and a supply of sediment of a
different nature will generally have been due to geographical changes
requiring much time. Nor will the closest inspection of a formation give
any idea of the time which its deposition has consumed. Many instances
could be given of beds only 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 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, i