elonging 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. After a thousand generations,
species (A) is supposed to have produced two fairly well-marked
varieties, namely a1 and m1. 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 a1 is supposed in the
diagram to have produced variety a2, which will, owing to the principle
of divergence, differ more from (A) than did variety a1. Variety m1 is
supposed to have produced