INTRODUCTION OF FOREIGN SPECIES. I5I 



subject, and that thej might serve as points of compar- 

 ison for future observers. Among the species which are 

 common to this and to other countries, we shall mention 

 Helix pulcliella and Bulimus lubricus, as being of foreign 

 origin, and as having been introduced ; but this we do, 

 rather in deference to received opinion, than because we 

 are satisfied of the fact ; for, in truth, so generally are 

 they diffused over the country, that we are inclined to 

 believe, as may be mferred from our remarks on a for- 

 mer page, that they are as truly native species as many 

 others which have never been considered to be other- 

 wise. 



The process of introducing foreign species is constantly 

 going on. The animals sometimes, but more commonly 

 their eggs, are transported in the soil of boxes of plants, 

 or about the roots of trees and shrubs imported for hor- 

 ticultural purposes, or in the cavities of woods used in 

 the arts. Another common mode of introduction is 

 among the ballast of vessels, collected on the shores of 

 foreign countries, and often discharged on our own. 

 Some of the larger European species, and particularly 

 Helix aspersa, are sometimes shipped to this country as 

 an article of commerce, and are used for food by foreign 

 residents. Others are brought as objects of curiosity, 

 and are preserved in gardens, and conservatories. Indi- 

 viduals introduced in some of these modes, escaping or 

 preserved by accident, are thus enabled to colonize the 

 species in new localities, where, if favored by propitious 

 circumstances, they obtain a permanent footing, and 



