280 
IMPERFECTION OF THE 
Chap. IX. 
pends 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 geo¬ 
logical formation and every stratum full of such inter¬ 
mediate 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 de¬ 
scendants. To give a simple illustration: the fantail and 
pouter pigeons have both descendedfrom 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 some¬ 
what 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 
