IN THE NORTH AND SOUTH. 377 



southern forms. And thus, when the two sets became com- 

 mingled 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 moun- 

 tains, and afterward 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 Lr 

 Plata, New Zealand, and to a lesser degree in Australia, and 

 have beaten the natives ; whereas extremely few southern 

 forms have become naturalized 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 rap- 

 idly sowing themselves, and becoming naturalized. Before 

 the last great Glacial period, no doubt the inter-tropical 

 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 naturalized ; and this is the first stage toward 

 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 inhab- 

 itants of real islands have everywhere yielded and are still 

 yielding to continental forms naturalized through man's 

 agency. 



The same principles apply to the distribution of terres- 

 trial animals and of marine productions, in the northern and 

 southern temperate zones, and on the inter-tropical moun- 

 tains. 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 cur- 

 rents^ while others might remain and survive in the colder 

 depths until the southern hemisphere was in its turn sub- 

 jected to a glacial climate and perinitted their further prog- 

 ress ; in nearly the same manner as, according to Forbes, 

 isolated spates inhabited by arctic productions exist to the 



