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. {119} 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 {120}
of divergent modification may have been increased in the successive
generations. This case would be represented in the diagram, if all the
lines proceeding from (A) were removed, exceptin