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lectures on evolution-第9章

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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。 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 togetherthey 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;
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