the other, at no less than sixty-eight different levels. Hence, when
the same species occur at the bottom, middle, and top of a formation,
the probability is that they have not lived on the same spot during the
whole period of deposition, but have disappeared and reappeared,
perhaps many times, during the same geological period. So that if such
species were to undergo a considerable amount of modification during
any one geological period, a section would not probably include all the
fine intermediate gradations which must on my theory have existed
between them, but abrupt, though perhaps very slight, changes of form.
It is all-important to remember that naturalists have
no golden rule by which to distinguish species and varieties; they
grant some little variability to each species, but when they meet with
a somewhat greater amount of difference between any two forms, they
rank both as species, unless they are enabled to connect them together
by close intermediate gradations. 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
pal ontologists 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,
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