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 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
naturalised through man's agency.
The same principles apply to the distribution of terrestrial 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 perhaps at once be able to migrate southwards, by keeping
to the cooler currents, while others might remain and survive 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 Arctic productions exist
to the present day in the deeper parts of the northern temperate seas.
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