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 lines proceeding from (A) were
removed, excepting that from a1 to a10. In the same way, for instance,
the English race-horse and English pointer have apparently both gone
on slowly diverging in character from their original stocks, without
either having given off any fresh branches or races.
After ten thousand generations, species (A) is supposed to have
produced three forms, a10, f10, and m10, which, from having diverged
in character during the successive generations, will have come to
differ largely, but perhaps unequally, from each other and from their
common parent.
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