20 



THE EVOLUTION OF SEX. 



in many parasitic species, what have been well called " pigmy " 

 males illustrate the contrast in an almost ludicrous degree. 



Two cases from aberrant worm types exhibit very vividly 

 this same antithesis of size. Among the common rotifers, the 

 males are almost always very different from the females, and 

 much smaller. Sometimes they seem to have dwindled out of 

 existence altogether, for only the females are known. In other 

 cases, though present, they entirely fail to accomplish their 

 proper function of fertilisation, and, as parthenogenesis obtains, 

 are not only minute, but useless. In a curious green marine 



Relative sizes of a male and female Rotifer (^1 1 ydatina scuta). 

 — From Leuiiis. 



worm, Bonellia, the male remains like a remote ancestor of the 

 female. It lives parasitically on or within the latter, and is 

 microscopic in size, measuring in fact only al)out one hundredth 

 part of the length of its host and mate. Somewhat similar to 

 the case of bonellia is that of a viviparous coccus insect {Lecauiuni 

 hespe7'idujii)^ where the males are very degenerate, small, blind, 

 and wingless. In spite of this condition, we should indeed 

 think because of it, they are very male, for even the larvce, 

 while still within the mother, have been shown to contain fully- 

 develo})ed s])ermat()zoa. 



It would be unfair to argue from such an extreme case as 

 that of Bonellia alone, but there is no doubt that up to the 



