友情提示:如果本网页打开太慢或显示不完整,请尝试鼠标右键“刷新”本网页!阅读过程发现任何错误请告诉我们,谢谢!! 报告错误
九色书籍 返回本书目录 我的书架 我的书签 TXT全本下载 进入书吧 加入书签

lectures on evolution-第11章

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!




the succession of living forms which is presented to us by

fossil remains。



Those who have attended to the progress of palaeontology are

aware that evidence of the character which I have defined has

been produced in considerable and continually…increasing

quantity during the last few years。 Indeed; the amount and the

satisfactory nature of that evidence are somewhat surprising;

when we consider the conditions under which alone we can hope to

obtain it。



It is obviously useless to seek for such evidence except in

localities in which the physical conditions have been such as to

permit of the deposit of an unbroken; or but rarely interrupted;

series of strata through a long period of time; in which the

group of animals to be investigated has existed in such

abundance as to furnish the requisite supply of remains; and in

which; finally; the materials composing the strata are such as

to ensure the preservation of these remains in a tolerably

perfect and undisturbed state。



It so happens that the case which; at present; most nearly

fulfils all these conditions is that of the series of extinct

animals which culminates in the horses; by which term I mean to

denote not merely the domestic animals with which we are all so

well acquainted; but their allies; the ass; zebra; quagga; and

the like。 In short; I use 〃horses〃 as the equivalent of the

technical name Equidae; which is applied to the whole

group of existing equine animals。



The horse is in many ways a remarkable animal; not least so in

the fact that it presents us with an example of one of the most

perfect pieces of machinery in the living world。 In truth; among

the works of human ingenuity it cannot be said that there is any

locomotive so perfectly adapted to its purposes; doing so much

work with so small a quantity of fuel; as this machine of

nature's manufacturethe horse。 And; as a necessary consequence

of any sort of perfection; of mechanical perfection as of

others; you find that the horse is a beautiful creature; one of

the most beautiful of all land…animals。 Look at the perfect

balance of its form; and the rhythm and force of its action。

The locomotive machinery is; as you are aware; resident in its

slender fore and hind limbs; they are flexible and elastic

levers; capable of being moved by very powerful muscles; and; in

order to supply the engines which work these levers with the

force which they expend; the horse is provided with a very

perfect apparatus for grinding its food and extracting therefrom

the requisite fuel。



Without attempting to take you very far into the region of

osteological detail; I must nevertheless trouble you with some

statements respecting the anatomical structure of the horse;

and; more especially; will it be needful to obtain a general

conception of the structure of its fore and hind limbs; and of

its teeth。 But I shall only touch upon those points which are

absolutely essential to our inquiry。



Let us turn in the first place to the fore…limb。 In most

quadrupeds; as in ourselves; the fore…arm contains distinct

bones called the radius and the ulna。 The corresponding region

in the horse seems at first to possess but one bone。 Careful

observation; however; enables us to distinguish in this bone a

part which clearly answers to the upper end of the ulna。 This is

closely united with the chief mass of the bone which represents

the radius; and runs out into a slender shaft which may be

traced for some distance downwards upon the back of the radius;

and then in most cases thins out and vanishes。 It takes still

more trouble to make sure of what is nevertheless the fact; that

a small part of the lower end of the bone of the horse's fore

arm; which is only distinct in a very young foal; is really the

lower extremity of the ulna。



What is commonly called the knee of a horse is its wrist。

The 〃cannon bone〃 answers to the middle bone of the five

metacarpal bones; which support the palm of the hand in

ourselves。 The 〃pastern;〃 〃coronary;〃 and 〃coffin〃 bones of

veterinarians answer to the joints of our middle fingers; while

the hoof is simply a greatly enlarged and thickened nail。 But if

what lies below the horse's 〃knee〃 thus corresponds to the

middle finger in ourselves; what has become of the four other

fingers or digits? We find in the places of the second and

fourth digits only two slender splint…like bones; about two…

thirds as long as the cannon bone; which gradually taper to

their lower ends and bear no finger joints; or; as they are

termed; phalanges。 Sometimes; small bony or gristly nodules are

to be found at the bases of these two metacarpal splints; and it

is probable that these represent rudiments of the first and

fifth toes。 Thus; the part of the horse's skeleton; which

corresponds with that of the human hand; contains one overgrown

middle digit; and at least two imperfect lateral digits; and

these answer; respectively; to the third; the second; and the

fourth fingers in man。



Corresponding modifications are found in the hind limb。

In ourselves; and in most quadrupeds; the leg contains two

distinct bones; a large bone; the tibia; and a smaller and more

slender bone; the fibula。 But; in the horse; the fibula seems;

at first; to be reduced to its upper end; a short slender bone

united with the tibia; and ending in a point below; occupying

its place。 Examination of the lower end of a young foal's shin

bone; however; shows a distinct portion of osseous matter; which

is the lower end of the fibula; so that the apparently single;

lower end of the shin bone is really made up of the coalesced

ends of the tibia and fibula; just as the; apparently single;

lower end of the fore…arm bone is composed of the coalesced

radius and ulna。



The heel of the horse is the part commonly known as the hock。

The hinder cannon bone answers to the middle metatarsal bone of

the human foot; the pastern; coronary; and coffin bones; to the

middle toe bones; the hind hoof to the nail; as in the fore…

foot。 And; as in the fore…foot; there are merely two splints to

represent the second and the fourth toes。 Sometimes a rudiment

of a fifth toe appears to be traceable。



The teeth of a horse are not less peculiar than its limbs。 The

living engine; like all others; must be well stoked if it is to

do its work; and the horse; if it is to make good its wear and

tear; and to exert the enormous amount of force required for its

propulsion; must be well and rapidly fed。 To this end; good

cutting instruments and powerful and lasting crushers are

needful。 Accordingly; the twelve cutting teeth of a horse are

close…set and concentrated in the fore…part of its mouth; like

so many adzes or chisels。 The grinders or molars are large; and

have an extremely complicated structure; being composed of a

number of different substances of unequal hardness。 The

consequence of this is that they wear away at different rates;

and; hence; the surface of each grinder is always as uneven as

that of a good millstone。



I have said that the structure of the grinding teeth is very

complicated; the harder and the softer parts being; as it were;

interlaced with one another。 The result of this is that; as the

tooth wears; the crown presents a peculiar pattern; the nature

of which is not very easily deciphered at first; but which it is

important we should understand clearly。 Each grinding tooth of

the upper jaw has an outer wall so shaped that; on the

worn crown; it exhibits the form of two crescents; one in front

and one behind; with their concave sides turned outwards。 From

the inner side of the front crescent; a crescentic front

ridge passes inwards and backwards; and its inner face

enlarges into a strong longitudinal fold or pillar。

From the front part of the hinder crescent; a back ridge

takes a like direction; and also has its pillar。



The deep interspaces or valleys between these ridges and

the outer wall are filled by bony substance; which is called

cement; and coats the whole tooth。



The pattern of the worn face of each grinding tooth of the lower

jaw is quite different。 It appears to be formed of two crescent…

shaped ridges; the convexities of which are turned outwards。

The free extremity of each crescent has a pillar; and

there is a large double pillar where the two crescents

meet。 The whole structure is; as it were; imbedded in cement;

which fills up the valleys; as in the upper grinders。



If the grinding faces of an upper and of a lower molar of the

same side are applied together; it will be seen that the opposed

ridges are nowhere parallel; but that they frequently cross;

and that thus; in the act of mastication; a hard surface in the

one is constantly applied to a soft surface in the other; and

vice versa。 They thus constitute a grinding apparatus of

great efficiency
返回目录 上一页 下一页 回到顶部 0 0
未阅读完?加入书签已便下次继续阅读!
温馨提示: 温看小说的同时发表评论,说出自己的看法和其它小伙伴们分享也不错哦!发表书评还可以获得积分和经验奖励,认真写原创书评 被采纳为精评可以获得大量金币、积分和经验奖励哦!