links less likely. Local varieties will not spread into other and
distant regions until they are considerably modified and improved;
and when they do spread, if discovered in a geological formation, they
will appear as if suddenly created there, and will be simply classed as
new species. Most formations have been intermittent in their
accumulation; and their duration, I am inclined to believe, has been
shorter than the average duration of specific forms. Successive
formations are separated from each other by enormous blank intervals of
time; for fossiliferous formations, thick enough to resist future
degradation, can be accumulated only where much sediment is deposited
on the subsiding bed of the sea. During the alternate periods of
elevation and of stationary level the record will be blank. 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 fossiliferous formations beneath the
lowest Silurian strata, I can only recur to the hypothesis given in the
ninth chapter. That the geological record is imperfect all will admit;
but that it is imperfect to the degree which I require, few will be
inclined to admit. If we look to long enough intervals of time, geology
plainly declares that all species have changed; and they have changed
in the manner which my theory requires, for they have changed slowly
and in a graduated manner. We clearly see this in the fossil remains
from consecutive formations invariably being much more closely related
to each other, than are the fossils from formations distant from each
other in time.
Such is the sum of the several chief objections and difficulties which
may justly be urged against my theory; and I have now briefly
recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to
doubt their weight. But it deserves especial notice that the more
important objections relate to questions on which we are confessedly
ignorant; nor do we know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and the most
perfect organs; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that we know
how imperfect the Geological Record is. Grave as these several
difficulties are, in my judgment they do not overthrow the theory of
descent with modification.
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