Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation,--facts, showing what wide, yet easily overlooked,
intervals have occurred in its accumulation. In other cases we have
the plainest evidence 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 never even have
been suspected, had not the trees chanced to have been preserved:
thus, Messrs. Lyell and Dawson found carboniferous beds 1400 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 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.
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