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and snake； nor between the snake and the crocodile； nor between
any two of these groups。 They are separated by absolute breaks。
If； then； it could be shown that this state of things had always
existed； the fact would be fatal to the doctrine of evolution。
If the intermediate gradations； which the doctrine of evolution
requires to have existed between these groups； are not to be
found anywhere in the records of the past history of the globe；
their absence is a strong and weighty negative argument against
evolution； while； on the other hand； if such intermediate forms
are to be found； that is so much to the good of evolution；
although； for reasons which I will lay before you by and by； we
must be cautious in our estimate of the evidential cogency of
facts of this kind。
It is a very remarkable circumstance that； from the commencement
of the serious study of fossil remains； in fact； from the time
when Cuvier began his brilliant researches upon those found in
the quarries of Montmartre； palaeontology has shown what she was
going to do in this matter； and what kind of evidence it lay in
her power to produce。
I said just now that； in the existing Fauna； the group of pig…
like animals and the group of ruminants are entirely distinct；
but one of the first of Cuvier's discoveries was an animal which
he called the Anoplotherium； and which proved to be； in a
great many important respects； intermediate in character between
the pigs； on the one hand； and the ruminants on the other。
Thus； research into the history of the past did； to a certain
extent； tend to fill up the breach between the group of
ruminants and the group of pigs。 Another remarkable animal
restored by the great French palaeontologist； the
Palaeotherium； 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 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 condition； that it is impossible to say
whether they differed from existing birds in any essential
character or not。 In your country the development of the
cretaceous series of rocks is enormous； the conditions under
which the later cretaceous strata have been deposited are highly
favourable to the preservation of organic remains； and the
researches； full of labour and risk； which have been carried on
by Professor Marsh in these cretaceous rocks of Western America；
have rewarded him with the discovery of forms of birds of which
we had hitherto no conception。 By his kindness； I am enabled to
place before you a restoration of one of these extraordinary
birds； every part of which can be thoroughly justified by the
more or less complete skeletons； in a very perfect state of
preservation； which he has discovered。 This Hesperornis
（Fig。 3）； which measured between five and six feet in length； is
astonishingly like our existing divers or grebes in a great many
respects； so like them indeed that； had the skeleton of
Hesperornis been found in a museum without its skull； it
probably would have been placed in the same group of birds as
the divers and grebes of the present day。
But Hesperornis differs from all existing birds； and so
far resembles reptiles； in one important particularit is
provided with teeth。 The long jaws are armed with teeth which
have curved crowns and thick roots （Fig。 4）； and are not set in
distinct sockets； but are lodged in a groove。 In possessing true
teeth； the Hesperornis differs from every existing bird；
and from every bird yet discovered in the tertiary formations；
the tooth…like serrations of the jaws in the Odontopteryx
of the London clay being mere processes of the bony substance of
the jaws； and not teeth in the proper sense of the word。 In view
of the characteristics of this bird we are therefore obliged to
modify the definitions of the classes of birds and reptiles。
Before the discovery of Hesperornis； the definition of
the class Aves based upon our knowledge of existing birds might
have been extended to all birds； it might have been said that
the absence of teeth was characteristic of the class of birds；
but the discovery of an animal which； in every part of its
skeleton； closely agrees with existing birds； and yet possesses
teeth； shows that there were ancient birds which； in respect of
possessing teeth； approached reptiles more nearly than any
existing bird does； and； to that extent； diminishes the
hiatus between the two classes。
Fig。 3Hesperornis regalis （Marsh）
Fig。 4Hesperornis regalis （Marsh）
（Side and upper views of half the lower jaw； side and end views
of a vertebra and a separate tooth。）
The same formation has yielded another bird； Ichthyornis
（Fig。 5）； which also possesses teeth； but the teeth are situated
in distinct sockets； while those of Hesperornis are not
so lodged。 The latter also has such very small； almost
rudimentary wings； that it must have been chiefly a swimmer and
a diver like a Penguin； while Ichthyornis has strong
wings and no doubt possessed corresponding powers of flight。
Ichthyornis also differed in the fact that its vertebrae
have not the peculiar characters of the vertebrae of existing
and of all known tertiary birds； but were concave at each end。
This discovery leads us to make a further modification in the
definition of the group of birds； and to part with another of
the characters by which almost all existing birds are
distinguished from reptiles。
Figure。 5Ichthyornis Dispar （Marsh）。
Side and upper views of half the lower jaw； and side and end
views of a vertebra。）
Apart from the few fragmentary remains from the English
greensand； to which I have referred； the Mesozoic rocks； older
than those in which Hesperornis and Ichthyornis
have been discovered； have afforded no certain evidence of
birds； with the remarkable exception of the Solenhofen slates。
These so…called slates are composed of a fine grained calcareous
mud which has hardened into lithographic stone； and in which
organic remains are almost as well preserved as they would be if
they had been imbedded in so much plaster of Paris。 They have
yielded the Archaeopteryx； the existence of which was
first made known by the finding of a fossil feather； or rather
of the impression of one。 It is wonderful enough that such a
perishable thing as a feather； and nothing more； s