204 Records of the Indian Museum. [^ol. XIY, 



Origin of the Fauna. 



The aquatic and quasi-aquatic fauna of the Inle district as a whole 

 was separated fioni the common fauna of the Oriental Region at a 

 remote but not extremely remote period. The fossil remaiiis of the 

 district are not sufficiently well known to cast nuich light on the precise 

 period at which this occurred. The local fauna developed in circum- 

 stances that favoured plasticity but did not render peculiar adaptations 

 to environment necessary. Primitive forms such as Chaudhuria^ were 

 able to survive side by side with highly modified forms such as Smvbwn, 

 Taia and Hydrobioides. In still, deep lakes, with transparent water 

 containing abundant salts of lime and other minerals, in a temperate 

 climate free from extremes of cold or heat, with a heavy but not 

 excessive rainfall, conditions were perhaps ideal for the rapid evolution 

 and the preservation of peculiar forms, modified superficially but not 

 chanoed in fundamental structure or adapted in direct correlation 

 with their mode of life. On the one hand competition was less keen 

 than in strictly tropical surroundings and physical barriers interfered 

 with the immigration of alien forms ; on the other there was nothing 

 to check the momentum of eccentricity and small peculiarities were 

 intensified rather than eliminated. The communities of difEerent 

 lakes developed slight racial or even specific peculiarities, but the 

 fauna as a whole remained uniform over a large area. With the 

 growth and deepening of the old lakes of the Shan Plateau this 

 process continued, but the day of the great lakes was soon over. 

 The lakes appeared and grew deep, changed their communications 

 from time to time, and finally shrank and for the most part dis- 

 appeared, owing to allied causes — the dissolving action of water, the 

 re-deposition in solid form of the salts dissolved, and the formation of 

 peat and of finely divided insoluble matter. The insoluble debris of 

 the rocks eaten away by water accumulated in the form of red soil 

 and masses of peat were heaped up as vegetable remains were trans- 

 formed into this substance, until most of the basins were filled. At 

 some places lakes dried up owing to their water eating through soluble 

 barriers of hard limestone, at others new lakes were formed by the 

 deposition of calcareous dams. The Inle Lake, which was probably the 

 largest of the system, has survived, shrunken in area and shallowed, 

 but still a lake. In it the fauna of the old lakes has become as it were 

 concentrated. This fauna has lost the power to resume the normal 

 characters of a swamp-fauna, or perhaps in its isolated state there has 

 been no reason why it should do so ; it retains the peculiar features 

 acquired in conditions quite other than those in which it now lives. 



1 Mr. R. H. Whitchouso, basing his view on a study of tlie .structure of the tail, 

 regards Chaudhuria as a highly specialized form. See p. 66 of this volume. 



