CHAPTER X - ON THE IMPERFECTION
OF THE GEOLOGICAL RECORD
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 intermediate varieties, which have formerly existed, 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 serious objection which can be urged against
the 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 the theory, have formerly existed. I
have found it difficult, when looking at any two species, to avoid
picturing to myself forms directly intermediate between them. But this
is a wholly false view; we should always look for forms intermediate
between each species and a common but unknown progenitor; and the
progenitor will generally have differed in some respects from all
its modified descendants. To give a simple illustration: the fantail
and pouter pigeons are both descended from the rock-pigeon; if we
possessed all the intermediate varieties which have ever existed, we
should have an extremely close series between both and the
rock-pigeon; but we should have no varieties directly intermediate
between the fantail and pouter; none, for instance, combining a tail
somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have
become so much modified, that, if we had no historical or indirect
evidence regarding their origin, it would not have been possible to
have determined, from a mere comparison of their structure with that
of the rock-pigeon, C. livia, whether they had descended from this
species or from some allied form, such as C. aenas.
So, with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links directly intermediate between them ever existed, but between
each and an unknown common parent. The common parent will have had
in its whole organisation much general resemblance to the tapir and to
the horse; but in some points of structure may have differed
considerably from both, even perhaps more than they differ from each
other. Hence, in all such cases, we should be unable to recognise
the parent-form of any two or more species, even if we closely
compared the structure of the parent with that of its modified
descendants, unless at the same time we had a nearly perfect chain
of the intermediate links.
It is just possible by theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case direct intermediate links will have existed between them.
But such a case would imply that one form had remained for a very long
period unaltered, whilst its descendants had undergone a vast amount
of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare
event; for in all cases the new and improved forms of life tend to
supplant the old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the natural and domestic varieties of
the same species at the present day; and these parent-species, now
generally extinct, have in their turn been similarly connected with
more ancient forms; and so on backwards, always converging to the
common ancestor of each great class. So that the number of
intermediate and transitional links, between all living and extinct
species, must have been inconceivably great. But assuredly, if this
theory be true, such have lived upon the earth.
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