274 DARWINISM chap. 



as types — the two sexes are nearly alike, the male being 

 sometimes more intensely coloured and with fewer pale 

 markings ; but in the American groups — represented by P. 

 seneas, P. sesostris, and allies — there is a wonderful diversity, 

 the males having a rich green or bluish patch on the fore wings, 

 while the females have a band or spots of pure white, not 

 always corresponding in position to the green spot of the 

 males. There are, however, transitional forms, by which a 

 complete series can be traced, from close similarity to great 

 diversity of colouring between the sexes ; and this may perhaps 

 be only an extreme example of the intenser colour and more 

 concentrated markings which are a very prevalent character- 

 istic of male butterflies. 



There are, in fact, many indications of a regular succession 

 of tints in which colour development has occurred in the 

 various groups of butterflies, from an original grayish or 

 brownish neutral tint. Thus in the " ^Eneas " group of 

 Papilios we have the patch on the upper wings yellowish in 

 P. triopas, olivaceous in P. bolivar, bronzy-gray with a white 

 spot in P. erlaces, more greenish and buff in P. iphidamas, 

 gradually changing to the fine blue of P. brissonius, and the 

 magnificent green of P. sesostris. In like manner, the intense 

 crimson spots of the lower wings can be traced step by step 

 from a yellow or buff tint, which is one of the most wide- 

 spread colours in the whole order. The greater purity and 

 intensity of colour seem to be usually associated with more 

 pointed wings, indicating greater vigour and more rapid flight. 



Sexual Selection as a supposed Cause of Colour 



Mr. Darwin, as is well known, imputed most of the 

 brilliant colours and varied patterns of butterflies' wings to 

 sexual selection — that is, to a constant preference, by female 

 butterflies, for the more brilliant males ; the colours thus 

 produced being sometimes transmitted to the males alone, 

 sometimes to both sexes. This view has always seemed to 

 me to be unsupported by evidence, while it is also quite 

 inadequate to account for the facts. The only direct evidence, 

 as set forth with his usual fairness by Mr. Darwin himself, is 

 opposed to his views. Several entomologists assured him 

 that, in moths, the females evince not the least choice of their 



