When viewed geographically, there are two heads under which the insects of every individual area may be classed: namely, those which were created within its bounds, and which constitute its true aborigines (in the strictest sense); and, secondly, those which _have reached it_, either by ordinary migration over an intervening land, or by accidental introduction through human or other agencies. Now it is to the members of the _latter_ of these ideal divisions, that we principally look for any positive evidence, whilst discussing the causes of variation: since, by the nature of the case, we _must_ have identical, or at any rate closely allied species to reason upon before any sound conclusions can be drawn concerning them from the circumstances and conditions to which they are severally exposed; and it is clear, that the fact of creatures being specifically coincident, and yet under influences remote, does, for the most part, actually _imply_ a transportation of them (from their primeval centres) beyond the limits of a naturally acquired range. Moreover, the autochthones of the soil (if we may be excused the idiom) are in all instances adjusted to the peculiarities of the region in which they were formed; and, consequently, where they have not (as very frequently happens) diffused themselves to a sufficient distance from the birthplace of their kind to be acted upon in two opposite manners from without, the date _they_ supply, during our inquiry into specific modifications as dependent on external disturbing elements, cannot be very considerable. In spite of this severe distinction, however, which I would urge between the insect _aborigines_ of a country and _those which_ (whether by compulsion or not) _have colonized it_, and of the preference which (as just stated) must be given to the latter whilst investigating the controlling principles of aberration, I would not wish to reject _in toto_ the testimony which the former likewise may indirectly furnish,--especially under the present section, in which climatal causes on a large scale have to be taken into account. True it is that we cannot hope to descry _physical results_ amongst phaenomena which are due to the _creative_ force alone; yet we may, in the contemplation of them, recognize such an amount of _design_, or a primary adaptation to conditions from without, as shall afford, through its permanence and method, fresh presumptive evidence that the "conditions" _themselves_ may have some inherent modifying power of their own on the aggressors from other districts, in which a contrary influence may perchance prevail, and for the overspreading of which they were, in the beginning, more peculiarly constituted and ordained.