CHAP. II.] SPECIES OF LARGER GENERA VARIABLE. 41 



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 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 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, circumstances have been favourable for varia- 

 tion ; 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 few. 



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 side, and those of the smaller genera on the other side, and it 

 has invariably proved to be the case that a larger proportion of 

 the species on the side of the larger genera presented varieties, 

 than on the side of the smaller genera. Moreover, the species of 

 the large genera which present any varieties, invariably present 

 a larger average number of varieties than do the species of the 

 small genera Both these results follow when another division 

 is made, anc' when all the least genera, with from only one to four 



