lectures on evolution-第13章

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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

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　le