12 BULLETIN 188, UNITED STATES NATIONAL MUSEUM 



aquatic species, wliicli may carry the eggs attached to their feet from one 

 watershed to another. Those who have paid particular attention to this mat- 

 ter, however, are definitely of the opinion that such a mode of dispersal of 

 freshwater fishes is normally highly improbable, even though there may be 

 records of such fortuitous dispersal in practically all groups of animals including 

 freshwater fishes. 



In connection with my work on the Siluroid fishes of India for a revised 

 edition of "Fishes" in the Fauna of British India series, I have been greatly 

 struck by the close similarity of the Indian forms to those found towards the 

 east in Indo-China, Siam, and the Malay Archipelago. As a result of a detailed 

 study of the genera and species inhabiting these regions I am definitely of the 

 opinion that the freshwater fish fauna of India in the main originated in South- 

 eastern Asia, most probably in Indo-China, and spread vpestwards by successive 

 waves of migration to India and later to Africa while the two masses of land 

 were connected with each other. Gregory's researches on the evolution of the 

 mountain and river systems of South-eastern Asia have shown that in this region 

 there were extensive river captures — the rivers on the west beheading the 

 rivers on the east ; these changes made possible the migrations of aquatic ani- 

 mals from the east to the west but not in the reverse direction. Gregory's 

 researches have further shown that all the rivers of Eastern Tibet drained into 

 the Gulf of Siam or the South China Sea before the present rivei- systems became 

 established. * * * The freshwater fauna of Eastern Asia at least may have 

 originated along the coasts of Indo-China, when the ocean water in this area 

 was greatly diluted by the drainage into it of several river systems. 



Professor Gregory's views about the capture of the eastern rivers by the 

 western rivers are, however, not accepted by all geologists ; * * * but a con- 

 siderable mass of evidence bearing on the close relationship and distribution 

 of the fishes of South-eastern Asia demands for its explanation a hypothesis 

 similar to that worked out by Gregory. 



Inasmuch as Thailand is geographically separated from parts of 

 Indo-China, Burma, and Malaya by only political barriers, with large 

 rivers constituting international boundaries, it is natural that there 

 should be many species of fresh- water fishes common to Siam and the 

 adjoining countries. With every new activity in the making of fish 

 collections in waters at or near the boundaries, the number of over- 

 lapping species becomes augmented, and some additions to the known 

 common faunas of these contiguous countries may be expected from 

 time to time for many years. 



That part of the Malay Peninsula that lies in Thailand is essen- 

 tially similar physically to the remaining part constituting the Fed- 

 erated and Non-federated Malay States. The boundary is artificial, 

 and the fish life on the two sides should, and does, present few 

 differences, and those differences will become less striking and may 

 largely disappear as the result of further observations and collecting. 

 The recent work of Herre and Myers (1937) has disclosed for Malaya 

 a very large number of species known also from Siam. 



In the forest-covered mountains of Northern Siam streams tribu- 

 tary to the Mekong, the Menam Chao Phya, and the Salwin basins 

 may have fishes that specifically are common to two or all three of 



