Nor do I suppose that the most divergent varieties
are invariably preserved: 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 increase. 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 allow 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.
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