264 



SEXUAL DIMORPHISM 



fications, and that such modifications are necessarily associated 

 with the sex or the period of life upon which alone the 

 particular conditions operate, the origin of the modifications 

 is explained, and all cases of sexual dimorphism are seen to 

 be special cases of the one general principle. 



Darwin refers to the extraordinary differences between 

 the sexes in the lower parasitic species of Crustacea, in which 

 the males are of small size and alone are furnished with 

 perfect swimming legs, antennae, and sense organs; the 

 females being destitute of these organs, with their bodies 

 often consisting of a mere distorted mass. Concerning these 

 he remarks that they are no doubt related to their widely 

 different habits of life, and consequently do not concern 

 the question of sexual selection. According to the views I 

 am here expressing the differences in the two cases are due 

 to causes of the same kind. Habits and actions of the 

 individual may be concerned in the quest for food, in the 

 escape from enemies, or in the intercourse of male and 

 female. The end achieved by the characters or organs 

 which are associated with certain habits or actions makes 

 no difference whatever in the explanation of the origin or 

 evolution of these characters and organs. The habits or 

 actions set up particular stimulations of particular parts of 

 the body. These stimulations produce certain reactions in 

 the growth and development of the parts. 



The following table of the subdivisions of the Crustacea 

 will help to explain the names used in this chapter : 



