to light multitudes of facts of the same order; and
at the present day, the investigations of such anatomists as Rutimeyer
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.

As we trace the history of birds back in time, we find their remains,
sometimes in great abundance, throughout the whole extent of the
tertiary rocks; but, so far as our present knowledge goes, the birds of
the tertiary rocks retain the same essential characters as the birds
of the present day. In other words, the tertiary birds come within the
definition of the class constituted by existing birds, and are as much
separated from reptiles as existing birds are. Not very long ago no
remains of birds had been found below the tertiary rocks, and I am not
sure but that some persons were prepared to demonstrate that they could
not have existed at an earlier period. But, in the course of the
last few years, such remains have been discovered in England; though,
unfortunately, in so imperfect and fragmentary a