nly a
few feet in thickness, representing formations, 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 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
{297} 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. 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 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 most closely
connected with either one or both forms by intermediate varieties. Nor
should it be forgotten, as before explained, that A might be the actual
progenitor {298