128 ACQUIRED CHARACTERS sec. 



expresses it, sets fire to the explosive substance, while the 

 nature and extent of the explosion caused depend on the 

 quality of the explosive material — this is proved by many 

 other facts. Were it not so, increased warmth would 

 necessarily in all butterflies change a given colour always in 

 the same way, always into the same other colour. Xhis is not 

 the case ; for while Polyommatus Phlaeas becomes black in the 

 south, Vanessa Urticae, which is likewise red, becomes blacker 

 in the north ; and many other examples well known to 

 entomoloE^ists can be adduced as evidence. On the other 

 hand, we find conversely that species of similar physical con- 

 stitution, that is to say, species nearly related, under the same 

 climatic conditions change in an analogous way." Of this 

 our white butterflies are mentioned as examples. "But 

 nothing can more strikingly prove that here everything 

 depends on physical constitution than the fact that in some 

 species the male individuals change differently to the female." 

 The females of Bryonise have undergone through climate a 

 greater change than the males,^ the converse is the case in 

 Phlaeas. 



" The external influences were exactly the same, but the 

 reaction of the organism was different. ... I bring this 

 specially forward because, in my opinion, Darwin ascribes 

 too great an influence to his sexual selection when he refers 

 to that alone the formation of secondary sexual differences. 

 The case of Bryonia^ teaches us that the latter can appear 

 from purely internal causes, and, until experiment has sup- 

 plied some fixed point as to the extent of the influence of 

 sexual selection, the view is justifiable that the sexual dimor- 



^ In answer to this I must point out that the warmth variety, P. Xapi, is the 

 newer, Bryonise the ancestral form. On my view of male preponderance the 

 female Bryoni^e mi;st be the farthest removed from Napi, and the male Napi the 

 farthest evolved. So it is true that the female of Bryoniae and the male of Napi 

 are at the greatest distance, but it is the latter, not the former, which has been 

 most changed by climate. 



