en 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.

Now let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the reproductive
system being eminently susceptible to changes in the conditions of life;
so that this system, when not rendered impotent, fails to