380 GEOGRAPHICAL DISTRIBUTION, Chap. XL 



that very many European productions cover the ground 

 in La Plata, and in a lesser degree in Australia, and 

 have to a certain extent beaten the natives ; whereas 

 extremely few southern forms have become naturalised 

 in any part of Europe, though hides, wool, and other 

 objects likely to carry seeds have been largely im- 

 ported into Europe during the last two or three cen- 

 turies from La Plata, and during the last thirty or forty 

 years from Australia. Something of the same kind 

 must have occurred on the intertropical mountains : no 

 doubt before the Glacial period they were stocked with 

 endemic Alpine forms ; but these have almost every- 

 where largely yielded to the more dominant forms, 

 generated in the larger areas and more efficient work- 

 shops of the north. In many islands the native pro- 

 ductions are nearly eqiialled or even outnumbered by 

 the naturalised ; and if the natives have not been actu- 

 ally exterminated, their numbers have been greatly 

 reduced, and this is the first stage towards extinction. 

 A mountain is an island on the land ; and the inter- 

 tropical mountains before the Glacial period must have 

 been completely isolated ; and I believe that the pro- 

 ductions of these islands on the land yielded to those 

 produced within the larger areas of the north, just in 

 the same way as the productions of real islands have 

 everywhere lately yielded to continental forms, natu- 

 ralised by man's agency. 



I am far from supposing that all difficulties are re- 

 moved on the view here given in regard to the range 

 and affinities of the allied species which live in the 

 northern and southern temperate zones and on the 

 mountains of the intertropical regions. Very many 

 difficulties remain to be solved. I do not pretend to 

 indicate the exact lines and means of migration, or the 

 reason why certain species and not others have migrated ; 



