30 



is evidence that a considerable amount of influence must 

 be conceded to the action of mere heat and cold, work- 

 ing permanently and according to fixed laws, on the 

 members of the insect world. Such being the case, it is 

 perhaps not surprising that a slight difficulty should 

 arise, through our employment of separate sections under 

 which to examine the causes of variation ; for, since it is 

 ordinarily by the union of several disturbing influences 

 that aberrations are brought about, it is for the most 

 part impossible to refer the results, however con- 

 spicuous they may be, to a solitary controlling element. 

 And hence, though we may be able at times to point 

 out perchance the single reason for certain phenomena 

 with comparative precision, it will generally happen that 

 two or three agents must be appealed to before we can 

 arrive at a conclusion by any means satisfactory. I 

 would desire, therefore, that the examples hereafter to 

 be noticed may be judged of in the mass ; and may not 

 be considered as severally assigned, of necessity, to an 

 isolated deranging cause, through the fact of their being 

 placed, for the sake of convenience, and because of the 

 predominance which special controlling principles have 

 had in maturing them, under sections, both, as it were, 

 exclusive and particular. 



That climate of itself possesses but a limited modify- 

 ing power on insect development, is evident from the 

 consideration (just alluded to), that numerous species of 

 comparatively wide distribution are totally unaffected by 

 it. Thus, for instance, the Pissodes notatus, Fab., a weevil 



