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. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely _a_1 and _m_1. These two varieties will
generally continue to be exposed to the same conditions which made
their parents variable,
and the tendency to variability is in itself hereditary, consequently
they will tend to vary, and generally to vary in nearly the same manner
as their parents varied. Moreover, these two varieties, being only
slightly modified forms, will tend to inherit those advantages which
made their common parent (A) more numerous than most of the other
inhabitants of the same country; they will likewise partake of those
more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And these
circumstances we know to be favourable to the production of new
varieties.
If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety _a_1 is supposed in the
diagram to have produced variety _a_2, which will, owing to the
principle of divergence, differ more from (A) than did variety _a_1.
Variety _m_1 is supposed to have produced two varieties, namely _m_2
and _s_2, differing from each other, and more considerably from their
common parent (A). We may continue the process by similar steps for any
length of time; some of the varieties, after each thousand generations,
producing only a single variety, but in a more and more modified
condition, some producing two or three varieties, and some failing to
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