interrupted. Nor will the closest inspection of a formation give us any
idea of the length of time which its deposition may have consumed.
Many instances could be given of beds, only a few feet in thickness,
representing formations which are elsewhere thousands of feet in
thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have even suspected
the vast lapse of time represented by the thinner formation. 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 not have been suspected, had not the trees
been preserved: thus Sir C. 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