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.
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