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 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.
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