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.
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