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 dotted lines unequally prolonged upwards.
But during the process of modification, represented in the diagram, another
of our principles, namely that of extinction, will have played an important
part. As in each fully stocked country natural selection necessarily acts
by the selected form having some advantage in the struggle for life over
other forms, there will be a constant tendency in the improved descendants
of any one species to supplant and exterminate in each stage of descent
their predecessors and their original progenitor.
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