VARIATION UNDER NATURE 51 



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. 



l'rom 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, circum- 

 stances 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 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 pro- 

 portion of the species on the side of the larger genera 

 present 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, and when all the smallest genera, 

 with from only one to four species, are absolutely 

 excluded from the tables. These facts are of plain 

 signification on the view that species are only strongly- 

 marked and permanent varieties ; for wherever many 

 species of the same genus have been formed, or where, 

 if we may use the expression, the manufactory of 

 species has been active, we ought generally to find 



