44 THE ORIGIN OF SPECIES 



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 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 something in the or- 

 ganic 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 inhab- 

 ited 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 or- 

 ganization 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 Distri- 

 bution. 



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 re- 

 lated species (*. 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 sap- 

 lings. 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 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 few. 



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

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



