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in great fossilised trees, still standing upright as they grew, of many long intervals of time and changes of level during the process of deposition, which would
have been
had not the trees
→chanced to have
been preserved:
Lyell and
found carboniferous beds 1400 feet thick in Nova Scotia, with ancient root-bearing strata, one above the
at no less than sixty-eight different levels. Hence, when the same species
at the bottom, middle, and top of a formation, the probability is that
not lived on the same spot during the whole period of deposition, but
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
a section would not
include all the fine intermediate gradations which must on
theory have
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
intermediate
from the reasons just
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
closely connected
→with
either one or both
Nor should it be forgotten, as before explained, that A might be the actual progenitor
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