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1872

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1872

we have seen 1859 1860 1861 1866 1869
as we saw 1872

of the species of 1859 1860 1861 1866 1869
species vary in 1872

rare species with restricted ranges. 1859 1860 1861
do the rare and restricted species. 1866 1869 1872

little fan of 1859 1860 1861
branching and 1866 1869 1872

have formed 1859 1860 1861 1866 1869
form it into 1872

generations; but it would have been better if each had represented ten thousand 1859 1860 1861 1866
generations, or ten thousand. 1869
or more 1872

genus, because we have seen in the second chapter,
that
that
on an average more of the species of large genera
vary
vary
than
in
of
small genera; and the varying species of the large genera present a greater number of varieties. We have, also, seen that the species, which are the commonest and the most
widely-diffused,
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
nor
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
being
derived from divergence of character comes in; for this will generally lead to the most different or divergent variations
(repre- sented
(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.
generations.
After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely
a 1
a 1
and m 1 . These two varieties will generally
still
continue to
be exposed to