130 ACQUIRED CHARACTERS sec. 



of natural selection. He says that it is on tliese grounds that 

 Darwin sought to derive the markings of butterflies, not from 

 ordinary, but from sexual selection ; but that sexual selection 

 in the origin of the colours of butterflies can be dispensed with. 

 He then raises the question whether species of Lepidoptera, 

 in so far as they arise through changes of climate, only alter- 

 nate between two forms — a cold-form and a warmth-form — or 

 whether each new chanije of climate does not rather, if it be 

 pronounced enough to cause a modification, give rise to a new 

 form. In the previous pages I have contended against the 

 view wliicli AVeismann now liolds on this important question, 

 that tlie alternation of external influences must hinder the 

 modification of species, forgetting that Weismann himself has 

 formerly upheld my view at least in respect of the changes of 

 climate. He says : " I believe that the old forms are never pro- 

 duced again by alternation of climate, but always still newer 

 ones ; that therefore a periodically repeated change of climate 

 is by itself sufficient in tlie course of a long period of time to 

 produce a series of successive new species. . . . The climate, 

 when it has acted in the same manner on several genera- 

 tions one after another, will f^raduallv brines about an altera- 

 tion in the physical constitution of the species whicli 

 will make itself visible by an alteration in colour and 

 marking. Now, when this newly-acquired physical constitu- 

 tion of the species, which we will suppose has been firmly 

 established by inheritance through a long series of generations, 

 is subjected again to a permanent change of climate, this 

 latter, even if it be exactly the same as at tlie time of the 

 first form of the species, cannot possibly bring back that first 

 form. The nature of the external influence is the same, it is 

 true, but the constitution of tlie species is by no means the 

 same. Just as a white butterfly, as was shown previously, 

 exhibits quite other alterations than a blue or one of the 

 Satyridai under the influence of the same change of 



