We have had to stretch the definition of the class of birds so as to
include birds with teeth and birds with paw-like fore limbs and long
tails. There is no evidence that _Compsognathus_ possessed feathers;
but, if it did, it would be hard indeed to say whether it should be
called a reptilian bird or an avian reptile.
As _Compsognathus_ walked upon its hind legs, it must have made tracks
like those of birds. And as the structure of the limbs of several of the
gigantic _Ornithoscelida,_ such as _Iguanodon,_ leads to the conclusion
that they also may have constantly, or occasionally, assumed the same
attitude, a peculiar interest attaches to the fact that, in the Wealden
strata of England, there are to be found gigantic footsteps, arranged
in order like those of the _Brontozoum,_ and which there can be no
reasonable doubt were made by some of the _Ornithoscelida,_ the remains
of which are found in the same rocks. And, knowing that reptiles that
walked upon their hind legs and shared many of the anatomical characters
of birds did once exist, it becomes a very important question whether
the tracks in the Trias of Massachusetts, to which I referred some time
ago, and which formerly used to be unhesitatingly ascribed to birds, may
not all have been made by ornithoscelidan reptiles; and whether, if we
could obtain the skeletons of the animals which made these tracks, we
should not find in them the actual steps of the evolutional process by
which reptiles gave rise to birds.
The evidential value of the facts I have brought forward in this Lecture
must be neither over nor under estimated. It is not historical proof of
the occurrence of the evolution of birds from reptiles, for we have no
safe ground for assuming that true birds had not made their appearance
at the commencement of the Mesozoic epoch. It is, in fact, quite
possible that all these more or less avi-form reptiles of the Mesozoic
epochs are not terms in the series of progression from birds to reptiles
at all, but simply the more or less modified descendants of Palaeozoic
forms through which that transition was actually effected.
We are not in a position to say that the known _Ornithoscelida_ are
intermediate in the order of their appearance on the earth between
reptiles and birds. All that can be said is that, if independent
evidence of the actual occurrence of evolution is producible, then these
intercalary forms remove every difficulty in the way of understanding
what the actual steps of the process, in the case of birds, may have
been.
That intercalary forms should have existed in ancient times is a
necessary consequence of the truth of the hypothesis of evolution; and,
hence, the evidence I have laid before you in proof of the existence of
such forms, is, so far as it goes, in favour of that hypothesis.
There is another series of extinct reptiles which may be said to be
intercalary between reptiles and birds, in so far as they combine some
of the characters of both these groups; and which, as they possessed the
power of flight, may seem, at first sight, to be nearer representatives
of the forms by which the transition from the reptile to the bird was
effected, than the _Ornithoscelida._
These are the _Pterosauria,_ or Pterodactyles, the remains of which are
met with throughout the series of Mesozoic rocks, from the lias to the
chalk, and some of which attained a great size, their wings having
a span of eighteen or twenty feet. These animals, in the form and
proportions of the head and neck relatively to the body, and in the
fact that the ends of the jaws were often, if not always, more or less
extensively ensheathed in horny beaks, remind us of birds. Moreover,
their bones contained air cavities, rendering them specifically lighter,
as is the case in most birds. The breast bone was large and keeled, as
in most birds and in bats, and the shoulder girdle is strikingly
similar to that of ordinary birds. But, it seems to me, that the special
resemblance of pterodactyles to birds ends here, unless I may add the
entire absence of teeth which characterises the great pterodactyles
_(Pteranodon)_ discovered by Professor Marsh. All other known
pterodactyles have teeth lodged in sockets. In the vertebral column and
the hind limbs there are no special resemblances to birds, and when
we turn to the wings they are found to be constructed on a totally
different principle from those of birds.
Fig. 8.--Pterodactylus Spectabilis (Von Meyer).
There are four fingers. These four fingers are large, and three of
them, those which answer to the thumb and two following fingers in my
hand--are terminated by claws, while the fourth is enormously prolonged
and converted into a great jointed style. You see at once, from what I
have stated about a bird's wing, that there could be nothing less like
a bird's wing than this is. It was concluded by general reasoning that
this finger had the office of supporting a web which extended between it
and the body. An existing specimen proves that such was really the case,
and that the pterodactyles were devoid of feathers, but that the fingers
supported a vast web like that of a bat's wing; in fact, there can be no
doubt that this ancient reptile flew after the fashion of a bat.
Thus, though the pterodactyle is a reptile which has become modified
in such a manner as to enable it to fly, and therefore, as might be
expected, presents some points of resemblance to other animals which
fly; it has, so to speak, gone off the line which leads directly from
reptiles to birds, and has become disqualified for the changes which
lead to the characteristic organisation of the latter class.
Therefore, viewed in relation to the classes of reptiles and birds, the
pterodactyles appear to me to be, in a limited sense, intercalary forms;
but they are not even approximately linear, in the sense of exemplifying
those modifications of structure through which the passage from the
reptile to the bird took place.