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lectures on evolution-第10章

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vice versa。 They thus constitute a grinding apparatus of

great efficiency; and one which is repaired as fast as it wears;

owing to the long…continued growth of the teeth。



Some other peculiarities of the dentition of the horse must be

noticed; as they bear upon what I shall have to say by and by。

Thus the crowns of the cutting teeth have a peculiar deep pit;

which gives rise to the well…known 〃mark〃 of the horse。 There is

a large space between the outer incisors and the front grinder。

In this space the adult male horse presents; near the incisors

on each side; above and below; a canine or 〃tush;〃 which is

commonly absent in mares。 In a young horse; moreover; there is

not unfrequently to be seen in front of the first grinder; a

very small tooth; which soon falls out。 If this small tooth be

counted as one; it will be found that there are seven teeth

behind the canine on each side; namely; the small tooth in

question; and the six great grinders; among which; by an unusual

peculiarity; the foremost tooth is rather larger than those

which follow it。



I have now enumerated those characteristic structures of the

horse which are of most importance for the purpose we have

in view。



To any one who is acquainted with the morphology of vertebrated

animals; they show that the horse deviates widely from the

general structure of mammals; and that the horse type is; in

many respects; an extreme modification of the general mammalian

plan。 The least modified mammals; in fact; have the radius and

ulna; the tibia and fibula; distinct and separate。 They have

five distinct and complete digits on each foot; and no one of

these digits is very much larger than the rest。 Moreover; in the

least modified mammals; the total number of the teeth is very

generally forty…four; while in horses; the usual number is

forty; and in the absence of the canines; it may be reduced to

thirty…six; the incisor teeth are devoid of the fold seen in

those of the horse: the grinders regularly diminish in size from

the middle of the series to its front end; while their crowns

are short; early attain their full length; and exhibit simple

ridges or tubercles; in place of the complex foldings of the

horse's grinders。



Hence the general principles of the hypothesis of evolution lead

to the conclusion that the horse must have been derived from

some quadruped which possessed five complete digits on each

foot; which had the bones of the fore…arm and of the leg

complete and separate; and which possessed forty…four teeth;

among which the crowns of the incisors and grinders had a simple

structure; while the latter gradually increased in size from

before backwards; at any rate in the anterior part of the

series; and had short crowns。



And if the horse has been thus evolved; and the remains of the

different stages of its evolution have been preserved; they

ought to present us with a series of forms in which the number

of the digits becomes reduced; the bones of the fore…arm and leg

gradually take on the equine condition; and the form and

arrangement of the teeth successively approximate to those which

obtain in existing horses。



Let us turn to the facts; and see how far they fulfil these

requirements of the doctrine of evolution。



In Europe abundant remains of horses are found in the Quaternary

and later Tertiary strata as far as the Pliocene formation。

But these horses; which are so common in the cave…deposits and

in the gravels of Europe; are in all essential respects like

existing horses。 And that is true of all the horses of the

latter part of the Pliocene epoch。 But; in deposits which belong

to the earlier Pliocene and later Miocene epochs; and which

occur in Britain; in France; in Germany; in Greece; in India; we

find animals which are extremely like horseswhich; in fact;

are so similar to horses; that you may follow descriptions given

in works upon the anatomy of the horse upon the skeletons of

these animalsbut which differ in some important particulars。

For example; the structure of their fore and hind limbs is

somewhat different。 The bones which; in the horse; are

represented by two splints; imperfect below; are as long as the

middle metacarpal and metatarsal bones; and; attached to the

extremity of each; is a digit with three joints of the same

general character as those of the middle digit; only very much

smaller。 These small digits are so disposed that they could have

had but very little functional importance; and they must have

been rather of the nature of the dew…claws; such as are to be

found in many ruminant animals。 The Hipparion; as the

extinct European three…toed horse is called; in fact; presents a

foot similar to that of the American Protohippus

(Fig。 9); except that; in the Hipparion; the smaller

digits are situated farther back; and are of smaller

proportional size; than in the Protohippus。



The ulna is slightly more distinct than in the horse; and the

whole length of it; as a very slender shaft; intimately united

with the radius; is completely traceable。 The fibula appears to

be in the same condition as in the horse。 The teeth of the

Hipparion are essentially similar to those of the horse;

but the pattern of the grinders is in some respects a little

more complex; and there is a depression on the face of the skull

in front of the orbit; which is not seen in existing horses。



In the earlier Miocene; and perhaps the later Eocene deposits of

some parts of Europe; another extinct animal has been

discovered; which Cuvier; who first described some fragments of

it; considered to be a Palaeotherim。 But as further

discoveries threw new light upon its structure; it was

recognised as a distinct genus; under the name

of Anchitherium。



In its general characters; the skeleton of Anchitherium

is very similar to that of the horse。 In fact; Lartet and De

Blainville called it Pal?otherium equinum or

hippoides; and De Christol; in 1847; said that it

differed from Hipparion in little more than the

characters of its teeth; and gave it the name of

Hipparitherium。 Each foot possesses three complete toes;

while the lateral toes are much larger in proportion to the

middle toe than in Hipparion; and doubtless rested on the

ground in ordinary locomotion。



The ulna is complete and quite distinct from the radius; though

firmly united with the latter。 The fibula seems also to have

been complete。 Its lower end; though intimately united with that

of the tibia; is clearly marked off from the latter bone。



There are forty…four teeth。 The incisors have no strong pit。

The canines seem to have been well developed in both sexes。

The first of the seven grinders; which; as I have said; is

frequently absent; and; when it does exist; is small in the

horse; is a good…sized and permanent tooth; while the grinder

which follows it is but little larger than the hinder ones。

The crowns of the grinders are short; and though the fundamental

pattern of the horse…tooth is discernible; the front and back

ridges are less curved; the accessory pillars are wanting; and

the valleys; much shallower; are not filled up with cement。



Seven years ago; when I happened to be looking critically into

the bearing of palaentological facts upon the doctrine of

evolution; it appeared to me that the Anchitherium; the

Hipparion; and the modern horses; constitute a series in

which the modifications of structure coincide with the order of

chronological occurrence; in the manner in which they must

coincide; if the modern horses really are the result of the

gradual metamorphosis; in the course of the Tertiary epoch; of a

less specialised ancestral form。 And I found by correspondence

with the late eminent French anatomist and palaeontologist;

M。 Lartet; that he had arrived at the same conclusion from the

same data。



That the Anchitherium type had become metamorphosed into

the Hipparion type; and the latter into the Equine

type; in the course of that period of time which is represented

by the latter half of the Tertiary deposits; seemed to me to be

the only explanation of the facts for which there was even a

shadow of probability。



And; hence; I have ever since held that these facts afford

evidence of the occurrence of evolution; which; in the sense

already defined; may be termed demonstrative。



All who have occupied themselves with the structure of

Anchitherium; from Cuvier onwards; have acknowledged its

many points of likeness to a well…known genus of extinct Eocene

mammals; Palaeotherium。 Indeed; as we have seen; Cuvier

regarded his remains of Anchitherium as those of a

species of Palaeotherium。 Hence; in attempting to trace

the pedigree of the horse beyond the Miocene epoch and the

Anchitheroid form; I naturally sought among the various species

of Palaeotheroid animals for its nearest ally; and I was led to

conclude that the Palaeotherium minus (Plagiolophus)

represented the next step more nearly than any form then known。



I think that this opinion was fully justifiable; but the

progress of investigation has thrown an unexpected light on the

question; and has brought us much nearer than could have been

anticipated to a knowledge of the true series of the progenitors

of the horse。



You are all aware that; when your country was first discovered

by Europeans; there were no traces of the existence of the horse

in any part of the American Continent。 The accounts of the

conquest of Mexico dwell upon the astonishment of the natives of

that country when they first became acquainted with that

astounding phenomenona man seated upon a horse。

Nevertheless; the investigations of American geologists have

proved that the remains of horses occur in the most superficial

deposits of both North and South America; just as they do in

Europe。 Therefore; for some reason or otherno feasible

suggestion on that subject; so far as I know; has been madethe

horse must have died out on this continent at some period

preceding the discovery of America。 Of late years there has been

discovered in your Western Territories that marvellous

accumulation of deposits; admirably adapted for the preservation

of organic remains; to which I referred the other evening; and

which furnishes us with a consecutive series of records of the

fauna of the older half of the Tertiary epoch; for which we have

no parallel in Europe。 They have yielded fossils in an excellent

state of conservation and in unexampled number and variety。

The researches of Leidy and others have shown that forms allied

to the Hipparion and the Anchitherium are to be

found among these remains。 But it is only recently that the

admirably conceived and most thoroughly 

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