352 THE DESCENT OF MAN. 



resembling the wings of L. agestis. Lastly, in Z. arion 

 both sexes are of a blue color and are very like, though in 

 the female the edges of the wings are rather duskier with 

 the black spots plainer; and in a bright-blue Indian species 

 both sexes are still more alike. 



I have given the foregoing details in order to show, in 

 the first place, that when the sexes of butterflies differ the 

 male as a general rule is the more beautiful and departs 

 more from the usual type of coloring of the group to which 

 the species belongs. Hence in most groups the females of 

 the several species resemble each other much more closely 

 than do the males. In some cases, however, to which I 

 shall hereafter allude, the females are colored more splen- 

 didly than the males. In the second place, these details 

 have been given to bring clearly before the mind that 

 within the same genus the two sexes frequently present 

 every gradation from no difference in color to so great a 

 difference that it was long before the two were placed by 

 entomologists in the same genus. In the third place, we 

 have seen that when the sexes nearly resemble each other 

 this appears due either to the male having transferred his 

 colors to the female, or to the male having retained or per- 

 haps recovered the primordial colors of the group. It also 

 deserves notice that in those groups in which the sexes 

 differ the females usually somewhat resemble the males, 

 so that when the males are beautiful to an extraordinary 

 degree the females almost invariably exhibit some degree 

 of beauty. From the many cases of gradation in the 

 amount of difference between the sexes, and from the prev- 

 alence of the same general type of coloration throughout 

 the whole of the same group, we may conclude that the 

 causes have generally been the same which have determined 

 the brilliant coloring of the males alone of some species, 

 and of both sexes of other species. 



As so many gorgeous butterflies inhabit the tropics it 

 has often been supposed that they owe their colors to the 

 great heat and moisture of these zones; but Mr. Bates* has 

 shown by the comparison of various closely-allied groups of 

 insects from the temperate and tropical regions that this 

 view cannot be maintained; and the evidence becomes con- 

 clusive when brilliantly colored males and plain colored 



* "The Naturalist on the Amazons," vol. i, 1863, p. 19, 



