We have, also, seen that the
species, which are the commonest and the most widely-diffused, vary
more than rare species with restricted ranges. Let (A) be a common,
widely-diffused, and varying species, belonging to a genus large in
its own country. The little fan of diverging dotted lines of unequal
lengths proceeding from (A), may represent its varying offspring. The
variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear
simultaneously, but often after long intervals of time; nor are they
all supposed to endure for equal periods. Only those variations which
are in some way profitable will be preserved or naturally selected.
And here the importance of the principle of benefit being derived from
divergence of character comes in; for this will generally lead to the
most different or divergent variations (represented by the outer
dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation is
supposed to have been accumulated to have formed a fairly well-marked
variety, such as would be thought worthy of record in a systematic
work.
The intervals between the horizontal lines in the diagram, may
represent each a thousand generations; but it would have been better
if each had represented ten thousand generations.
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