586 



explained by a wholly difFerent set of causes, as will appear fiorn the 

 following observations. 



"When we compare the floras, that is, the lists of native plants, of 

 two countries closely alike in physical conditions, we generally find a 

 difference resulting from the absence of certain kinds in one which 

 exist in the other, and vice versa ; moreover, these may be kinds, 

 which when introduced into the country where they were wanting will 

 flourish there with a luxuriance equalling that in their native habita- 

 tion. This shows it is not the physical or external conditions which 

 have prevented their growth, and we therefore ask why were not they 

 present at first in both ?" — P. 4. 



The author in continuation briefly alludes to the opinion now en- 

 tertained by many, that the earth has gradually become clothed with 

 vegetation " partly by the spreading of some special kinds from cen- 

 tres within those countries where they were originally exclusively 

 created, and while these have spread outward into the neighbouring 

 regions, colonists from like centres lying in the surrounding countries 

 have invaded and become intermingled with the indigenous inhabi- 

 tants." Thus, by the side of the climatic and other physical influ- 

 ences exist " a second and totally different set of conditions, which 

 must be thoroughly investigated before we can clearly understand the 

 manner in which the vegetable inhabitants of the world have acquired 

 their present positions and relations toward each other." 



Another influence controlling the distribution of plants, and that a 

 most powerful one, is the agency of man. Mr. Henfrey truly states 

 that " Important as the eflfects of climate are, and not to be combated 

 beyond a certain point, yet is the struggle with the elements vigo- 

 rously sustained, above all in the north of Europe ; curious and 

 striking as are the phenomena of the spontaneous migrations of plants, 

 they sink into insignificance beside the operations of man's improving 

 hand. Barren plains are forced to yield a crop of food-plants ; bogs 

 and marshes are drained and turned into arable land ; vast tracts 

 wrested from the sea and brought under the dominion of the husband- 

 man. The bare escarpments of the rocky banks of the rivers of Ger- 

 many are terraced by the patient hand of industry, and converted into 

 smiling vineyards. The whole face of the more populous countries 

 is changed, and a view of the vegetation of Europe would be defi- 

 cient in some of its most attractive and important features, without a 

 sketch of the distribution and characters of the cultivated plants." — 

 P. 9. 



A slight allusion follows to the "history of the changes which have 



