142 INTRODUCTION. 



striking example among them occurs in Helix 2mlc7ieUa. 

 This diminutive species is spread throughout the con- 

 tment of Europe ; it is common in the north of Africa, 

 and in some parts of the south of Asia ; * it is found in 

 Cuba, and others of the West Indian islands ; and is 

 abundant in nearly every part of the United States and 

 Canada. The various positions which it is thus found 

 to occupy, and the dissimilar circumstances in which it 

 exists, together with the difference of chmate and other 

 physical agents to which it is subjected, and the vast dis- 

 tances both of land and ocean which intervene between 

 these localities, render it doubtful whether its general 

 dispersion is not due to other causes than those which 

 have been named, and whether, indeed, it can be ex- 

 plained on any acknowledged principles. Its condition 

 in this country tends to increase these doubts. Its occur- 

 rence might, of course, be expected, in any country 

 which has been closely connected with Europe by com- 

 merce, but this would be no reason for meeting it in the 

 interior of North America, far removed from the settle- 

 ments of white men, and in places still inhabited by the 

 aboriginal races, and only occasionally visited by the 

 wandering hunter. Yet, in 1820, on the arrival of 

 Major Long's explormg expedition at Council Bluffs, on 

 the Missouri River, five hundred miles above its embou- 



1 Specimens of Helix pulchella were brought to tliis country by Mr. 

 Lyell, which were obtained by Lieut. Lyell, in the neighborhood of Can- 

 dahar. They difler in no respect from common European and American 

 specimens. 



