INTRODUCTION OF FOREIGN SPECIES. 151 
subject, and that they 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 pulchella and Bulimus lubricus, as being of foreign 
origin, and as having been introduced : but tliis 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 inferred 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 avpersa, 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 
