lectures on evolution- 第12部分

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　for