424 ORIGIN OF SPECIES 



fection, or dominating power, than the southern forms. And 

 thus, when the two sets became commingled in the equatorial 

 regions, during the alternations of the Glacial periods, the 

 northern forms were the more powerful and were able to 

 hold their places on the mountains, and afterwards to mi- 

 grate southward with the southern forms ; but not so the 

 southern in regard to the northern forms. In the same 

 manner at the present day, we see that very many European 

 productions cover the ground in La Plata, New Zealand, and 

 to a lesser degree in Australia, and have beaten the natives; 

 whereas extremely few southern forms have become natu- 

 ralised in any part of the northern hemisphere, 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 forty or fifty 

 years from Australia. The Neilgherrie mountains in India, 

 however, offer a partial exception ; for here, as I hear from 

 Dr. Hooker, Australian forms are rapidly sowing themselves 

 and becoming naturalised. Before the last great Glacial 

 period, no doubt the intertropical mountains were stocked 

 with endemic Alpine forms; but these have almost every- 

 where 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 those which have become naturalised; 

 and this is the first stage towards their extinction. Moun- 

 tains are islands on the land, and their inhabitants have 

 yielded to those produced within the larger areas of the 

 north, just in the same way as the inhabitants of real islands 

 have everywhere yielded and are still yielding to continental 

 forms naturalised through man's agency. 



The same principles apply to the distribution of terrestrial 

 animals and of marine productions, in the northern and 

 southern temperate zones, and on the intertropical mountains. 

 When, during the height of the Glacial period, the ocean- 

 currents were widely different to what they now are, some 

 of the inhabitants of the temperate seas might have reached 

 the equator ; of these a few would perhaps at once be able to 

 migrate southward, by keeping to the cooler currents, whilst 

 others might remain and survive in the colder depths until 



