SPECIES OF LARGER GENERA VARIABLE 71 



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 favourable 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 organisation 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- 

 organised 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 spe- 

 cies ought, as a general rule, to be now forming. Where 

 m.any large trees grow, we expect to find saplings. Where 

 many species of a genus have been formed through varia- 

 tion, circumstances have been favourable for variation; and 



