re. 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 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, 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 have 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, 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 h