dure 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
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 form it into 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 or more 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 still
be exposed to the same conditions which made their parents variable, and
the tendency to variability is in itself hereditary; consequently they
will likewise tend to vary, and commonly in nearly the same manner as
did their parents. Moreover, these two varieties, being only slightly
modified forms, will tend to inherit those advantages which made their
parent (A) more numerous than most of the other inhabitants of the same
country; they will also partake of those more general advantages which
made the genus to which the parent-species belonged, a large genus
in its own country. And all these circumstances are 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 two varieties, namely m2 and s2, 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 produce any. Thus
the varieties or modified descendants of the common parent (A), will
generally go on in