ds that
of other species; so that, while in many respects he appears to be
exerting his power to blend and confound the various provinces of
indigenous species, he is, in other ways, instrumental in obstructing
the fusion into one group of the inhabitants of contiguous provinces.

Thus, for example, when two botanical regions exist in the same great
continent, such as _the European region_, comprehending the central
parts of Europe, and those surrounding the Mediterranean, and _the
Oriental region_, as it has been termed, embracing the countries
adjoining the Black Sea and the Caspian, the interposition between these
of thousands of square miles of cultivated lands, opposes a new and
powerful barrier against the mutual interchange of indigenous plants.
Botanists are well aware that garden plants naturalize and diffuse
themselves with great facility in comparatively unreclaimed countries,
but spread themselves slowly and with difficulty in districts highly
cultivated. There are many obvious causes for this difference; by
drainage and culture the natural variety of stations is diminished, and
those stray individuals by which the passage of a species from one fit
station to another is effected, are no sooner detected by the
agriculturist, than they are uprooted as weeds. The larger shrubs and
trees, in particular, can scarcely ever escape observation, when they
have attained a certain size, and will rarely fail to be cut down if
unprofitable.

The same observations are applicable to the interchange of the insects,
birds, and quadrupeds of two regions situated like those above alluded
to. No beasts of prey are permitted to make their way across the
intervening arable tracts. Many birds, and hundreds of insects, which
would have found some palatable food amongst the various herbs and trees
of the primeval wilderness, are unable to subsist on the olive, the
vine, the wheat, and a few trees and grasses favored by man. In
addition, therefore, to his direct intervention, man, in this case,
operates indirectly to impede the dissemination of plants, by
intercepting the migration of animals, many of which would otherwise
have been active in transporting seeds from one province to another.

Whether, in the vegetable kingdom, the influence of man will tend, after
a considerable lapse of ages, to render the geographical range of
_species in general_ more extended, as De Candolle seems to anticipate,
or whether the compensating agen