0, vi. p. 414 he uses his
newly-acquired knowledge of pigeons to illustrate this point.

{310} Compare the _Origin_, Ed. i. p. 281, vi. p. 414.

What evidence{311} is there of a number of intermediate forms having
existed, making a passage in the above sense, between the species of the
same groups? Some naturalists have supposed that if every fossil which
now lies entombed, together with all existing species, were collected
together, a perfect series in every great class would be formed.
Considering the enormous number of species requisite to effect this,
especially in the above sense of the forms not being _directly_
intermediate between the existing species and genera, but only
intermediate by being linked through a common but often widely different
ancestor, I think this supposition highly improbable. I am however far
from underrating the probable number of fossilised species: no one who
has attended to the wonderful progress of palaeontology during the last
few years will doubt that we as yet have found only an exceedingly small
fraction of the species buried in the crust of the earth. Although the
almost infinitely numerous intermediate forms in no one class may have
been preserved, it does not follow that they have not existed. The
fossils which have been discovered, it is important to remark, do tend,
the little way they go, to make good the series; for as observed by
Buckland they all fall into or between existing groups{312}. Moreover,
those that fall between our existing groups, fall in, according to the
manner required by our theory, for they do not directly connect two
existing species of different groups, but they connect the groups
themselves: thus the Pachydermata and Ruminantia are now separated by
several characters, <for instance> the Pachydermata{313} have both a
tibia and fibula, whilst Ruminantia have only a tibia; now the fossil
Macrauchenia has a leg bone exactly intermediate in this respect, and
likewise has some other intermediate characters. But the Macrauchenia
does not connect any one species of Pachydermata with some one other of
Ruminantia but it shows that these two groups have at one time been less
widely divided. So have fish and reptiles been at one time more closely
connected in some points than they now are. Generally in those groups in
which there has been most change, the more ancient the fossil, if not
identical with recent, the more often it falls between existing groups