1838.] Land and Fresh-water Shells of the Himalaya. 2 1 3 



elevation, for where the temperature assimilates to that of the plains, 

 there may we expect to find the plants and animals adapted to it. Such 

 an expectation is, at least, fulfilled, in the portion of the hills I have 

 been fortunate enough to visit ; thus for instance at Ruru about 5000 

 feet above the sea, in the valley of the Pahbar, where the temperature 

 in the summer months is somewhat high, plants which flourish abun- 

 dantly around Neemuvh, are intermingled with others peculiar to the 

 hills, and again around Subathu at an elevation rather beyond 3000 

 feet, that beautiful flower the " Gloriosa Superba" and many species of 

 convolvulus, so common in the jungles of Meywar, are seen, as well as 

 the bhela, bamboo, and other plants of the provinces, surrounded by 

 the vegetation of the mountains. Thus, then, at elevations where the 

 plants of the hills and plains are seen mingled together, it is natural to 

 look for a similar fact in the animal kingdom. And with regard to the 

 Mollusca we shall find it so. 



Yet though we find some species common to the plains extending far 

 into the hills, there is nevertheless a well marked line of elevation be- 

 yond which the welfare of the general number forbids them to pass. 

 Thus, while " Nanina vesicula" is found roaming throughout the hills, 

 apparently at all elevations up to 10,500 feet, other species .which in 

 the plains of India are found in company with it and at the same sea- 

 sons, never mount beyond a third of that height, confining themselves 

 to the warmer valleys at about 3 to 4000 feet above the sea. Among 

 these are " Nanina vitrinoides" and " Succinea crassiuscula" which 

 around Subathu are found in company with species peculiar to the 

 hills, such as " Helicarion cassida" and " Pupa pulchella" which are 

 never 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, rich in 

 the plants of hills and plains and producing a species peculiar to itself, 

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

 the lowlands. 



" When temperature, says Lyell, forms the barrier which arrests the 

 progress of an animal or plant in a particular 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, espe- 

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

 whose habitations are continuous, some being unable to proceed further 

 without encountering too much heat, others too much cold. Indivi- 

 duals, which are thus on the borders of the regions proper to their re- 

 2 p 



