DISTRIBUTION OF INDIAN FEESHWATEK FISHES. 569 



and when African forms were enabled to travel eastwards and at- 

 tain a firm hold in India. The immigration from the west must 

 have been considerable ; for it seems to have greatly checked the 

 further development o£ the Malayan fauna, which remained pre- 

 served only on the more elevated hills, chiefly those consisting of 

 gneissose and metamorphic rocks " (Proc. As. Soc. 1871, p. 84). 



In short, many zoologists consider that the Indian fauna was 

 formerly very similar to the Malayan ; that something occurred 

 which acted injuriously on that fauna ; while a communication 

 occurring with Africa, and perhaps due to the Indian climate be- 

 coming more tropical, a development of African forms occurred, 

 but that this commingling did not take place on the more elevated 

 regions ; that afterwards there was a large irruption of Malayan 

 forms due to a connexion being formed between Burma and 

 Eastern Bengal, and that they overran the Hindustan subregion. 



The distribution of the freshwater fishes in these regions ought 

 to give us some facts which support or refute these opinions ; for 

 although marine fishes can ascend into fresh water, and should 

 their retreat to the sea be cut ofi", they are able to make it their 

 home, it is not so with true freshwater forms, which never breed 

 in the sea, and cannot exist in it for any length of time. Thus 

 the freshwater forms are unable to pass from the mainland to 

 islands' ; they must have a freshwater channel up which they can 

 proceed ; but for this to exist, a land-continuity becomes neces- 

 sary. Land connexion alone between two continents may not 

 always be sufB.cient ; as even if such were present, it does not ne- 

 cessarily follow that freshwater would be also there. Again, a 

 mountain-chain may extend across the isthmus over which fresh- 

 water fishes would not be able to pass. 



It has been advanced that freshwater fishes have two modes of 

 dispersal : — (1) carried by external agency out of one river-system 

 to another, or from the mainland to islands ; (2) by river-sytems 

 due to some cause commingling and permitting the fish to 

 migrate. 



Under the head of external agency, the action of hurricanes 

 and whirlwinds have been adduced, when with the downpour of 



^ Wallace observes of Amphibia : — " Salt water is fatal to tbem as well as to 

 their eggs ; and deserts and oceans would probably form the most effectual 

 barriers to their 'dispersal ; whereas both snakes and lizards abound in deserts, 

 and have some means of occasionally passing the ocean which frogs and sala- 

 manders do not seem to possess." 



