INl'RODUCTION. 



65 



to possess two kinds of characters, one essential and 

 depending upon organization, the other contingent, and 

 modified bj the external cu-cumstances under the influ- 

 ence of which it exists ; and while the type of the 

 species, or the unity resulting from the totality of its 

 characters, remams constantly the same, nature permits 

 a great amount of variation in the subordinate par- 

 ticulars, which radiate, as it were, in every du'cction, 

 and at greater or less distance, from the specific cen- 

 tre. If we consider the physical condition of the ter- 

 restrial mollusks, we are prepared to find in them the 

 maximum development of such variations ; for, owing 

 to their limited locomotive powers, individuals are con- 

 fined to a very narrow range, and during their whole 

 lives, and often during a succession of generations, are 

 subjected to absolutely the same influences. Yet, in the 

 course of time, they have spread to gi-eat distances from 

 each other, and now occupy very different localities. 

 Hence, while one set of individuals is exposed to the con- 

 tinued action of one agent, other sets, occupying other 

 localities, may be acted upon by other and very different 

 ones, and in each locality a considerable but different 

 modification of the specific character may consequently 

 take place. We have only to suppose the existence 

 of a certain number of localities, each distinguished by 

 the presence of different causes, operating through a 

 period of some duration on the species occupying it, 

 and the result Avill almost necessarily be the production 

 of as many well-marked varieties. Such is in fact 



VOL. I. 8 



