And this from the reasons just assigned we
can seldom hope to effect in any one geological section. Supposing B
and C to be two species, and a third, A, to be found in an underlying
bed; even if A were strictly intermediate between B and C, it would
simply be ranked as a third and distinct species, unless at the same
time it could be most closely connected with either one or both forms
by intermediate varieties. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet
might not at all necessarily be strictly intermediate between them in
all points of structure. So that we might obtain the parent-species
and its several modified descendants from the lower and upper beds of
a formation, and unless we obtained numerous transitional gradations,
we should not recognise their relationship, and should consequently be
compelled to rank them all as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D'Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on my
theory we ought to find. Moreover, if we look to rather wider
intervals, namely, to distinct but consecutive stages of the same
great formation, we find that the embedded fossils, though almost
universally ranked as specifically different, yet are far more closely
allied to each other than are the species found in more widely
separated formations; but to this subject I shall have to return in
the following chapter.
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