be explained as due to 

 the more sedentary habits required by such duties, but the 

 cause in other cases such as that of the eels is not so obvious. 



