le to cross the Andes, so as to reach the
eastern plains; nor, for a similar reason, would those first established
in the Pampas, or the valleys of the Amazon and the Orinoco, ever arrive
at the shores of the Pacific.

In the ocean an analogous state of things would prevail; for there,
also, climate would exert a great influence in limiting the range of
species, and the land would stop the migrations of aquatic tribes as
effectually as the sea arrests the dispersion of the terrestrial. As
certain birds, insects, and the seeds of plants, can never cross the
direction of prevailing winds, so currents form natural barriers to the
dissemination of many oceanic races. A line of shoals may be as
impassable to deep-water species, as are the Alps and the Andes to
plants and animals peculiar to plains; while deep abysses may prove
insuperable obstacles to the migrations of the inhabitants of shallow
waters.

_Supposed centres, or foci, of creation._--It is worthy of observation,
that one effect of the introduction of single pairs of each species must
be the confined range of certain groups in spots, which, like small
islands, or solitary inland lakes, have few means of interchanging their
inhabitants with adjoining regions. Now this congregating in a small
space of many peculiar species, would give an appearance of _centres_ or
_foci_ of creation, as they have been termed, as if they were favourite
points where the creative energy has been in greater action than in
others, and where the numbers of peculiar organic beings have
consequently become more considerable.

I do not mean to call in question the soundness of the inferences of
some botanists, as to the former existence of certain limited spots
whence species of plants have been propagated, radiating, as it were, in
all directions from a common centre. On the contrary, I conceive these
phenomena to be the necessary consequences of the plan of nature before
suggested, operating during the successive mutations of the surface,
some of which the geologist can prove to have taken place subsequently
to the period when many species now existing were created. In order to
exemplify how this arrangement of plants may have been produced, let us
imagine that, about three centuries before the discovery of St. Helena
(itself of submarine volcanic origin), a multitude of new islands had
been thrown up in the surrounding sea, and that these had each become
clothed with plants emigrating