lectures on evolution-第9章

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There　are　four　fingers。　These　four　fingers　are　large；　and　three

of　them；　those　which　answer　to　the　thumb　and　two　following

fingers　in　my　handare　terminated　by　claws；　while　the　fourth　is

enormously　prolonged　and　converted　into　a　great　jointed　style。

You　see　at　once；　from　what　I　have　stated　about　a　bird's　wing；

that　there　could　be　nothing　less　like　a　bird's　wing　than　this

is。　It　was　concluded　by　general　reasoning　that　this　finger　had

the　office　of　supporting　a　web　which　extended　between　it　and　the

body。　An　existing　specimen　proves　that　such　was　really　the　case；

and　that　the　pterodactyles　were　devoid　of　feathers；　but　that　the

fingers　supported　a　vast　web　like　that　of　a　bat's　wing；　in　fact；

there　can　be　no　doubt　that　this　ancient　reptile　flew　after　the

fashion　of　a　bat。



Thus；　though　the　pterodactyle　is　a　reptile　which　has　become

modified　in　such　a　manner　as　to　enable　it　to　fly；　and　therefore；

as　might　be　expected；　presents　some　points　of　resemblance　to

other　animals　which　fly；　it　has；　so　to　speak；　gone　off　the　line

which　leads　directly　from　reptiles　to　birds；　and　has　become

disqualified　for　the　changes　which　lead　to　the　characteristic

organisation　of　the　latter　class。　Therefore；　viewed　in　relation

to　the　classes　of　reptiles　and　birds；　the　pterodactyles　appear

to　me　to　be；　in　a　limited　sense；　intercalary　forms；　but　they　are

not　even　approximately　linear；　in　the　sense　of　exemplifying

those　modifications　of　structure　through　which　the　passage　from

the　reptile　to　the　bird　took　place。







LECTURES　ON　EVOLUTION



III

THE　DEMONSTRATIVE　EVIDENCE　OF　EVOLUTION





The　occurrence　of　historical　facts　is　said　to　be　demonstrated；

when　the　evidence　that　they　happened　is　of　such　a　character　as

to　render　the　assumption　that　they　did　not　happen　in　the　highest

degree　improbable；　and　the　question　I　now　have　to　deal　with　is；

whether　evidence　in　favour　of　the　evolution　of　animals　of　this

degree　of　cogency　is；　or　is　not；　obtainable　from　the　record　of

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　grindin