SEXUAL SELECTION 351 



often consisting of a mere distorted mass. But these ex- 

 traordinary difierences between the two sexes are, no doubt, 

 related to their widely different habits of life, and, conse- 

 quently, do not concern us. In various crustaceans, be- 

 longing to distinct families, the anterior antennas are fur- 

 nished with peculiar thread-like bodies, which are believed 

 to act as smelling organs, and these are much more numer- 

 ous 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 pro- 

 vided males having been the more successful in finding 

 partners and in producing offspring. Fritz Muller has de- 

 scribed 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 fur- 

 nished with more numerous smelling-threads, and in the 

 other form with more powerful and more elongated chelse, 

 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 originated in certain individ- 

 uals having varied in the number of the smelling-threads, 

 while other individuals varied in the shape and size of their 

 chelffi; 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 resembling in its simple tapering joints the 

 antennse of the female. In the male the modified antenna 

 is either swollen in the middle or angularly bent, or con- 



8 "Pacts and Arguments for Darwin," Eng. translat., 1869, p. 20. See the 

 previous discussion on the olfactory threads. Sars has described a somewhat 

 analogous case (as quoted in "Nature." 1870, p. 455) in a Norwegian crusta- 

 cean, the Pontoporeia affinis. 



