Tertiary period consisted of genera which are still living.[141] But from
what is now known of the rate of sub-aerial denudation, we are sure, that
during each division of this period many mountain chains must have been
considerably lowered, while we know that some of the existing ranges have
been greatly elevated. Ancient volcanoes, too, have been destroyed by
denudation, and new ones have been built up, so that we may be quite sure
that ample means for the transmission of temperate plants across the
tropics, may have existed in countries where they are now no longer to be
found. The great mountain masses of Guiana and Brazil, for example, must
have been far more lofty before the sedimentary covering was denuded from
their granitic bosses and metamorphic peaks, and may have aided the
southern migration of plants before the final elevation of the Andes. And
if Africa presents us with an example of a continent of vast antiquity, we
may be sure that its great central plateaux once bore far loftier mountain
ranges before they were reduced to their present condition by long ages of
denudation.

_Proofs of Migration by Way of the Andes._--We are now prepared to apply
the principles above laid down to the explanation of the character and
affinities of the various portions of the north temperate flora in the
southern hemisphere, and especially in Australia and New Zealand.

At the present time the only unbroken chain of highlands and mountains
connecting the Arctic and north temperate with the Antarctic lands is to be
found in the American continent, the only break of importance being the
comparatively low Isthmus of Panama, where there is {521} a distance of
about 300 miles occupied by rugged forest-clad hills, between the lofty
peaks of Veragua and the northern extremity of the Andes of New Grenada.
Such distances are, as we have already seen, no barrier to the diffusion of
plants; and we should accordingly expect that this great continuous
mountain-chain has formed the most effective agent in aiding the southward
migration of the Arctic and north temperate vegetation. We do find, in
fact, not only that a large number of northern genera and many species are
scattered all along this line of route, but that at the end of the long
journey, in Southern Chile and Fuegia, they have established themselves in
such numbers as to form an important part of the flora of those countries.
From the lists given in the works already referre