按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
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。
Fig。 7。Restoration of Compsognathus Longipes
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 handare 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。
LECTURES ON EVOLUTION
III
THE DEMONSTRATIVE EVIDENCE OF EVOLUTION
The occurrence of historical facts is said to be demonstrated；
when the evidence that they happened is of such a character as
to render the assumption that they did not happen in the highest
degree improbable； and the question I now have to deal with is；
whether evidence in favour of the evolution of animals of this
degree of cogency is； or is not； obtainable from the record of
the succession of living forms which is presented to us by
fossil remains。
Those who have attended to the progre