52 8PECIE8 OF LAliGER GENERA VARIABLE. 



larger genera (i. e., those iucludiug 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 in- 

 habiting any country, shows that there is something in 

 the organic or inorganic conditions of that country favor- 

 able to the genus; and, consequently, we might have ex- 

 pected to have found in the larger genera, or those includ- 

 ing 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 dif- 

 fused, 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 wherever many closely related species (i.e., species of 

 the same genus) have been formed, many varieties or incip- 

 ient species ought, as a general rule, to be now forming. 

 Where many large trees grow, we expect to find saplings. 

 Where many species of a genus have been formed through 

 variation, circumstances have been favorable for variation; 

 and hence we might expect that the circumstances would 

 generally still be favorable to variation. On tlie other 

 hand, if we look at each species as a special act of creation, 

 there is no apparent reason why more varieties should 

 occur in a group having many species, than in one having 

 few. 



To test the truth of this anticipation I have arranged 



