168 ALTERNATE GLACIAL PERIODS [Chap. XII. 



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 work- 

 shops 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. 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 



