<jhap. xiii. Islands to those of the Mainland. 



357 



may safely infer that Charles Island is well stocked with its own 

 species, for annually more eggs are laid and young birds hatched, 

 than can possibly be reared ; and we may infer that the mocking- 

 thrush peculiar to Charles Island is at least as well fitted for its 

 home as is the species peculiar to Chatham Island. Sir C. Lyell 

 and Mr. Wollaston have communicated to me a remarkable fact 

 bearing on this subject ; namely, that Madeira and the adjoining 

 islet of Porto Santo possess many distinct but representative 

 species of land-shells, some of which live in crevices of stone ; 

 and although large quantities of stone are annually transported 

 from Porto Santo to Madeira, yet this latter island has not become 

 colonised by the Porto Santo species: nevertheless both islands 

 have been colonised by European land-shells, which no doubt had 

 some advantage over the indigenous species. From these con- 

 siderations I think we need not greatly marvel at the endemic 

 species which inhabit the several islands of the Galapagos Archi- 

 pelago, not having all spread from island to island. On the same 

 continent, also, preoccupation has probably played an important 

 part in checking the commingling of the species which inhabit 

 different districts with nearly the same physical conditions. Thus, 

 the south-east and south-west corners of Australia have nearly the 

 same physical conditions, and are united by continuous land, 

 yet they are inhabited by a vast number of distinct mammals, 

 birds, and plants ; so it is, according to Mr. Bates, with the butter- 

 flies and other animals inhabiting the great, open, and continuous 

 valley of the Amazons. 



The same principle which governs the general character of the 

 inhabitants of oceanic islands, namely, the relation to the source 

 whence colonists could have been most easily derived, together with 

 their subsequent modification, is of the widest application through- 

 out nature. We see this on every mountain-summit, in every lake 

 and marsh. For Alpine species, excepting in as far as the same 

 species have become widely spread during the Glacial epoch, are 

 related to those of the surrounding lowlands ; thus we have in 

 South America, Alpine humming-birds, Alpine rodents, Alpine 

 plants, &c, all strictly belonging to American forms ; and it is 

 obvious that a mountain, as it became slowly upheaved, would be 

 colonised from the surrounding lowlands. So it is with the inha- 

 bitants of lakes and marshes, excepting in so far as great facility 

 of transport has allowed the same forms to prevail throughout 

 large portions of the world. We see this same principle in the 

 character of most of the blind animals inhabiting the caves of 

 America and of Europe. Other analogous facts could be given. 



