ountry, 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 large 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 have
generally 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 {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 va