lectures on evolution- 第10部分

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；　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

wh