Chap. II. SPECIES OF LARGE GENERA VARIABLE. 55 
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 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 proportion 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 spe¬ 
cies, are absolutely excluded from the tables. These 
