Dr. Thomson on the Climate and Vegetation of the Himalaya. 197 



may be said, that to the eastward the vegetation is very much more 

 luxuriant and tropical, and that it changes very gradually in advancing 

 to the westward, in exact proportion to the diminution in the quantity of 

 rain. The same gradual transition in the vegetable world may also be 

 observed in advancing up the valleys, or in passing across the mountains 

 from the outer valleys to those which are further removed from the In- 

 dian plain ; though in the latter case, of course, the effects of gradually 

 increasing elevation must be taken into consideration as partly the cause 

 of the change as well as the decrease of humidity. 



The plains of northern India which skirt the base of the Himalaya, do 

 not (if we except the belt immediately at the base of the mountains,) 

 present by any means a rich flora. From their situation nearly on the 

 tropics, their distance as a whole from the sea, and their proximity to the 

 mountains, they are not very damp, and their climate has too decided a 

 lowering of temperature in the cold season to permit them to be clothed 

 with the dense forest vegetation which clothes the tropical plains of South 

 America. They are in general open plains without much wood, and 

 where not under cultivation, are covered either with a dense jungle of dif- 

 ferent species of arundo and saccharum, or with scattered trees of various 

 tropical families, acacias and zizyphi being very common genera. Here 

 and there only there are patches of forest gcnei-ally low and scrubby, and 

 without much underwood, or any of the fine parasitical plants and ferns 

 which are so ornamental in tropical woods. 



In the lower parts of Bengal, the proximity of the sea somewhat modi- 

 fies this general character; a number of ferns, one or two species of 

 pothos, and a few Orchideae, among which Vanda Eoxburghii and a large 

 and fine Cymbidium are the most common, are to be found. In the same 

 way the valleys of Silhet and Assam are exceptional in character, but 

 from their being inclosed with mountains of some elevation on all sides, 

 they are scarcely to be regarded as part of the Indian plain, but may 

 more properly be considered as wide mountain valleys, and they in fact 

 closely resemble in vegetation the valleys of the larger Himalayan rivers 

 in the east part of the chain. 



Close to the foot of the chain of moimtains throughout its whole course 

 from east to west, there lies a belt of forest and swampy land, which is 

 well known in India by the name of Terai, and which, where it is de- 

 veloped to any considerable extent, bears a very bad character for un- 

 healthiness, and is indeed in many places quite impassable for Europeans 

 at most seasons of the year. This forest belt seems to be due to the 

 greater humidity of atmosphere, and at the same time greater equability 

 of temperature, which is produced by the proximity of the mountains. Its 

 width is very various, from forty or fifty miles, to which I believe it attains 

 in some parts of Nepal, to eight or ten miles, which is a more common 

 width. Westward of the Jumna it almost disappears, being represented 

 by a line of swampy or marshy ground, and a low jungle of bushes of the 

 common plain species of trees. 



