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. If we
suppose the amount of change between each horizontal line in our diagram
to be excessively small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful category of
sub-species; but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to convert these
three forms into well-defined species: thus the diagram illustrates
the steps by which the small differences distinguishing varieties
are increased into the larger differences distinguishing species. By
continuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we get eight
species, marked by the letters between a14 and m14, all descended from
(A). Thus, as I believe, species are multiplied and genera are formed.

In a large genus it is probable that more than one species would vary.
In the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of
change supposed to be represented between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n14 to z14, are supposed to have been produced. In each genus, the
species, which are already extremely different in character, will
generally tend to produce the greatest number of modified descendants;
for these will have the be