In the last chapter we offered a few passing remarks on insect-aberration generally, whether regarded as a _universal fact_ (which, however, even supposing such to be true, it is not the object of the present treatise to substantiate), or as an _occasional_ one,--that is to say, as existing at all times to that extent (as an hereditary principle), that it is _liable_ to manifest itself, or not, according as external agencies may favour or oppose its occurrence. In the latter case, which alone I propose to consider, this inherent tendency may be displayed, either through the expression of "varieties" well defined, or by a mere proneness to wander, irregularly and at large, from an assumed diagnostic type. In the following pages, the _former_ of these resultant conditions (namely, that in which "varieties," technically so called, though _more or less_ isolated in their character, are apparent) will be especially discussed; since my principal desire is, to point out the influence of _local disturbing causes_ in regulating, to a greater or less extent, though of course within certain specific limits, the outward contour of the insect tribes,--and it requires no argument to prove that, where those local elements (whatsoever they may be) prevail, the _same_ effects will, for the most part (in the same species), be produced; and that, therefore, modifications which are characteristic of countries and regions far removed from each other have an _à priori_ claim for stability, above those which circumstances less important than geographical ones, and which are consequently more fluctuating in their combinations, may from time to time (as it were, accidentally) shape out. Having then examined our premises, and prepared ourselves, with an unbiassed mind, for the reception of phænomena which should be constant (and in some instances, also, conspicuous) _in proportion as_ the conditions which unite in bringing them about are significant; let us advert to a few of the more prominent cases in which our instinct would seem to warrant the belief that aberrations are to be usually anticipated. And since it will hardly be denied that, like the representatives of other departments of the animate world, insects _may_, in their outward configuration and development, be in some measure under the control of the external influences to which they are immediately exposed, we will take a rapid glance at a few of the circumstances and conditions which are known to have more or less of a qualifying effect on the members of large and opposite sections of the organic creation; and then see how far we are enabled, by means of facts, to trace out results for the Insecta, corresponding to those which are admitted to obtain in the other groups. And, since the existence of analogous results infers, to a certain extent, the similarity of the agents which have brought them about, our "causes of variation" (provided the effects can be shown) may be in reality almost demonstrated.