During these latter periods there will
probably be more variability in the forms of life; during periods of
subsidence, more extinction.
With respect to the absence of strata rich in fossils beneath the Cambrian
formation, I can recur only to the hypothesis given in the tenth chapter;
namely, that though our continents and oceans have endured for an enormous
period in nearly their present relative positions, we have no reason to
assume that this has always been the case; consequently formations much
older than any now known may lie buried beneath the great oceans. With
respect to the lapse of time not having been sufficient since our planet
was consolidated for the assumed amount of organic change, and this
objection, as urged by Sir William Thompson, is probably one of the gravest
as yet advanced, I can only say, firstly, that we do not know at what rate
species change, as measured by years, and secondly, that many philosophers
are not as yet willing to admit that we know enough of the constitution of
the universe and of the interior of our globe to speculate with safety on
its past duration.
That the geological record is imperfect all will admit; but that it is
imperfect to the degree required by our theory, few will be inclined to
admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the manner
required by the theory, for they have changed slowly and in a graduated
manner.
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