478 On the Creation, Diffusion, and 



with species peculiar to the hills, such as " Helicarion cas- 

 sida," (Nob.) and " Pupa pulchella," (Nob.) neither of which 

 are found in the plains, while the former are not met with 

 in the higher and colder hills. To this region also " Pupa 

 pulchella" would seem more properly to belong. 



Here then we have a well marked frontier line of elevation, 

 separating, as it were, on either hand, the animals of the 

 highlands from the lowlands. 



" When temperature," observes Lyell, " is the barrier 

 which arrests the progress of an animal or plant in a parti- 

 cular direction, the individuals are fewer and less vigorous 

 as they approach the extreme confines of the geographical 

 range of the species. In almost every district, especially if 

 it be mountainous, there are a variety of species, the limits 

 of whose habitations are conterminous,, some being unable 

 to proceed farther without encountering too much heat, 

 others too much cold. Individuals which are thus on the 

 borders of the regions proper to their respective species, 

 are like the outposts of hostile armies, ready to profit by 

 every slight change of circumstances in their favour, and to 

 advance upon the ground occupied by their neighbours and 

 opponents."* 



With respect to the Mollusca of these regions, it will be 

 found, and indeed the remark is generally applicable to all 

 classes of animate beings, that although the same genera 

 may occur in widely separated countries, the species are 

 with few exceptions, entirely distinct. 



Thus, although in the Himalaya genera, Pupa, Bulimus, 

 Helix, Carychium, and Vertigo occur, yet the species are 

 distinct from those of Europe, and with the exception of one 

 or two species common to both regions, # they are totally 

 distinct also from those of the plains of India. 



If climate, therefore, is instrumental in gradually chang- 



* Lyell's Principles of Geology. 



