Nor should it be forgotten, as before explained, that A might be the actual
progenitor of B and C, and yet would not necessarily be strictly
intermediate between them in all respects. So that we might obtain the
parent-species and its several modified descendants from the lower and
upper beds of the same formation, and unless we obtained numerous
transitional gradations, we should not recognise their blood-relationship,
and should consequently rank them 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 the theory we
ought to find. Look again at the later tertiary deposits, which include
many shells believed by the majority of naturalists to be identical with
existing species; but some excellent naturalists, as Agassiz and Pictet,
maintain that all these tertiary species are specifically distinct, though
the distinction is admitted to be very slight; so that here, unless we
believe that these eminent naturalists have been misled by their
imaginations, and that these late tertiary species really present no
difference whatever from their living representatives, or unless we admit,
in opposition to the judgment of most naturalists, that these tertiary
species are all truly distinct from the recent, we have evidence of the
frequent occurrence of slight modifications of the kind required.
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