tween
existing species, but intermediate with a common parent. It was admitted
that if even all the preserved fossils and existing species were
collected, such a series would be far from being formed; but it was
shown that we have not _good_ evidence that the oldest known deposits
are contemporaneous with the first appearance of living beings; or that
the several subsequent formations are nearly consecutive; or that any
one formation preserves a nearly perfect fauna of even the hard marine
organisms, which lived in that quarter of the world. Consequently, we
have no reason to suppose that more than a small fraction of the
organisms which have lived at any one period have ever been preserved;
and hence that we ought not to expect to discover the fossilised
sub-varieties between any two species. On the other hand, the evidence,
though extremely imperfect, drawn from fossil remains, as far as it does
go, is in favour of such a series of organisms having existed as that
required. This want of evidence of the past existence of almost
infinitely numerous intermediate forms, is, I conceive, much the
weightiest difficulty{512} on the theory of common descent; but I must
think that this is due to ignorance necessarily resulting from the
imperfection of all geological records.

{511} Part II begins with Ch. IV. See the Introduction, where the
absence of division into two parts (in the _Origin_) is discussed.

{512} In the recapitulation in the last chapter of the _Origin_,
Ed. i. p. 475, vi. p. 651, the author does not insist on this point
as the weightiest difficulty, though he does so in Ed. i. p. 299.
It is possible that he had come to think less of the difficulty in
question: this was certainly the case when he wrote the 6th
edition, see p. 438.

In the fifth chapter it was shown that new species gradually{513}
appear, and that the old ones gradually disappear, from the earth; and
this strictly accords with our theory. The extinction of species seems
to be preceded by their rarity; and if this be so, no one ought to feel
more surprise at a species being exterminated than at its being rare.
Every species which is not increasing in number must have its
geometrical tendency to increase checked by some agency seldom
accurately perceived by us. Each slight increase in the power of this
unseen checking agency would cause a corresponding decrease in the
average numbers of that species, and