400 



GEOGEAPIIICAL DISTRIBUTION AND 



[Ch. XL. 



unintentionally, or against liis will, liis intervention is strictly 

 analogous to tliat of tlie species so extirpated. 



I may observe, moreover, tliat if, at former periods, the 

 animals inhabiting any given district liave been partially 

 altered by the extinction of some species, and the introduction 



immi 



must 



plants conveyed about with them to foreign countries. As, 



mi 



birds is substituted 

 for another, the countries from and to which seeds are trans- 

 ported are immediately changed. Vicissitudes, therefore, 

 analogous to those which man has occasioned, may have 

 previously attended the springing up of new relations between 

 species in the vegetable and animal worlds. 



It may also be remarked, that if man is the most active 

 agent in enlarging, so also 



circumscr 



graphical boundaries of particular plants. He promotes the 

 migration of some, he retards 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. 



Botanists are well aware that garden plants naturalise 

 and diffuse themselves with great facility in comparatively 

 unreclaimed countries, but spread themselves slowly and with 

 diflaculty 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 large 

 shrubs and trees, in particular, can scarcely ever escape obser- 

 vation, 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 wav across the intervening arable tracts. Many 



