21 



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 de- 

 fined, or by a mere proneness to wander, irregularly and 

 at large, from an assumed diagnostic type. In the fol- 

 lowing pages, the former of these resultant conditions 

 (namely, that in which " varieties," technically so called, 

 though more or less isolated in their character, are appa- 

 rent) 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 argu- 

 ment to prove that, where those local elements (whatso- 

 ever 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 coun- 

 tries and regions far removed from each other have an 

 a priori claim for stability, above those which circum- 

 stances less important than geographical ones, and 

 which are consequently more fluctuating in their com- 

 binations, may from time to time (as it were, accident- 

 ally) shape out. Having then examined our premises, 

 and prepared ourselves, with an unbiassed mind, for the 

 reception of phenomena which should be constant (and 

 in some instances, also, conspicuous) in proportion as 

 the conditions which unite in bringing them about are 



