aracters from both parent-forms.
Finally, then, although we are as ignorant of the precise cause of the
sterility of first crosses and of hybrids as we are why animals and
plants removed from their natural conditions become sterile, yet the
facts given in this chapter do not seem to me opposed to the belief that
species aboriginally existed as varieties.

CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day--On the
nature of extinct intermediate varieties; on their number--On the lapse
of time, as inferred from the rate of denudation and of deposition
number--On the lapse of time as estimated by years--On the poorness of
our palaeontological collections--On the intermittence of geological
formations--On the denudation of granitic areas--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--Antiquity of the habitable earth.

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 depends, on the very
process of natural selection, through which new varieties continually
take the places of and supplant their parent-forms. But just in
proportion as this process of extermination has acted on an enormous
scale, so must the number of inte