256 SEXUAL DIMORPHISM 



the habits of another species, Lethrus cephalotes, in which the 

 male and female cohabit in the same burrow, and the male, 

 which has enlarged mandibles, gives battle to all intruders. 

 We have therefore in this case, as in others, the evidence that 

 the special modification or hypertrophy of particular parts 

 in one sex is associated with special mechanical stimulation, 

 and we have no reason to believe that the male would have 

 been thus different from the female unless as a consequence 

 of this special stimulation. 



Conspicuous as the excrescences of male Coleoptera are 

 in themselves, they are even more extraordinary in their 

 variability within the same species. Secondary sexual 

 characters are often variable, but there are no other 

 animals in which such differences in the degree of their 

 development in the same species are constantly found as in 

 these two families of Coleoptera. In the Dynastides every 

 degree of development may occur, from males which are 

 almost similar to the females in everything except that they 

 are slightly larger, to males with an extreme development of 

 horns, and a size of body much greater than that of the 

 female. In the Lucanidse very often the degrees of develop- 

 ment fall into two or more distinct stages, intermediate 

 conditions being rare or wanting. In this family the 

 mandibles of the females are often of considerable size, and 

 those of the best developed males are not only much larger but 

 of different shape. In one species, for instance, the mandibles 

 in the females are fairly straight and parallel, while those of 

 the males are curved so as to meet only at the ends. Now 

 the least developed form of the males resembles the females 

 not only in the size but in the shape of the mandibles, while 

 the intermediate " forms " or states are of course intermediate. 

 Thus the males themselves are polymorphic. No theory 

 based on selection has succeeded in explaining this remark- 

 able state of things. Darwin considered that the great range 



