lruses and the eared seals are intercalary forms between the
fissipede Carnivora and the ordinary seals, so the Zeuglodonts are
intercalary between the _Carnivora_, as a whole, and the _Cetacea_.
Whether the Zeuglodonts are also linear types in their relation to
these two groups cannot be ascertained, until we have more definite
knowledge than we possess at present, respecting the relations in time
of the _Carnivora_ and _Cetacea_.

Thus far we have been concerned with the intercalary types which
occupy the intervals between Families or Orders of the same class; but
the investigations which have been carried on by Professor Gegenbaur,
Professor Cope, and myself into the structure and relations of the
extinct reptilian forms of the _Ornithoscelida_ (or _Dinosauria_ and
_Compsognatha_) have brought to light the existence of intercalary
forms between what have hitherto been always regarded as very distinct
classes of the vertebrate sub-kingdom, namely _Reptilia_ and _Aves_.
Whatever inferences may, or may not, be drawn from the fact, it is now
an established truth that, in many of these _Ornithoscelida_, the hind
limbs and the pelvis are much more similar to those of Birds than
they are to those of Reptiles, and that these Bird-reptiles, or
Reptile-birds, were more or less completely bipedal.

When I addressed you in 1862, I should have been bold indeed had I
suggested that palaeontology would before long show us the possibility
of a direct transition from the type of the lizard to that of the
ostrich. At the present moment we have, in the _Ornithoscelida_, the
intercalary type, which proves that transition to be something more
than a possibility; but it is very doubtful whether any of the genera
of _Ornithoscelida_ with which we are at present acquainted are the
actual linear types by which the transition from the lizard to the
bird was effected. These, very probably, are still hidden from us in
the older formations.

Let us now endeavour to find some cases of true linear types, or forms
which are intermediate between others because they stand in a direct
genetic relation to them. It is no easy matter to find clear and
unmistakable evidence of filiation among fossil animals; for, in order
that such evidence should be quite satisfactory, it is necessary that
we should be acquainted with all the most important features of the
organization of the animals which are supposed to be thus related, and
not merely with the fragm