lectures on evolution-第14章

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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　and　patiently　worked…out

investigations　of　Professor　Marsh　have　given　us　a　just　idea　of

the　vast　fossil　wealth；　and　of　the　scientific　importance；　of

these　deposits。　I　have　had　the　advantage　of　glancing　over　the

collections　in　Yale　Museum；　and　I　can　truly　say　that；　so　far　as

my　knowledge　extends；　there　is　no　collection　from　any　one　region

and　series　of　strata　comparable；　for　extent；　or　for　the　care

with　which　the　remains　have　been　got　together；　or　for　their

scientific　importance；　to　the　series　of　fossils　which　he　has

deposited　there。　This　vast　collection　has　yielded　evidence

bearing　upon　the　question　of　the　pedigree　of　the　horse　of　the

most　striking　character。　It　tends　to　show　that　we　must　look　to

America；　rather　than　to　Europe；　for　the　original　seat　of　the

equine　series；　and　that　the　archaic　forms　and　successive

modifications　of　the　horse's　ancestry　are　far　better　preserved

here　than　in　Europe。



Professor　Marsh's　kindness　has　enabled　me　to　put　before　you　a

diagram；　every　figure　in　which　is　an　actual　representation　of

some　specimen　which　is　to　be　seen　at　Yale　at　this　present　time

（Fig。　9）。





Fig。　9。





The　succession　of　forms　which　he　has　brought　together　carries　us

from　the　top　to　the　bottom　of　the　Tertiaries。　Firstly；　there　is

the　true　horse。　Next　we　have　the　American　Pliocene　form　of　the

horse　（Pliohippus）；　in　the　conformation　of　its　limbs　it

presents　some　very　slight　deviations　from　the　ordinary　horse；

and　the　crowns　of　the　grinding　teeth　are　shorter。　Then　comes　the

Protohippus；　which　represents　the　European

Hipparion；　having　one　large　digit　and　two　small　ones　on

each　foot；　and　the　general　characters　of　the　fore…arm　and　leg　to

which　I　have　referred。　But　it　is　more　valuable　than　the　European

Hipparion　for　the　reason　that　it　is　devoid　of　some　of　the

peculiarities　of　that　formpeculiarities　which　tend　to　show

that　the　European　Hipparion　is　rat