CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
1. ON THE ABSENCE OF INTERMEDIATE VARIETIES AT THE PRESENT DAY.
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 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 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 other allied
species, such as C. oenas.