DISTRIBUTION OF PLANTS. 283 



assertion deserves no particular refutation. Streams, indeed, 

 can carry down seeds ; and plants from those higher regions 

 through which the streams flow are accordingly often found 

 growing on their banks. But the Flora on the banks of one 

 and the same stream, is very different in the different districts 

 through which it passes, as is seen in the clearest manner up- 

 on the shores of the Elbe ; for in Bohemia very different 

 plants appear from those which spring in the neighbourhood 

 of Dresden, — others, again, make their appearance near Wit- 

 tenberg and Barby, — and a yet different set near Lauenburgh 

 and Hamburgh. 



These considerations lead us to conclude, that the vege- 

 table world has neither descended from one common birth- 

 place, nor diffused itself from one country into another ; but 

 that vegetation is in every case the product of the joint in- 

 fluence of temperature, soil, and the particular composition 

 of the moisture of the earth. 



Nor is the conclusion of Brown (on Congo, p. 50.), that 

 the native country of a genus is always where the greatest 

 variety of species is found, by any means to be admitted, 

 since the example of Nicotiana shews the contrary. The 

 greatest number of its species are found in South America ; 

 yet the Nicotiana Chinensis, Lehm. txxid Jruticosa are cer- 

 tainly indigenous to Eastern Asia. 



CHAP. VI. 



ON MALFORMATIONS AND DISEASES OF PLANTS. 



Linne, Philosophia botanica. S. 119, 131. 

 Jager, Uber die Missbildungcn der Gewachsc. 

 Gallesio, Theoric der vegetabilischen Reproduction. 

 Keith'3 System of Physiological Botany. 



