i860.] LYELL'S CRITICISMS. 335 



continuously united with the mainland ; it was mere base 

 subservience, and terror of Hooker and Co. 



With respect to atolls, I think mammals would hardly 

 survive very long, even if the main islands (for as I have 

 said in the Coral Book, the outline of groups of atolls 

 do not look 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 during their existence as atolls. 



I cannot conceive any existing reptile being converted into 

 a mammal. From homologies I should look at it as certain 

 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 characters 

 (and in a lesser degree bird character) with mammalian. We 

 must imagine some form as intermediate, as is Lepidosiren 

 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 

 inheritance 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 

 anticipated. The argument of littoral Miocene shells at the 

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



