23 



I. Climatal causes generally, whether dependent on 



latitude or altitude. 



Perhaps, judging superficially, climatal causes gene- 

 rally would appear to have more effect on insect deve- 

 lopment than any with which we are acquainted ; yet, 

 powerful as they unquestionably are, experience teaches 

 us that such is not the case. In combination with other 

 modifying principles, hereafter to be noticed, they may 

 be (and probably are) exceedingly important ; yet, when 

 taken singly and alone, we have no evidence to show 

 that their consequences are of such primary significance 

 as might be anticipated. Mr. Darwin, in describing the 

 fauna (which includes many mundane forms) of the 

 Galapagos Archipelago, situated immediately under the 

 equator, remarks : " The birds, plants, and insects have 

 a desert character, and are not more brilliantly coloured 

 than those from Patagonia ; we may therefore conclude, 

 that the usual gaudy colouring of the intertropical pro- 

 ductions is not related either to the heat or light of 

 those zones, but to some other cause, perhaps to the 

 conditions of existence being generally favourable to 

 life*." 



Although it is true, in a broad sense, that the nearer 

 we approach the Line the grander and more gorgeous 

 are the animate beings which tenant the surface of our 

 earth, there are at the same time so many exceptions to 

 this law, that it cannot be regarded as by any means 

 * Journal of Researches (London, 1852), p. 381. 



