2 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



excepting in cases where specimens have been accidentally introduced 

 and remaining undisturbed have multiplied into a colony. In this re- 

 spect they present a remarkable contrast to some of the same animals in 

 Europe, which not only are very common in open and cultivated tracts, 

 but are particularly numerous in fields and gardens, where some of the 

 species commit much mischief, and in cellars, drains, and other similar 

 situations, in immediate contiguity with man. The species which have 

 been introduced from Europe, and naturalized in this country, are dis- 

 tinguished by the same habits as the stock from which they are derived, 

 and differ as much from the native species. Thus, Tachea hortensis 

 lives in open and exposed situations destitute of shelter, except that 

 afforded by grass and shrubs ; Zonites cellarius occupies gardens and 

 cellars ; Limax flavus inhabits cellars and damp places about drains ; 

 and L. agrestis is common everywhere in gardens, fields, cellars, nnd 

 houses. It infests the roadside, and the neighborhood of our dwellings, 

 and has in some places become the pest of the horticulturist. 



Whether this difference of habitat arises from original constitution, or 

 is the consequence of the long-continued operation of external causes, is 

 a curious subject of inquiry. The preference for the forest over the open 

 country exhibited by the native species, even in situations where both 

 have been for a long time equally accessible to them, seems to indicate 

 that the former supposition is correct ; and this opinion is strengthened 

 by the disappearance of nearly every species with the progress of agri- 

 culture. If their habits were not insuperable, they might be expected 

 to have been somewhat modified ere now, and to have become adapted 

 to the new physical conditions to which they are subjected. That they 

 have not been, suggests the thought that, like the aboriginal race of 

 men, and some of the larger quadrupeds, they are destined to give way 

 before the advance of civilization, and to have their places filled by for- 

 eign species. On the other hand, there are some facts which tend to 

 show that accidental causes may have produced a slow and gradual rev- 

 olution in the habits of the European species, corresponding with the 

 changes, which, within the historical period, have taken place over the 

 surface of the greater part of Europe ; and that in process of time the 

 same influences will produce similar results on the habits of the North 

 American species. All those parts of Europe which are now the most 



the most common species is Patula altcrnata, Zmiitrs arboreus, and Limax campestris. 

 Iu Savannah it is Triodopsisappressa ; in Norfolk, Va., it is Mesoclow. albolabris ; in Macon. 

 Ga., it is M. major. The original introduction was no doubt accidental. 



