188 Heredity. 



in one manner, and some in another: both forms having 

 derived certain special but nearly equal advantages from 

 their differently shaped organs. 



Dr. Hagen has called attention to the fact that in 

 certain of our American species of cray-fishes, there are 

 two slightly different male forms, and Fritz Muller, 

 who pointed out the existence of the two male forms of 

 Orchestia, has also described a remarkable dimorphic 

 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 numer- 

 ous smelling threads, and in the other form with more 

 powerful and more elongated claws to hold the female. 

 Fritz Muller suggests that these differences between the 

 two male forms of the same species must have originat- 

 ed in certain individuals having varied in the number 

 of their smelling threads, while other individuals varied 

 in the shape and size of their claws, 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 when found, 

 have left the greatest number of progeny to inherit their 

 respective advantages. 



Whenever a number of species of a genus have any 

 part more developed in the male than it is in the female, 

 this part, as a rule, varies in the males of the different 

 species, and is therefore of great systematic importance, 

 since it furnishes diagnostic characters for distinguish- 

 ing the species from each other. This rule is of general 

 application, in all groups of animals with separate sexes, 

 and every one who is at all familiar with the systematic 

 zoology of our higher animals knows how difficult it is 

 to identify species without mature male specimens. 



The Crustacea furnish an abundant supply of illus- 

 trations of this law, luit we have space for onh T one. 



