be closely connected by intermediate varieties
with either one or both forms. 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. If we look to rather wider intervals
of time, namely, to distinct but consecutive stages of the same great
formation, we find that the embedded fossils, though universally ranked
as specifically different, yet are far more closely related to each
other than are the species found in more widely separated formations;
so that here again we have undoubted evidence of change in the direction
required by the theory; but to this latter subject I shall return in the
following chapter.

With animals and plants that propagate rapidly and do not wander
much, there is reason to suspect, as we have formerly seen, that their
varieties are generally at first local; and that such local varieties
do not spre