188 SEXUAL DIMORPHISM 



after the generative products have been once discharged 

 The increased size of the eyes in the mature male may be 

 due to the greater use made of the organs, the ripe male 

 perhaps ceasing to conceal himself and swimming about 

 actively in search of the female. 



A similar difference in size occurs in the Eel. The largest 

 male eel described did not exceed 19 inches in length, while 

 females, when adult, are from 27 to 39 inches long. Eels 

 only become sexually mature in the sea, and like the conger 

 die after breeding once. According to Grassi and Calan- 

 druccio, the discoverers of the Leptocephalus larva of the 

 eel, the eyes are larger in mature eels, both male and female y 

 than in the immature specimens which inhabit fresh water. 



Sub-order Anacanthini. Pleuronectidce. In this family 

 the males are invariably smaller than the females. This fact 

 can be demonstrated in two ways : (a) by ascertaining the 

 average size of a large number of males and a large number 

 of females caught on grounds frequented by the adults of both 

 sexes ; (b) by ascertaining the size of the smallest mature and 

 largest immature specimens of both sexes, and the proportion 

 of mature and immature specimens at intermediate sizes. Thus 

 in the northern part of the North Sea the smallest mature 

 female plaice were 13 in. long, the smallest mature males, 9 in. 

 The largest immature females were 18 in. long, the largest 

 immature males 16 in. According to Dr. Fulton the ratio of 

 the average length of female plaice to that of males was 114 

 to 100, of turbot 118 to 100. 



This difference of size is the opposite to that which occurs 

 frequently in mammals and birds, and cannot be attributed 

 to a general inferiority of size in the male sex. On the 

 other hand, it is difficult to discover a reason for the larger 

 size of female fishes, although the larger size of male 

 mammals and birds can be attributed to their combats, etc. 

 There is no reason to suppose that sexual selection takes 



