64 SPECIES OF LARGER GENERA VARIABLE. Chap. II. 



spect to cominoiinoss or the number of individuals of any spe- 

 cies, the comparison, of course, relates only to the members of 

 the same group. A plant may be said to be dominant if it 

 be more numerous in individuals and more widely diffused than 

 the other plants of the same country, not livini^^ under widely- 

 different conditions of Hfe. Such a plant is not the less domi- 

 nant in the sense here used, because some conferva inhabiting 

 the water or some parasitic fungus is infinitely more numerous 

 in individuals, and more widely diffused ; if one kind of conferva 

 or parasitic fungus exceeded its allies in the above respects, 

 it would be a dominant form within its own class. 



JSjjecies of the Larger Genera in each Country vary more 

 frequently than the Species of the Smaller Genera. 



If the plants inhabiting a country and descrilicd in any 

 Flora be divided into two ecjual masses, all those in the larger 

 genera (i. e., those including many species) being placed on 

 one side, and all those in the smaller genera on the other side, 

 a somewhat larger number of the very common and much-dif- 

 fused or dominant species will be found on the side of the 

 larger genera. This, again, might have been anticipated ; for 

 the mere fact of many species of the same genus inhabiting any 

 country, shows that there is something in the organic or inor- 

 ganic conditions of that country favorable to the genus ; and, 

 consequently, we might have expected to have found in the 

 larger genera, or those including many species, a large propor- 

 tional number of dominant species. But so many causes tend 

 to obscure this result, that I am surprised that my tables show 

 even a small majority on the side of the larger genera. I will 

 here allude to only two causes of obscurity. Fresh-water and 

 salt-loving plants have generally very wide ranges and are 

 much diffused, but this seems to be connected with the nature 

 of the stations inhabited by them, and has little or no relation 

 to the size of the genera to which the species belong. Again, 

 plants low in the scale of organization are generally much 

 more widely diffused than plants higher in the scale ; and here, 

 again, there is no close relation to the size of the genera. Tlie 

 cause of lowly-organized plants ranging widely will be discussed 

 in our chapter on Geographical Distribution. 



From looking at species as only strongly-marked and well- 

 defined varieties, I was led to anticipate tliat the species of the 

 larger genera in each country would oftener present varieties, 



