e of extinct intermediate varieties; on their number--On the vast
lapse of time, as inferred from the rate of deposition and of
denudation--On the poorness of our palaeontological collections--On the
intermittence of geological formations--On the absence of intermediate
varieties in any one formation--On the sudden appearance of groups of
species--On their sudden appearance in the lowest known fossiliferous
strata.

In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them have
now been discussed. One, namely the distinctness of specific forms, and
their not being blended together by innumerable transitional links, is a
very obvious difficulty. I assigned reasons why such links do not commonly
occur at the present day, under the circumstances apparently most
favourable for their presence, namely on an extensive and continuous area
with graduated physical conditions. I endeavoured to show, that the life of
each species depends in a more important manner on the presence of other
already defined organic forms, than on climate; and, therefore, that the
really governing conditions of life do not graduate away quite insensibly
like heat or moisture. I endeavoured, also, to show that intermediate
varieties, from existing in lesser numbers than the forms which they
connect, will generally be beaten out and exterminated during the course of
further modification and improvement. The main cause, however, of
innumerable intermediate links not now occurring everywhere throughout
nature {280} depends on the very process of natural selection, through
which new varieties continually take the places of and exterminate their
parent-forms. But just in proportion as this process of extermination has
acted on an enormous scale, so must the number of intermediate varieties,
which have formerly existed on the earth, be truly enormous. Why then is
not every geological formation and every stratum full of such intermediate
links? Geology assuredly does not reveal any such finely graduated organic
chain; and this, perhaps, is the most obvious and gravest objection which
can be urged against my theory. The explanation lies, as I believe, in the
extreme imperfection of the geological record.

In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have found
it difficult