produce any. 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.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake of the same
advantages which made their parent successful in life, they will
generally go on multiplying in number as well as diverging in
character: this is represented in the diagram by the several divergent
branches proceeding from (A). The modified offspring from the later and
more highly improved branches in the lines of descent, will, it is
probable, often take the place of, and so destroy, the earlier and less
improved branches: this is represented in the diagram by some of the
lower branches not reaching to the upper horizontal lines. In some
cases I do not doubt that the process of modification will be confined
to a single line of descent, and the number of the descendants will not
be increased; although the amount
of divergent modification may have been increased in the successive
generations. This case would be represented in the diagram, if all the
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