VARIATION UNDER NATURE 9l 



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 (^.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 

 somewhat larger number of the very common and much 

 diffused or dominant species. This might have been an- 

 ticipated; for the mere fact of many species of the same 

 genus inhabiting any country, shows that there is some- 

 thing in the organic or inorganic conditions of that coun- 

 try 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 num- 

 ber 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 obscu- 

 rity. 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 



