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lectures on evolution-第10章

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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。



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