GEOGRAPHICAL DISTRIBUTION. H7 
tide of population is already rolling, offer, in the great 
extent of their table-land and in the height to which they 
rise, a vast field of research to future naturalists, where 
they will be able to solve many of the most important 
questions connected with the geographical distribution 
of the terrestrial mollusks of our country ; and only a 
few years will elapse before these mountains will be 
accessible, with comparative ease, to those who may wish 
to explore them. 
Having thus noticed the most important agents which 
are supposed to influence the extension of these animals, 
it may be proper to give such general results respecting 
then- actual distribution, as seem to be justified by our 
present scanty knowledge, and to be in accordance with 
the preceding principles. And if, as before observed, 
but little confidence can be placed in such general infer- 
ences, they may nevertheless be useful by exciting 
further investigation, and may lead the way to more 
certain information. The facts hitherto observed in re- 
lation to species, indicate the existence of several distinct 
zoological divisions or sections of country, of which the 
topographical characters are in general well marked, and 
hi which, respectively, causes are seen to exist adapted, 
according to our views, to exercise the influences we have 
named. It is not pretended that the productions of each 
of these, even in the limited department of the terres- 
trial mollusks on which alone they are founded, differ en- 
tirely from those of the others ; but, while they are more 
or less blended in general, there are certain species which 
VOL. I. 31 
