402 ALTERNATE GLACIAL PERIODS 



more powerful and were able to hold their places on the 

 mountains, 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 La Plata, New Zealand, and to a lesser degree 

 in Australia, and have beaten the natives; whereas 

 extremely few soathern 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 cen- 

 turies from La Plata and during the last forty or fifty years 

 from Australia. The K"eilgherrie Mountains in India, 

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

 Dr. Hooker, Australian forms are rapidly sowing them- 

 selves and becoming naturalized. 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 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 

 inhabitants 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 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 per- 

 haps at once be able to migrate southward, by keeping to 

 the cooler currents, while others might remain and sur- 

 vive 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 Arctio 



