GIANT TORTOISES AND THEIR DISTRIBUTION 317 



To explain, on the theory of their continental origin, the 

 absence of practically all terrestrial mammalian life from the 

 Galapagos group and the smaller islands of the Indian Ocean 

 is a task I am not on this occasion disposed to attempt. It 

 may, however, be mentioned, that in the case of the Seychelles 

 Dr. Wallace has suggested drowning out as a vera causa for 

 the absence of mammals; but such a submergence would 

 assuredly have been equally fatal to the tortoises which are 

 now known to have been indigenous to that group, unless 

 indeed they reached all the islands in the Indian Ocean at 

 an epoch far too late to allow for their differentiation into 

 insular species. 



Bibliography 



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Madagascar, Trans. Zool. Soc. vol. xviii. p. 59, London, 190S. 



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