138 INTRODUCTION. 
immediate influence of large bodies of water, it would 
not have been unreasonable to have considered this 
influence, the only one apparently common to all of 
them, as the cause of the great multiplication of indi- 
viduals ; and hence to have inferred, that the proximity 
of the sea or of the Great Lakes was favorable to their 
increase. But, as other islands, seemingly subject to 
the same influences, are totally destitute of shells, and 
the main land in their immediate vicinity maintains them 
only in scanty numbers, we must necessarily seek some 
other cause adequate to explain the facts. According 
to our views, we find it hi the circumstance that either 
these islands are from their position inaccessible, or 
from their diminutive extent unworthy of attention, and 
hence not subjected to agriculture, and not inhabited by 
man and the domesticated animals which accompany 
him. The mollusks are therefore in undisturbed pos- 
session, and being free from the attacks of enemies, 
they multiply to such an extent as the climate and 
facility of obtaining food will permit ; and consequently, 
in process of time, become very abundant. These facts, 
then, do not corroborate the opinion held abroad as to 
the influence of the sea. 
Local causes. There are probably other causes of a 
local character not understood, but affecting the increase 
of individuals in particular circumscribed spots, pro- 
moting the increase of species and individuals in some 
localities, and restraining it in others. Such causes may 
exist in the intimate composition of the soil, or in the 
