INTRODUCTION. 
65 
to possess two kinds of characters, one essential and 
depending upon organization, the other contingent, and 
modified by the external circumstances 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, remains constantly the same, nature permits 
a great amount of variation hi the subordinate par- 
ticulars, which radiate, as it were, in every direction, 
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 great distances from 
each other, and now occupy very different localities. 
Hence, whde one set of individuals is exposed to the con- 
tinued action of one agent, other sets, occupying other 
localities, may he 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 will almost necessarily be the production 
of as many well-marked varieties. Such is in fact 
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