The Evidence from Sexual Characters. 187 



fied descendants of an ancestral form which had, like 

 the cray-fish, a long free tail, which was used in swim- 

 ming. The fact that the young crab does have such 

 an abdomen, is one of the proofs of the correctness of 

 this view; but as the crab grows up, the abdomen be- 

 comes curled forwards under the body; it ceases to be 

 used as a swimming organ; its separate rings become 

 fused together, and its appendages become rudimen- 

 tary or disappear. This very instructive change goes 

 further in the male than it does in the female, for in 

 the latter, more of the rings remain distinct; a greater 

 number of appendages persist in the adult, and these 

 are much more like those of the young, or of the cray- 

 fish, than are those of the male. 



The great modification of 'the male as compared with 

 the female is well shown, among the Crustacea, by the 

 fact that there may be in the same species two different 

 male forms. This sexual dimorphism, as it is called, is 

 well shown in a Brazilian amphipod, OrcTiestia Dar- 

 winii, in which species there are two male forms which 

 differ from each other in the structure of their large 

 claws. These claws are used for holding the female, but 

 as both forms are now used for this purpose, either 

 shape would certainly have sufficed as well as the other, 

 and this case therefore differs greatly from that of the 

 social insects, where one form performs a certain duty 

 in the community, while another form is adapted to fill 

 a different place and perform a different duty. The 

 two male forms in Orchestia seem to be due simply to 

 the tendency of the male organism to become modified 

 more rapidly than the female, and not to any great ad- 

 vantage which has resulted from the divergent modifi- 

 cation. In discussing this case Darwin says that the 

 two male forms have originated by some having varied 



