rmediate 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.

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