25 



When viewed geographically, there are two heads 

 under which the insects of every individual area may be 



x 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 varia- 

 tion : 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 con- 

 cerning 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 avro^Oove^ 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 which they supply, during our inquiry into 

 specific modifications as dependent on external disturbing 

 elements, cannot be very considerable. 



c 



