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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
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
→and older bed;
even if A were strictly intermediate between
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