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 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
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
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