e plants of the south from spreading themselves to the
northward; but it has been remarked that some species have made their
way through the gorges of these chains, and are found on their northern
sides, principally in those places where they are lower and more
interrupted."[991] Now the chains here alluded to have probably been of
considerable height ever since the era when the existing vegetation
began to appear, and were it not for the deep fissures which divide
them, they might have caused much more abrupt terminations to the
extension of distinct assemblages of species.

Parts of the Italian peninsula, on the other hand, have gained a
considerable portion of their present height since a majority of the
marine species now inhabiting the Mediterranean, and probably, also,
since the terrestrial plants of the same region were in being. Large
tracts of land have been added, both on the Adriatic and Mediterranean
side, to what originally constituted a much narrower range of mountains,
if not a chain of islands running nearly north and south, like Corsica
and Sardinia. It may therefore be presumed that the Apennines have been
a centre whence species have diffused themselves over the contiguous
_lower_ and _newer_ regions. In this and all analogous situations, the
doctrine of Wildenow, that species have radiated from the mountains as
from centres, may be well founded.

_Introduction of New Species._

If the reader should infer, from the facts laid before him in the
preceding chapters, that the successive extinction of animals and plants
may be part of the constant and regular course of nature, he will
naturally inquire whether there are any means provided for the repair of
these losses? Is it part of the economy of our system that the habitable
globe should, to a certain extent, become depopulated both in the ocean
and on the land; or that the variety of species should diminish until
some new era arrives when a new and extraordinary effort of creative
energy is to be displayed? Or is it possible that new species can be
called into being from time to time, and yet that so astonishing a
phenomenon can escape the observation of naturalists?

Humboldt has characterized these subjects as among the mysteries which
natural science cannot reach; and he observes that the investigation of
the origin of beings does not belong to zoological or botanical
geography. To geology, however, these topics do strictly appertain; and
this