98 



countries in which the creatures are placed may necessi- 

 tate their action. I will not recapitulate the evidence 

 which I have already adduced, proving that islands have 

 an especial capability of their own, either for increasing 

 or neutralizing, as it may happen, the powers of flight 

 (in which latter case, however, a compensation is usually 

 made for the loss) ; but I will point to the data which 

 are there brought together, in support of the hypothesis 

 for which I am now pleading, believing that they will 

 be found sufficient, on inquiry, to establish the doctrine 

 of alary mutability, so far at least as it is connected with 

 isolation as an element of control. If, however (irre- 

 spectively of its cause), the thing itself be recognized, the 

 principle is at once established; and we may reason 

 upon it as a matter of fact. So that, if we can ensure 

 this concession or acknowledgment, the occasional 

 proneness to variation of these thoracic appendages is, as 

 a law, admitted. The only questions which would then 

 appear immediately to suggest themselves, are : Under 

 what circumstances do they principally fluctuate ? and 

 why should it happen that organs which are apparently 

 so necessary as a medium of subsistence, should be 

 subject to inconstancy ? 



Both of these have, in reality, been already replied to 

 in the preceding chapter. Nevertheless, we may briefly 

 repeat, that, so far as the first is concerned, it is in 

 islands that we detect the maximum of instability to 

 which the wings of the Insecta are liable, and that it is 

 in seasons of extraordinary heat that their development 



