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 m 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 wUl 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 aflectmg the increase 

 of individuals in particular circumscribed spots, pro- 

 moting the increase of species and individuals in some 

 localities, and restraming it in others. Such causes may 

 exist in the intimate composition of the soil, or in the 



