g that from a^1 to a^{10}.
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, a^{10}, f^{10}, and m^{10}, 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 a^{14} and m^{14}, 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 (w^{10} and z^{10}) or two species, according to the amount of
change supposed to be represented {121} between the horizontal lines. After
fourteen thousand generations, six new species, marked by the letters
n^{14} to z^{14}, 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 best chance of filling new and widely different places in the
polity of nature: hence in the diagram I have chosen the extreme species
(A), and the nearly extreme species (I), as those which have largely
varied, and have given rise to new varieties and species. The other nine
species (marked by capital letters) of our original genus, may for a long
period continue to transmit unaltered descendants;