348 ALTERNATE GLACIAL PERIODS Chap. XI. 



tlie mountains of Borneo and Abyssinia. I suspect that this 

 preponderant niij^ration from the north to t!ie south is due to 

 the f^rcatcr extent of hmd in the north, and to tlie noi'tbeni 

 forms liaviuii;' existed in their own homes in g-reater numbers, 

 and havini^ eonsccfucntly been advanced throu<>;h natural selec- 

 tion and comjM'tition to a liigher stage of perfection, or domi- 

 nating jKnver, tliiui the soutliern 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 afterward to migrate southward with the south- 

 ern forms ; but not so the southern in regard to the northern 

 forms. In the same manner we see, at the present day, that 

 very many European productions cover the groimd in La 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 

 ccnturietj from La Plata, and during the last thirty or forty 

 years from Australia. The Ncilgherric Mountains in India, 

 however, offer a part ial exception ; for here, as I hear from Dr. 

 Hooker, Australian forms are rapidly sowing themselves and 

 becoming naturalized. Before the last great Glacial period, 

 no doubt the intertropical mountains Avere stocked with en- 

 demic 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 pi'oductions are nearly equalled, or even outnumbered, 

 by those which have become naturalized ; and this is the llrst 

 stage toward their extinction. Mountains are islands on the 

 land, and their inhabitants have yielded to those produced with- 

 in 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 naturalizetl through man's 

 agency. 



The same principles apply to the distribution of terrestrial 

 animals and of marine productions, in the northern and south- 

 ern temperate zones, and on the intertropical mountains. 

 When during the height of the Glacial period the ocean-cur- 

 rents Avere Avidely diiferent to Avhat thev noAV are, some of the 

 inhabitants of the temperate seas might have reached the equa- 

 tor ; of these a fcAV would perhaps at once be able to migrate 



