EMYS TECTA. 387 



geology and geograpliy, and not to determine the species 

 in any degree from either. Anything short of this is but 

 tampering with the right evidence. 



Unless we are to continue to take modifications of form 

 and constant external characters as sufficient tests of common 

 origin or like descent, there is an end to the power of dis- 

 criminating species, and the foundations of the science are 

 undermined. But the majority of naturalists are agreed 

 that with existing forms the evidence is sufficient, and no 

 adequate reasons have yet been brought forward to show 

 why the same should not be believed to hold in the case of 

 extinct forms. In fact, a &na belief in it is the bases on 

 which all our investigations are conducted. 



Under this view, conceiving that we are not justified in 

 constituting a difference where we do not fiind it, we infer 

 that the fossil Emys is specifically identical with the existing 

 Emys tecta, for it agrees as closely with the forms grouped 

 under that name as these do with one another. 



The Emys tecta, according to OTir present state of know- 

 ledge, is confined to India, where it is found in great abimd- 

 ance in the Ganges and in ponds in the North-western 

 Provinces, and in Bengal. 



The fossil came from the tertiary strata of the Sewalik 

 hills, where it was associated with the remains of Golosso- 

 chelys, Trionyx, and other Chelonians ; and of Mastodon, 

 Hippopotamus, Sivatherium, Camel, &c. The precise locality 

 is not knovpn. 



If the view taken of its identity with Emys tecta prove 

 correct, the fossil will furnish a good instance of the per- 

 manence, and, within certain limits, invariableness, of species 

 during a long course of time extending through more than 

 one geological epoch. Such instances are so abundant among 

 the Mollusca, as to have constituted the basis of Mr. Lyell's 

 classification of the tertiary strata. But they are so ex- 

 tremely rare among the Vertebrata, that there is perhaps 

 hardly a single well authenticated case on record. Agassiz 

 goes the length of stating that he has found his conclusion 

 regarding the absence of a species from two geological 

 formations, and even from two different paths of one forma- 

 tion, to have been ^invariably confirmed by fossil fishes and 

 Ecliinodermata.' 



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