GEOGRAPHICAL DISTRIBUTION 341 



in greater numbers, and having 1 consequently been 

 advanced through natural selection and competition to 

 a higher state of perfection or dominating power, than 

 the southern forms. And thus, when they became 

 commingled during the Glacial period, the northern 

 forms were enabled to beat the less powerful southern 

 forms. Just in the same manner as we see at the 

 present day, 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 imported into Europe during the 

 last two or three centuries from La Plata, and during 

 the last thirty or forty years from Australia. Some- 

 thing 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 everywhere largely yielded to 

 the more dominant forms, generated in the larger 

 areas and more efficient workshops of the north. In 

 many islands the native productions are nearly equalled 

 or even outnumbered by the naturalised ; and if the 

 natives have not been actually 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 intertropical mountains 

 before the Glacial period must have been completely 

 isolated ; and I believe that the productions 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, naturalised 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 



