44 SPECIES OF LARGER GENERA VARIABLE. [Chap. If. 



Species of the Larger Genera in each Country vary more frequently 

 than the Sjjecies 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 somewhat larger number of the very common and 

 much diffused or dominant species. This might have been antici- 

 pated ; for the mere fact of many species of the same genus inhabit- 

 ing 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 wherever many closely related species (i.e., 

 species of the same genus) have been formed, many varieties or 

 incipient 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, cir- 

 cumstances have been favourable for variation ; and hence we might 

 expect that the circumstances would generally be still favourable 

 to variation. On the 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 tew. 



To test the truth of this anticipation I have arranged the plants 

 of twelve countries, and the coleopterous insects of two districts, into 

 two nearly equal masses, the species of the larger genera on one 



