Fig. 4--Hesperornis 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. 5--Ichthyornis 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, should be discovered; yet, for a long time, nothing was known
of this bird except its feather. But by and by a solitary skeleton
was discovered which is now in the British Museum. The skull of
this solitary specimen is unfortunately wanting, and it is therefore
uncertain whether the _Archaeopteryx_ possessed teeth or not. [2] But
the remainder of the skeleton is so well preserved as to leave no doubt
respecting the main features of the animal, which are very singular. The
feet are not only altogether bird-like, but have the special characters
of the feet of perching birds, while the body had a clothing of true
feathers. Nevertheless, in some other respects, _Archaeopteryx_ is
unlike a bird and like a reptile. There is a long tail composed of many
vertebrae. The structure of the wing differs in some very remarkable
respects from that which it presents in a true bird. In the latter, the
end of the wing answers to the thumb and two fingers of my hand; but
the metacarpal bones, or those which answer to the bones of the fingers
which lie in the palm of the hand, are fused together into one mass; and
the whole apparatus, except the last joints of the thumb, is bound up in
a sheath of integument, while the edge of the hand carries the principal
quill-feathers. In the _Archaeopteryx,_ the upper-arm bone is like that
of a bird; and the two bones of the forearm are more or less like those
of a bird, but the fingers are not bound together--they are free. What
their number may have been is uncertain; but several, if not all,
of them were terminated by strong curved claws, not like such as are
sometimes found in birds, but such as reptiles possess; so that, in the
_Archaeopteryx,_ we have an animal which, to a certain extent, occupies
a midway place between a bird and a reptile. It is a bird so far as
its foot and sundry other parts of its skeleton are concerned; it is
essentially and thoroughly a bird by its feathers; but it is much more
properly a reptile in the fact that the region which represents the
hand has separate bones, with claws resembling those which terminate the
forelimb of a reptile. Moreover, it has a long reptile-like tail with
a fringe of feathers on each side; while, in all true birds hitherto
known, the tail is relatively short, and the vertebrae which constitute
its skeleton are generally peculiarly modified.
Like the _Anoplotherium_ and the _Palaeotherium,_ therefore,
_Archaeopteryx_ tends to fill up the interval between groups which, in
the existing world, are widely separated, and to destroy the value
of the definitions of zoological groups based upon our knowledge of
existing forms. And such cases as these constitute evidence in favour
of evolution, in so far as they prove that, in former periods of the
world's history, there were animals which overstepped the bounds of
existing groups, and tended to merge them into larger assemblages. They
show that animal organisation is more flexible than our knowledge of
recent forms might have led us to believe; and that many structural
permutations and combinations, of which the present world gives us no
indication, may nevertheless have existed.
But it by no means follows, because the _Palaeotherium_ has much in
common with the horse, on the one hand, and with the rhinoceros on the
other, that it is the intermediate form through which rhinoceroses have
passed to become horses, or _vice versa;_ on the contrary, any such
supposition would certainly be erroneous. Nor do I think it likely that
the transition from the reptile to the bird has been effected by such
a form as _Archaeopteryx._ And it is convenient to distinguish these
intermediate forms between two groups, which do not represent the actual
passage from the one group to the other, as _intercalary_ types, from
those _linear_ types which, more or less approximately, indicate the
nature of the steps by which the transition from one group to the other
was effected.
I conceive that such linear forms, constituting a series of natural
gradations between the reptile and the bird, and enabling us to
understand the manner in which the reptilian has been metamorphosed
into the bird type, are really to be found among a group of ancient and
extinct terrestrial reptiles known as the _Ornithoscelida._ The remains
of these animals occur throughout the series of mesozoic formations,
from the Trias to the Chalk, and there are indications of their
existence even in the later Palaeozoic strata.
Most of these reptiles, at present known, are of great size, some having
attained a length of forty feet or perhaps more. The majority resembled
lizards and crocodiles in their general form, and many of them were,
like crocodiles, protected by an armour of heavy bony plates. But, in
others, the hind limbs elongate and the fore limbs shorten, until
their relative proportions approach those which are observed in the
short-winged, flightless, ostrich tribe among birds.
The skull is relatively light, and in some cases the jaws, though
bearing teeth, are beak-like at their extremities and appear to have
been enveloped in a horny sheath. In the part of the vertebral column
which lies between the haunch bones and is called the sacrum, a number
of vertebrae may unite together into one whole, and in this respect,
as in some details of its structure, the sacrum of these reptiles
approaches that of birds.
But it is in the structure of the pelvis and of the hind limb that some
of these ancient reptiles present the most remarkable approximation to
birds, and clearly indicate the way by which the most specialised and
characteristic features of the bird may have been evolved from the
corresponding parts of the reptile.
In Fig. 6, the pelvis and hind limbs of a crocodile, a three-toed bird,
and an ornithoscelidan are represented side by side; and, for facility
of comparison, in corresponding positions; but it must be recollected
that, while the position of the bird's limb is natural, that of the
crocodile is not so. In the bird, the thigh bone lies close to the
body, and the metatarsal bones of the foot (ii., iii., iv., Fig. 6)
are, ordinarily, raised into a more or less vertical position; in the
crocodile, the thigh bone stands out at an angle from the body, and the
metatarsal bones (i., ii., iii., iv., Fig. 6) lie flat on the ground.
Hence, in the crocodile, the body usually lies squat between the legs,
while, in the bird, it is raised upon the hind legs, as upon pillars.
In the crocodile, the pelvis is obviously composed of three bones
on each side: the ilium (_Il._), the pubis (_Pb._), and the ischium
(_Is._). In the adult bird there appears to be but one bone on each
side. The examination of the pelvis of a chick, however, shows that
each half is made up of three bones, which answer to those which remain
distinct throughout life in the crocodile. There is, therefore, a
fundamental identity of plan in the construction of the pelvis of both
bird and reptile; though the difference in form, relative size, and
direction of the corresponding bones in the two cases are very great.
But the most striking contrast between the two lies in the bones of the
leg and of that part of the foot termed the tarsus, which follows upon
the leg. In the crocodile, the fibula (_F_) is relatively large and its
lower end is complete. The tibia (_T_) has no marked crest at its upper
end, and its lower end is narrow and not pulley-shaped. There are
two rows of separate tarsal bones (_As., Ca., &c._) and four distinct
metatarsal bones, with a rudiment of a fifth.
In the bird, the fibula is small and its lower end diminishes to a
point. The tibia has a strong crest at its upper end and its lower
extremity passes into a broad pulley. There seem at first to be no
tarsal bones; and only one bone, divided at the end into three heads
for the three toes which are attached to it, appears in the place of the
metatarsus.
In the young bird, however, the pulley-shaped apparent end of the tibia
is a distinct bone, which represents the bones marked _As., Ca.,_ in the
crocodile; while the apparently single metatarsal bone consists of three
bones, which early unite with one another and with an additional bone,
which represents the lower row of bones in the tarsus of the crocodile.
In other words, it can be shown by the study of development that the
bird's pelvis and hind limb are simply extreme modifications of the
same fundamental plan as that upon which these parts are modelled in
reptiles.
On comparing the pelvis and hind limb of the ornithoscelidan with that
of the crocodile, on the one side, and that of the bird, on the other
(Fig. 6), it is obvious that it represents a middle term between the
two. The pelvic bones approach the form of those of the birds, and
the direction of the pubis and ischium is nearly that which is
characteristic of birds; the thigh bone, from the direction of its head,
must have lain close to the body; the tibia has a great crest; and,
immovably fitted on to its lower end, there is a pulley-shaped bone,
like that of the bird, but remaining distinct. The lower end of the
fibula is much more slender, proportionally, than in the crocodile.
The metatarsal bones have such a form that they fit together immovably,
though they do not enter into bony union; the third toe is, as in
the bird, longest and strongest. In fact, the ornithoscelidan limb is
comparable to that of an unhatched chick.
Fig. 6.--Bird. Ornithoscelidan. Crocodile. The letters have
the same signification in all the figures. _Il.,_ Ilium;
_a._ anterior end; _b._ posterior end; _Ia._ ischium; _Pb.,_
pubis; _T,_ tibia; _F,_ fibula; _As.,_ astragalus; _Ca.,_
calcaneum; I, distal portion of the tarsus; i., ii., iii.,
iv., metatarsal bones.
Taking all these facts together, it is obvious that the view, which was
entertained by Mantell and the probability of which was demonstrated by
your own distinguished anatomist, Leidy, while much additional evidence
in the same direction has been furnished by Professor Cope, that some of
these animals may have walked upon their hind legs as birds do, acquires
great weight. In fact, there can be no reasonable doubt that one of the
smaller forms of the _Ornithoscelida, Compsognathus,_ the almost entire
skeleton of which has been discovered in the Solenhofen slates, was a
bipedal animal. The parts of this skeleton are somewhat twisted out of
their natural relations, but the accompanying figure gives a just view
of the general form of _Compsognathus_ and of the proportions of its
limbs; which, in some respects, are more completely bird-like than those
of other _Ornithoscelida._