} 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 {299} 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 oftenest given rise, first to local varieties and ultimately to
new species; and this again would greatly lessen the chance of our being
able to trace the stages of transition in any one geological formation.

It should not be forgotten, that at the pre