214 MAMMALS. 



distribution of fresh-water fish. The minnow is a fresh-water fish ; it cannot live in sea water ; 

 neither can its spawn, and j^et it is found in ahnost every streamlet in Britain, although 

 belono-ing to opposite watersheds and different sj-stems. So is it with the loche, the stickleback, 

 and most of our other fresh-water fishes — we are so familiar with the phenomenon, that it seldom 

 occurs to us to inquire how this wonderful fact has been brought about. We shall have to 

 make up our minds upon this when we come to the Fishes, but in the meantime the present 

 case is still more extraordinary. The Dolphins are not only in different systems of rivers, but 

 belong to a marine genus. We have to account, not only for a marine animal being developed 

 into a fresh-water si^ecies, but also into two allied species in two unconnected rivers. 



They are so closely allied, that they must have taken their origin from a common and not 

 very distant ancestor ; and it docs not seem at all a satisfactory explanation to suppose, that this 

 common ancestor frequented the shores of India, and first gave off one new species in the delta 

 of the Indus, and then repeated the process on the other side of the peninsida at the mouth of 

 the Ganges. Moreover, it implies a step taken by the common ancestor, which is, according to 

 my reading, against the first principles regulating the origination of species. The animal never 

 voluntarily seeks the change which produces the development of a new species. It is most 

 comfortable in the country in which it was born, and for which it is adapted — Inertia is strong 

 to keep it where it is. It is like Sydney Smith, when he said that where etiquette prevented 

 him doing things disagreeable to himself, he was a perfect martinet. Where personal comfort 

 retained the Dolphin within the bounds of its original habitat, it would not readily seek to wander 

 beyond them ; therefore, I cannot conceive it possible that a Doljohin, or rather a school of Dol- 

 phins, (for the reader will remember that my theory of the origin of species only acts upon a 

 large body) should desert the congenial open sea, for the imcongenial muddy flats of a delta, and 

 by remaining there voluntarily for a sufiicient nimiber of ages, give rise to new fresh-water river 

 Dolphins. In general, the change originating a species comes to the animal — not the animal 

 to the change. Hunger and the struggle for life may make exceptions to the rule, but on looking 

 back at the past historj' of the globe, it seems very evident that cases resulting from such causes 

 are very exceptional indeed. So I argue that here the change from salt to fresh water must 

 have been forced upon the Dolphins. 



The hypothesis which seems to me best to account for the facts is this : — - 



1. The five rivers, the Indus, the Jhelum, the Chenab, the Ravi, and the Ghora, flow through 

 a desert which also extends over a great space on the west of the Indus in its lower course. 

 This immense sandy waste was undoubtedly at no very distant date in geological time the bottom 

 of a sea continuous with the Arabian Sea ; it still bears the aspect of the bottom of the sea ; and 

 I assume (no very great assumption) that when India was last raised above the waters, that 

 great sandy desert was left as a great gulf or bay, extending up almost to the Himmalayahs . 

 and that in it numerous marine dolphins played and sported as they do now in the Arabian 

 Sea or the Bay of Bengal. 



2. As the land continued to rise, this gulf was shut off from the sea by the elevation of the 

 coast between Bombay and Kurrachee, then of course became a salt lake, without exit, in which 

 also of course were shut up our marine Dolphins. 



3. Into this lake flowed the waters which now supply the five rivers above mentioned, as 

 M'ell as the sources of the Ganges, and of its upper tributaries, from the snows of the Ilim- 

 malavah. 



