cies and the immediate pre-existence of
closely allied species on the same area--or, at most, on closely
contiguous areas.

Where a continuous area has long been circumscribed by barriers of any
kind, which prevent the animals from wandering beyond it, then we find
that all the species, both extinct and living, constitute more or less a
world of their own; while, on the other hand, where the animals are free
to migrate from one area to another, the course of their migrations is
marked by the origination of new species springing up _en route_, and
serving to connect the older, or metropolitan, forms with the younger,
or colonising, forms in the way of a graduated series. This principle,
however, admits of being traced only in certain cases of species
belonging to the same genus, of genera belonging to the same family, or,
at most, of families belonging to the same order. In other words, the
more general the structural affinity, the more general is the
geographical extension--as we should expect to be the case on the theory
of descent with branching modifications, seeing that the larger, the
older, and the more diverse the group of organisms compared, the greater
must be their chances of dispersal.

These general considerations led us to contemplate more in detail the
correlation between structural affinity and barriers to free migration.
Such barriers, of course, differ in the cases of different organisms.
Marine organisms are stopped by land, unsuitable temperature, or
unsuitable depths; fresh-water organisms by sea and by mountain-chains;
terrestrial organisms chiefly by water. Now it is a matter of fact
which admits of no dispute, that in each of these cases we meet with a
direct correlation between the kind of barrier and the kind of organisms
whose structural affinities are affected thereby. Where we have to do
with marine organisms, barriers such as the Isthmus of Panama and the
varying depth of the Western Pacific determine three very distinct
faunas, ranging north and south in closely parallel lines, and under
corresponding climates. Where we have to do with fresh-water organisms,
we find that a mountain-chain only a few miles wide has more influence
in determining differences of organic type on either side of it than is
exercised by even thousands of miles of a continuous land-area, if this
be uninterrupted by any mountains high enough to prevent water-fowl,
whirlwinds, &c., from dispersing the ova. Again, wher