two varieties, namely m2 and s2, differing
from each other, and more considerably from their common parent (A). We
may continue the process by similar steps for any length of time; some
of the varieties, after each thousand generations, producing only
a single variety, but in a more and more modified condition, some
producing two or three varieties, and some failing to 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