320 ALTERNATE GLACIAL PERIODS [CHAP. XII. 



wards to migrate 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 naturalised 

 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 excep- 

 tion; 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 

 everywhere 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 out- 

 numbered, by those which have become naturalised ; and this is 

 the first stage towards their extinction. Mountains 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 the southern hemisphere was 

 in its turn subjected to a glacial climate and permitted their 

 further progress ; in nearly the same manner as, according to 

 Forbes, isolated spaces inhabited by Arctic productions exist 

 to the present day in the deeper parts of the northern temperate 

 seas. 



I am far from supposing that all the difficulties in regard to the 

 distribution and affinities of the identical and allies species, which 

 now live so widely separated in the north and south, and some- 

 times on the intermediate mountain-ranges, are removed on the 

 views above given. The exact lines of migration cannot be 

 indicated. We cannot say why certain species and not others 

 have migrated ; why certain species have been modified and have 

 given rise to new forms, whilst others have remained unaltered 



