t 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.

One other consideration is worth notice: with animals and plants that
can propagate rapidly and are not highly locomotive, 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 spread widely and
supplant their parent-forms until they have been modified and perfected
in some considerable degree. According to this view, the chance of
discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes
are supposed to have been local or confined to some one spot. Most
marine animals have a wide range; and we have seen that with plants it
is those which have the widest range, that oftenest present varieties;
so that with shells and other marine animals, it is probably those
which have had the widest range, far exceeding the limits of the known
geological formations of Europe, which have