46

AGENCY OF MAN IN THE

[Book III.

It may also be remarked, that if man is the mostactive agent in enlarging, so also is he in circumscrib-ing the geographical boundaries of particular plants.He promotes the migration of some, he retards that ofother species, so that, while in many respects he ap-pears to be exerting his power to blend and confoundthe various provinces of indigenous species, he is, inother ways, instrumental in obstructing the fusioninto one group of the inhabitants of contiguous pro-vinces.

Thus, for example, when two botanical regionsexist in the same great continent, such as the European region, comprehending the central parts of Europe and those surrounding the Mediterranean, and theOriental region, as it has been termed, embracing thecountries adjoining the Black Sea and the Caspian,the interposition between these of thousands of squaremiles of cultivated lands, opposes a new and powerfulbarrier against the mutual interchange of indigenousplants. Botanists are well aware that garden plantsnaturalize and diffuse themselves with great facility incomparatively unreclaimed countries, but spread them-selves slowly and with difficulty in districts highlycultivated. There are many obvious causes for thisdifference: by drainage and culture the natural varietyof stations is diminished, and those stray individualsby which the passage of a species from one fit stationto another is effected, are no sooner detected by theagriculturist, than they are uprooted as weeds. Thelarger shrubs and trees, in particular, can scarcelyever escape observation, when they have attained acertain size, and will rarely fail to be cut down ifunprofitable.

The same observations are applicable to the inter-