Yet he who objected to
consider as intermediate the extinct genera, which thus link together the
living genera of three families, would be partly justified, for they are
intermediate, not directly, but only by a long and circuitous course
through many widely different forms. If many extinct forms were to be
discovered above one of the middle horizontal lines or geological
formations--for instance, above No. VI.--but none from beneath this line,
then only two of the families (those on the left hand a14, etc., and b14,
etc.) would have to be united into one; and there would remain two families
which would be less distinct from each other than they were before the
discovery of the fossils. So again, if the three families formed of eight
genera (a14 to m14), on the uppermost line, be supposed to differ from each
other by half-a-dozen important characters, then the families which existed
at a period marked VI would certainly have differed from each other by a
less number of characters; for they would at this early stage of descent
have diverged in a less degree from their common progenitor. Thus it comes
that ancient and extinct genera are often in a greater or less degree
intermediate in character between their modified descendants, or between
their collateral relations.
Under nature the process will be far more complicated than is represented
in the diagram; for the groups will have been more numerous; they will have
endured for extremely unequal lengths of time, and will have been modified
in various degrees.
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