SPECIES OF LARGER GENERA VARIABLE. 49 



game conditions. A plant of this kind is not the less domi- 

 nant because some conferva inhabiting the water or some 

 parasitic fungus is infinitely more numerous in individuals, 

 and more widely diffused. But if the conferva or parasitic 

 fungus exceeds its allies in the above respects, it will then 

 be dominant within its own class. 



SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY 

 MORE FREQUENTLY THAN THE SPECIES OF THE SMALLER 

 GENERA. 



If the plants inhabiting a country, as described in any 

 Flora, be divided into two equal 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, the former will be found to include a some- 

 what larger number of the very common and much diffused 

 or dominant species. This 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 

 inorganic 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 larger 

 proportional 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 generally 

 have 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. The 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 that the species 

 of the larger genera in each country would oftener present 

 varieties, than the species of the smaller genera ; for wher- 

 ever many closely related species (i. e., species of the same 

 genus) have been formed, many varieties or incipient species 

 pu.gbtj as a general rnle ? to be. now forming f^bere many 



