262 THE DESCENT OF MAN. 



tributed to their less vigorous condition at that time. All these 

 worm-like animals apparently stand too low in the scale for the 

 individuals of either sex to exert any choice in selecting a part- 

 ner, or for the individuals of the same sex to struggle together 

 in rivalry. 



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

 great class we first meet mth undoubted secondary sexual char- 

 acters, often developed in a remarkable 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 consisting 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 con- 

 sequently do not concern us. In various crustaceans, belonging 

 to distinct families, the anterior antennae are furnished with pe- 

 culiar thread-like bodies, which are believed to act as smelling- 

 organs, and these are much more numerous 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 smelling- 

 threads has probably been acquired through sexual selection, by 

 the better provided males having been the more successful in 

 finding partners and in producing offspring. Fritz Muller has 

 described a remarkable dimorphic species of Tanais, in which the 

 male is represented by two distinct forms, which never graduate 

 into each other. In the one form the male is furnished with 

 more numerous smelling-threads, and in the other form with 

 more powerful and more elongated chelffi or pincers, which serve 

 to hold the female. Fritz Muller suggests that these differences 

 between the two male forms of the same species may have origi- 

 nated in certain individuals having varied in the number of the 

 smelling-threads, whilst other individuals varied in the shape 

 and size of their chelae; so that of the former, those which were 

 best able to find the female, and of the latter, those which were 

 best able to hold her, have left the greatest number of progeny 

 to inherit their respective advantages.' 



In some of the lower crustaceans, the right anterior antenna 

 of the male differs greatly in structure from the left, the latter 



* 'Facts and Arguments for Darwin,' English translat. 1869, p. 20. 

 See the previous discussion on the olfactory threads. Sars has de- 

 scribed a somewhat analogous case (as quoted in 'Nature,' 1870, p. 455) 

 in a Norwegian crustacean, the Pontoporeia afflnis. 



