We have already stated, that climate, when taken alone, does not appear to produce any very decided modifying effect on insect form, seeing that there are vast numbers of species of a wide geographical range which do not display, on their northern and southern limits, differences sufficiently constant to be regarded as purely climatal ones; and it is clear that, if climatal causes of themselves were of real primary significance, we should probably seldom fail to trace out, from their long-continued operation, some steady and positive result. Yet when combined with other principles, there is evidence that a considerable amount of influence must be conceded to the action of mere heat and cold, working 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 conspicuous 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 phaenomena 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.