chap. xi. Mammiferotis Remains of. 361 



mamrniferous inhabitants. I do not doubt that large 

 animals could now exist, as far as the amount, not kind, 

 of vegetation is concerned, on the sterile plains of Bahia 

 Blanca and of the R. Negro, as well as on the equally 

 if not more sterile plains of Southern Africa. The 

 climate, however, may perhaps have somewhat deterior- 

 ated since the mammifers embedded at Bahia Blanca 

 lived there ; for we must not infer, from the continued 

 existence of the same shells on the present coasts, that 

 there has been no change in climate ; for several of 

 these shells now range northward along the shores of 

 Brazil, where the most luxuriant vegetation flourishes 

 under a tropical temperature. With respect to the 

 extinction,, which at first fills the mind with astonish- 

 ment, of the many great and small mammifers of this 

 period, I may also refer to the work above cited (second 

 edit. p. 173), in which I have endeavoured to show that, 

 however unable we may be to explain the precise cause, 

 we ought not properly to feel more surprised at a species 

 becoming extinct than at one being rare ; and yet we 

 are accustomed to view the rarity of any particular 

 species as an ordinary event, not requiring any extra- 

 ordinary agency. 



The several mammifers embedded in the Pampean 

 formation, which mostly belong to extinct genera, and 

 some even to extinct families or orders, and which differ 

 nearly, if not quite, as much as do the Eocene mammifers 

 of Europe from living quadrupeds having existed con- 

 temporaneously with Mollusca, all still inhabiting the 

 adjoining sea, is certainly a most striking fact. It is, 

 however, far from being an isolated one ; for, during 

 the late tertiary deposits of Britain, an elephant, rhi- 

 noceros, and hippopotamus co-existed with many recent 

 land and fresh-water shells ; and, in North America, 

 we have the best evidence that a mastodon, elephant, 



