236 A NATURALIST IN HIMALAYA 



naturalists, and many interesting methods have been 

 discovered by means of which the various species 

 increase their geographical range. 



In this district man, by his mode of cultivation, 

 seems to be the most important agent in effecting 

 the dispersal of shells both of the land and the fresh 

 water. On the broad fields in the beds of the valleys 

 and on the narrow tiers of terraces rising up the slopes 

 of the mountains the cultivator sows his crops of rice. 

 Rice fields need an abundance of water ; in fact the 

 crop, in order to thrive, must stand in a watery swamp. 

 To effect this the cultivator diverts the water from 

 the mountain streams and carries it in channels over 

 the land. From the main channel smaller water- 

 courses conduct the fluid to the cultivated area, and 

 still smaller channels connect field with field. Thus 

 vast expanses of land are fed with the water from one 

 stream, and, by means of channels, all the fields of 

 that large area are in intercommunication one with 

 the other. Each field is swarming with molluscs, 

 chiefly Limncea, Planorbis and bivalves, and, as the 

 water flows from field to field, many of the smaller 

 shells must be swept along with it. The molluscs 

 also crawl on to twigs or adhere to the under surface 

 of floating leaves, and are thus drifted along the 

 channels from field to field. I think numbers must 

 be floated away on leaves, for I collected many from 

 a clump of herbage that had temporarily obstructed 

 one of the channels. 



No living agent is so powerful as man in influencing 

 the course of organic nature. Here the husbandmen 

 sow large areas of their land with rice, and to nourish 

 it they conduct the water from the streams, lead it by 



