lines proceeding from (A) were removed, excepting 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 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
transmitting unaltered descendants; and this is shown in the diagram by
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