Thus the varieties or modified descendants,
proceeding from the common parent (A), will generally go on increasing
in number and diverging in character. In the diagram the process is
represented up to the ten-thousandth generation, and under a condensed
and simplified form up to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in
itself made somewhat irregular. I am far from thinking that the most
divergent varieties will invariably prevail and multiply: a medium
form may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not
perfectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will be increased. In our diagram the line of
succession is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently distinct
to be recorded as varieties. But these breaks are imaginary, and might
have been inserted anywhere, after intervals long enough to have
allowed the accumulation of a considerable amount of divergent
variation.
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