creasing 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, nor that it goes on continuously; it is far more
probable that each form remains for long periods unaltered, and then
again undergoes modification. 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. In some cases no
doubt the process of modification will be confined to a single line of
descent, and the number of modified descendants will not be increased;
although the amount of divergent modification may have been augmented.
This case would be represented in the diagram, if all the lines
proceeding from