CHAPTER II
VARIATION UNDER NATURE
Species of the Larger Genera in each Country vary more frequently
than the Species of the Smaller Genera
If the plants inhabiting a country, as described in any Flora, be
divided into two equal masses, all those in the larger genera (i.e.,
those including many species) being placed on one side, and 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 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 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, and
when all the least genera, with from only one to four species, are
altogether 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 the
manufactory still in action, more especially as we have every reason
to believe the process of manufacturing new species to be a slow
one. And this certainly holds true, if varieties be looked at as
incipient species; for my tables clearly show as a general rule
that, wherever many species of a genus have been formed, the species
of that genus present a number of varieties, that is of incipient
species, beyond the average. It is not that all large genera are now
varying much, and are thus increasing in the number of their
species, or that no small genera are now varying and increasing; for
if this had been so, it would have been fatal to my theory; inasmuch
as geology plainly tells us that small genera have in the lapse of
time often increased greatly in size; and that large genera have often
come to their maxima, declined, and disappeared. All that we want to
show is, that when many species of a genus have been formed, on an
average many are still forming; and this certainly holds good.
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