340 ALTERNATE GLACIAL PERIODS [Chap. XII. 



case. We must, also, bear in mind the occurrence in both hemi- 

 spheres of former Glacial periods ; for these will account, in 

 accordance with the same principles, for the many quite distinct 

 species inhabiting the same widely separated areas, and belonging to 

 genera not now found in the intermediate torrid zones. 



It is a remarkable fact strongly insisted on by Hooker in regard 

 to America, and by Alph. de Candolle in regard to Australia, that 

 many more identical or slightly modified species have migrated from 

 the north to the south, than in a reversed direction. We see, 

 however, a few southern forms on the mountains of Borneo and 

 Abyssinia. I suspect that this preponderant migration from the 

 north to the south is due to the greater extent of land in the north, 

 and to the northern forms having existed in their own homes in 

 greater numbers, and having consequently been advanced through 

 natural selection and competition to a higher stage of perfection, 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 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 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 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 naturalised ; and this is the first stage towards their 

 extinction. Mountains are islands on the land, and their inhabi- 

 tants have yielded to those pi'oduced 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 forma 

 naturalised through man's agency. 



