It is perhaps unnecessary that the action of temporary heat and cold, of an unusual degree, should be considered under a separate head from that of climatal causes generally; nevertheless, since the latter are, in a certain sense, permanent in their operation, it may be thought desirable that I should offer a few words on the effect of sudden exceptions to the ordinary routine of things, such as, for instance, seasons of peculiar intensity. It does not however appear that any very important modifications do often occur from conditions thus abnormal, and as it were _accidentally_ brought about: on the contrary, indeed, it is a well-known fact, that the members of the insect world are singularly independent of such contingencies; and that, in the same manner as their times of maturation are neither hastened nor retarded by them, their external development is for the most part free from their control. Yet, in spite of this, specific results _are_ wont to happen, ever and anon, from such circumstances, as though it were a fundamental axiom, that every agent which Nature can press (regularly or irregularly) into her service should have, though it may not always exercise its privilege, some qualifying voice. I believe that almost the only deviation from the typical state, in insect form, which has been observed to originate, _par excellence_, from the occasional continuance of undue heat or cold, is curiously enough an organic one,--having reference to the enlargement of the wings. Every entomologist must be aware that a vast proportion of the Coleoptera (especially the _Carabidae_) are subject to great inconstancy in their metathoracic organs of flight. Many species, as the common _Calathus mollis_ of our own country (to which my attention has been more particularly drawn by the Rev. J. F. Dawson), have the hind wings at one time ample, at another rudimentary, and at a third nearly obsolete. Now, although other causes, hereafter to be noticed, would seem to have far greater power than climatal ones in _permanently_ regulating the size and capacity of these appendages; I think it will be found on examination (and I may add that Mr. Westwood is of the same opinion[14]), that the greater or less development of them may be frequently explained by the unusual severity of the seasons. My own researches would certainly tend to prove, that _heat_ does (in the main) favour, and _cold_ retard, their presence. Exceptions (often rendered intelligible from the evident working of counter influences) will of course arise in abundance to this hypothesis; yet my impression is that, upon a broad scale, it will stand the ordeal of a rigid inquiry.