328 SEXUAL SELECTIOX. Part II. 



Sub-kingdom of the Arthropoda: Class, Crustacea. — 

 In this great class we first meet with undoubted se- 

 condary sexual characters, often developed in a remark- 

 able manner. Unfortunately the habits of crustaceans 

 are very imperfectly known, and we cannot explain the 

 uses of many structures peculiar to one sex. With 

 the lower parasitic species the males are of small size, 

 and they alone are furnished with perfect swimming- 

 legs, antennae and sense-organs ; the females being 

 destitute of these organs, with their bodies often consist- 

 ing of a mere distorted mass. But these extraordinary 

 differences between the two sexes are no doubt related 

 to their widely different habits of life, and consequently 

 clo not concern us. In various crustaceans, belonging 

 to distinct families, the anterior antennas are furnished 

 with peculiar thread-like bodies, which are believed to 

 act as smelling-organs, and these are much more nume- 

 rous in the males than in the females. As the males, 

 without any unusual development of their olfactory 

 organs, would almost certainly be able sooner or later 

 to find the females, the increased number of the smell- 

 ing-threads has probably been acquired through sexual 

 selection, by the better provided males having been the 

 most successful in finding partners and in leaving off- 

 spring. Fritz Miiller has described a remarkable dimor- 

 phic species of Tanais, in which the male is represented 

 by two distinct forms, never graduating into each other. 

 In the one form the male is furnished with more 

 numerous smel ling-threads, and in the other form with 

 more powerful and more elongated chelae or pincers 

 which serve to hold the female. Fritz Miiller suo-gests 

 that these differences between the two male forms of the 

 same species must have originated in certain individuals 

 having varied in the number of the smelling-threads, 

 whilst other individuals varied in the shape and size of 



