restored by the great French palæontologist, the _Palæotherium_,
similarly tended to connect together animals to all appearance so
different as the rhinoceros, the horse, and the tapir. Subsequent
research has brought to light multitudes of facts of the same order;
and, at the present day, the investigations of such anatomists as
Rütimeyer and Gaudry have tended to fill up, more and more, the gaps in
our existing series of mammals, and to connect groups formerly thought
to be distinct.

But I think it may have an especial interest if, instead of dealing with
these examples, which would require a great deal of tedious osteological
detail, I take the case of birds and reptiles; groups which, at the
present day, are so clearly distinguished from one another that there
are perhaps no classes of animals which, in popular apprehension, are
more completely separated. Existing birds, as you are aware, are covered
with feathers; their anterior extremities, specially and peculiarly
modified, are converted into wings, by the aid of which most of them are
able to fly; they walk upright upon two legs; and these limbs, when they
are considered anatomically, present a great number of exceedingly
remarkable peculiarities, to which I may have occasion to advert
incidentally as I go on, and which are not met with, even approximately,
in any existing forms of reptiles. On the other hand, existing reptiles
have no feathers. They may have naked skins, or be covered with horny
scales, or bony plates, or with both. They possess no wings; they
neither fly by means of their fore-limbs, nor habitually walk upright
upon their hind-limbs; and the bones of their legs present no such
modifications as we find in birds. It is impossible to imagine any two
groups more definitely and distinctly separated, notwithstanding certain
characters which they possess in common.