Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick
in Nova Scotia, with ancient root-bearing strata, one above the other, at
no less than sixty-eight different levels. Hence, when the same species
occurs at the bottom, middle, and top of a formation, the probability is
that it has not lived on the same spot during the whole period of
deposition, but has disappeared and reappeared, perhaps many times, during
the same geological period. Consequently if it were to undergo a
considerable amount of modification during the deposition of any one
geological formation, a section would not include all the fine intermediate
gradations which must on our theory have existed, but abrupt, though
perhaps 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 the closest
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 older and 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
closely connected by intermediate varieties with either one or both forms.
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