128 THE 'ORIGIN OF SPECIES.' [i36o. 



like a former continent) had been tenanted by mammals, from 

 the extremely small area, the very peculiar conditions, and 

 the probability that during subsidence all or nearly all atolls 

 have been breached and flooded by the sea many times dur- 

 ing their existence as atolls. 



I cannot conceive any existing reptile being converted 

 into a mammal. From homologies I should look at it as cer- 

 tain that all mammals had descended from some single pro- 

 genitor. What its nature was, it is impossible to speculate. 

 More like, probably, the Ornithorhynchus or Echidna than 

 any known form ; as these animals combine reptilian charac- 

 ters (and in a less degree bird character) with mammalian. 

 We must imagine some form as intermediate, as is Lepidosi- 

 ren now, between reptiles and fish, between mammals and 

 birds on the one hand (for they retain longer the same em- 

 bryological character) and reptiles on the other hand. With 

 respect to a mammal not being developed on any island, 

 besides want of time for so prodigious a development, there 

 must have arrived on the island the necessary and peculiar 

 progenitor, having a character like the embryo of a mammal ; 

 and not an already developed 'reptile, bird or fish. 



We might give to a bird the habits of a mammal, but in- 

 heritance would retain almost for eternity some of the bird- 

 like structure, and prevent a new creature ranking as a true 

 mammal. 



I have often speculated on antiquity of islands, but not 

 with your precision, or at all under the point of view of 

 Natural Selection not having done what might have been an- 

 ticipated. The argument of littoral Miocene shells at the 

 Canary Islands is new to me. I was deeply impressed (from 

 the amount of the denudation) [with the] antiquity of St. 

 Helena, and its age agrees with the peculiarity of the flora. 

 With respect to bats at New Zealand (N. B. There are two 

 or three European bats in Madeira, and I think in the Canary 

 Islands) not having given rise to a group of non-volant bats, 

 it is, now you put the case, surprising ; more especially as 

 the genus of bats in New Zealand is very peculiar, and there- 



