poLPiiixs. 215 



4. Under the operation of such an immense influx of fresh water, the lake gradually became 

 fresh water. 



5. The change from salt to fresh water was not so rapid as to destroy the Dolphins, but 

 sufficiently so to induce a change in the species. It must have been more rapid than in Lake 

 Baikal, and under its influence the Dolphin became changed into a fresh-water Plat.\nista. 



6. The dividing water-shed which now seiJaratcs the sources of the Ganges from the sources 

 of the Indus, had not yet been sufficiently elevated to divide the two, and as soon as the lake 

 was full to overflowing it overflowed, and the waters escaped into the line of the Ganges. 

 There would then only be one great river in the north of India, and that the Ganges. By 

 the time this happened, the transformation of the Dolphin into the Platanista had been com- 

 pleted, — it may have been either Platanist.\ Indi or P. Gaxgetica that was produced, or it 

 may have been a common ancestor of both. When this hajjpened, the Platanista, whatever 

 its species, would inhabit both the lake and the Ganges, but they could not go back to the sea, 

 via the Ganges, for by this time they had been changed into fresh-water species. 



7. But the land continued to rise, and the llimmalayahs, in their rise, also raised" that portion 

 of land lying between the sources of the Ganges and this great lake. Of course, this cut off tlic 

 exit by the lake into the Ganges. Those individuals of Platanista, which were out in the waters 

 of the river, woidd find themselves cut off from their natm-al home or reserve, and restricted to a 

 river- life in the Ganges ; a new condition, perhaps, of sirfficient importance to induce a second 

 change into Platanista Gangetica. 



8. The lake, cut ofi" from its exit by the Ganges, continues to rise until it again overflows 

 elsewhere, and this time finds an exit where the mouth of the Indus now is, and the Indus 

 flows through the midst of it ; old channels show that the Indus once so flowed, and not, as 

 now, to the west of it. The surviving shoals of Platanista, in their tiu'n, would find their lake-life 

 turned into a river one, and Platanista Indi is the residt. 



I have no doubt that Inia Amazonica, the Amazonian species, was produced bj^ a similar 

 concurrence of circumstances, with the exception that there it was not a double event, but only 

 a single-barrelled phenomenon, at least so far as species is concerned. A species has indeed 

 been described under the name of I. Boliviensis, but it is imderstood to be only a sjTionyme of 

 the I. Amazonica. We know that the Amazon flows in the course of an ancient arm of the 

 sea, that Brazil and Guiana, &c., were once islands, and the ancestors of Inia must have been 

 caught in a sea gulf turned by a rise of land, into an inland lake without an outlet, and in 

 this lake been converted into fresh species in the same way that Platanista first was. The fact 

 that it is found at a great distance from the sea, and above cataracts which must have proved 

 an absolute barrier to its ever having ascended from it by the present channel of the river, 

 sufficiently proves this. In some natural history books, it is said in general terms to inhabit the 

 great rivers of South America,* conveying the im^u'ession that it occurs in more than in the 

 Amazon and its tributaries, and I have a recollection of seeing somewhere the Orinoko given as 

 one of its habitats, but I cannot find the reference. If it really does inhabit the Orinoko as well 

 as the Amazon, it would infer something like a repetition of the history of the Platanistas of the 

 Ganges and Indus, with the exception that only one species has been developed. 



* Dallas, W. S. " Natural History of the Animal Kinsdom,"' 1856, p. G83. 



