Chap. IX. 
GEOLOGICAL KECORD. 
297 
in great fossilised trees, still standing upriglit 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 geo¬ 
logical 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 va¬ 
rieties ; 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 to¬ 
gether 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 
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