(A) were removed, excepting that from a1 to a10. In the
same way the English racehorse 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; 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 doubtful or at least 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 any genus, the
species which are already very different in character from each
other, will generally tend to produce the greatest number of modified
descendants; for these will have the best chance of seizing on 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 long but unequal periods continue to transmit
unaltered descendants; and this is shown in the diagram by the do