1843—1882] N. AMERICAN FLORA 423 



America, but if so, for comparison with other parts of world, Letter 325 

 I would exclude the Arctic and Alpine- Arctic, as belonging to 

 a quite distinct category. When excluding the naturalised, 

 I think De Candolle must be right in advising the exclusion 

 (giving list) of plants exclusively found in cultivated land, even 

 when it is not known that they have been introduced by man. 

 I would give list of temperate plants (if any) found in Eastern 

 Asia, China, and Japan, and not elsewhere. Nothing would 

 eive me a better idea of the flora of United States than the 

 proportion of its genera to all the genera which are confined 

 to America ; and the proportion of genera confined to America 

 and Eastern Asia with Japan ; the remaining genera would 

 be common to America and Europe and the rest of world ; I 

 presume it would be impossible to show any especial affinity 

 in genera, if ever so few, between America and Western 

 Europe. America might be related to Eastern Asia (always 

 excluding Arctic forms) by a genus having the same species 

 confined to these two regions ; or it might be related by the 

 genus having different species, the genus itself not being found 

 elsewhere. The relation of the genera (excluding identical 

 species) seems to me a most important element in geographical 

 distribution often ignored, and I presume of more difficult 

 application in plants than in animals, owing to the wider 

 ranges of plants ; but I find in New Zealand (from Hooker) 

 that the consideration of genera with representative species 

 tells the story of relationship even plainer than the identity of 

 the species with the different parts of the world. I should 

 like to see the genera of the United States, say 500 (exclud- 

 ing Arctic and Alpine) divided into three classes, with the 

 proportions given thus : — 



375$ American genera ; 



fgs Old World genera, but not having any identical 



species in common ; 

 If!" Old World genera, but having some identical 



species in common ; 



Supposing that these 200 genera included 600 U.S. plants, 

 then the 600 would be the denominator to the fraction of 

 the species common to the Old World. But I am running 

 on at a foolish length. 



There is an interesting discussion in De Candolle (about 



