xii Report on the Zoological Survey of India 



whicli a freshwater fauna replaces a marine one as soon as physical 

 conditions become suitable, and (ii) the fact that marine animals are 

 always attemj)ting to colonize fresh water, and occasionally succeed in 

 doing so when the transition is sufficiently gradual, as a rule becoming 

 modified structurally in the process. All these lakes are of recent 

 geological origin and have, probably in historical times, been more 

 intimately connected with the open sea than they are at present. 



" The Chilka Lake, which still opens directly into the sea, is imper- 

 fectly separated into an inner and an outer region and contains water 

 that, at any rate at certain seasons, remains distinctly brackish. Its 

 fauna is essentially marine, with but a small permanent freshwater 

 element ; but the species are mostly dwarfed and modified. They 

 resemble those found in smaller lagoons and backwaters on the coast 

 rather than those of marine origin that have long established them- 

 selves in Indian estuaries and rivers. 



" The Tale Saj) is definitely separated into an inner and an outer 

 region connected by an intricate channel. The outer region is in direct 

 communication with the sea and contains water of considerable but 

 variable salinity, while the inner region is affected only indirectly by 

 marine factors and its water is ^permanently fresh or practically so. 

 The fauna of the outer region closely resembles that of the Chilka Lake, 

 but that of the inner region, though including a fairly large marine 

 element, does not differ fundamentally from that of other bodies of 

 fresh water in the vicinity. 



" The Tai Hu is a body of fresh water occupying a shallow depres- 

 sion in the alluvium of the Yangtse delta. It is only connected with 

 the sea by complicated channels and is not subject to tidal influence. 

 Its fauna is in the main normal lacustrine but still retains a small but 

 distinct marine element. 



(6) Inland River-Basins. 



" By an Inland River-Basin is meant a lake that forms the final 

 repository of the waters of a river which never reaches the sea. Many 

 such lakes (e.g., the Dead Sea) are practically lifeless owing to the con- 

 centration of mineral salts in their water. Others, such as the Sea of 

 Aral, which have once been in direct communication with the sea, possess 

 an impoverished marine fauna ; while a few, like the Hamun-i-Hel- 

 7nand, which have never been connected with the sea, remain fairly fresh 

 owing to occasional flushing by floods and have a normal freshwater 

 fauna. The fauna of the Hamun-i-Helmand is a very poor one and 

 consists mainly of species brought down by the river from the moun- 

 tains of Afghanistan. The species are not at all or very little modified. 



(c) Normal Inland Lakes. 



" A normal Inland Lake is one containing fresh water, with a fairly 

 deep bed and connected either with the sea or with some other large 

 body of water by means of a river or rivers flowing out of it. Condi- 

 tions of life in it are such that while various faunistic zones can always 



