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 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.
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