lectures on evolution- 第9部分

In　the　bird；　the　fibula　is　small　and　its　lower　end　diminishes　to

a　point。　The　tibia　has　a　strong　crest　at　its　upper　end　and　its

lower　extremity　passes　into　a　broad　pulley。　There　seem　at　first

to　be　no　tarsal　bones；　and　only　one　bone；　divided　at　the　end

into　three　heads　for　the　three　toes　which　are　attached　to　it；

appears　in　the　place　of　the　metatarsus。



In　the　young　bird；　however；　the　pulley…shaped　apparent　end　of

the　tibia　is　a　distinct　bone；　which　represents　the　bones　marked

As。；　Ca。；　in　the　crocodile；　while　the　apparently　single

metatarsal　bone　consists　of　three　bones；　which　early　unite　with


one　another　and　with　an　additional　bone；　which　represents　the

lower　row　of　bones　in　the　tarsus　of　the　crocodile。



In　other　words；　it　can　be　shown　by　the　study　of　development　that

the　bird's　pelvis　and　hind　limb　are　simply　extreme　modifications

of　the　same　fundamental　plan　as　that　upon　which　these　parts　are

modelled　in　reptiles。



On　comparing　the　pelvis　and　hind　limb　of　the　ornithoscelidan

with　that　of　the　crocodile；　on　the　one　side；　and　that　of　the

bird；　on　the　other　（Fig。　6）；　it　is　obvious　that　it　represents　a

middle　term　between　the　two。　The　pelvic　bones　approach　the　form

of　those　of　the　birds；　and　the　direction　of　the　pubis　and

ischium　is　nearly　that　which　is　characteristic　of　birds；

the　thigh　bone；　from　the　direction　of　its　head；　must　have　lain

close　to　the　body；　the　tibia　has　a　great　crest；　and；　immovably

fitted　on　to　its　lower　end；　there　is　a　pulley…shaped　bone；　like

that　of　the　bird；　but　remaining　distinct。　The　lower　end　of　the

fibula　is　much　more　slender；　proportionally；　than　in　the

crocodile。　The　metatarsal　bones　have　such　a　form　that　they　fit

together　immovably；　though　they　do　not　enter　into　bony　union；

the　third　toe　is；　as　in　the　bird；　longest　and　strongest。

In　fact；　the　ornithoscelidan　limb　is　comparable　to　that　of　an

unhatched　chick。





Fig。　6。Bird。　Ornithoscelidan。　Crocodile。

The　letters　have　the　same　signification　in　all　the　figures。

Il。；　Ilium；　a。　anterior　end；　b。　posterior

end；　Ia。　ischium；　Pb。；　pubis；　T；　tibia；

F；　fibula；　As。；　astragalus；　Ca。；　calcaneum；

I；　distal　portion　of　the　tarsus；　i。；　ii。；　iii。；　iv。；　metatarsal

bones。





Taking　all　these　facts　together；　it　is　obvious　that　the　view；

which　was　entertained　by　Mantell　and　the　probability　of　which

was　demonstrated　by　your　own　distinguished　anatomist；　Leidy；

while　much　additional　evidence　in　the　same　direction　has　been

furnished　by　Professor　Cope；　that　some　of　these　animals　may　have

walked　upon　their　hind　legs　as　birds　do；　acquires　great　weight。

In　fact；　there　can　be　no　reasonable　doubt　that　one　of　the

smaller　forms　of　the　Ornithoscelida；　Compsognathus；　the

almost　entire　skeleton　of　which　has　been　discovered　in　the

Solenhofen　slates；　was　a　bipedal　animal。　The　parts　of　this

skeleton　are　somewhat　twisted　out　of　their　natural　relations；

but　the　accompanying　figure　gives　a　just　view　of　the　general

form　of　Compsognathus　and　of　the　proportions　of　its

limbs；　which；　in　some　respects；　are　more　completely　bird…like

than　those　of　other　Ornithoscelida。





Fig。　7。Restoration　of　Compsognathus　Longipes





We　have　had　to　stretch　the　definition　of　the　class　of　birds　so

as　to　include　birds　with　teeth　and　birds　with　paw…like　fore

limbs　and　long　tails。　There　is　no　evidence　that

Compsognathus　possessed　feathers；　but；　if　it　did；　it

would　be　hard　indeed　to　say　whether　it　should　be　called　a

reptilian　bird　or　an　avian　reptile。



As　Compsognathus　walked　upon　its　hind　legs；　it　must　have

made　tracks　like　those　of　birds。　And　as　the　structure　of　the

limbs　of　several　of　the　gigantic　Ornithoscelida；　such　as

Iguanodon；　leads　to　the　conclusion　that　they　also　may

have　constantly；　or　occasionally；　assumed　the　same　attitude；　a

peculiar　interest　attaches　to　the　fact　that；　in　the　Wealden

strata　of　England；　there　are　to　be　found　gigantic　footsteps；

arranged　in　order　like　those　of　the　Brontozoum；　and　which

there　can　be　no　reasonable　doubt　were　made　by　some　of　the

Ornithoscelida；　the　remains　of　which　are　found　in　the

same　rocks。　And；　knowing　that　reptiles　that　walked　upon　their

hind　legs　and　shared　many　of　the　anatomical　characters　of　birds

did　once　exist；　it　becomes　a　very　important　question　whether　the

tracks　in　the　Trias　of　Massachusetts；　to　which　I　referred　some

time　ago；　and　which　formerly　used　to　be　unhesitatingly　ascribed

to　birds；　may　not　all　have　been　made　by　ornithoscelidan

reptiles；　and　whether；　if　we　could　obtain　the　skeletons　of　the

animals　which　made　these　tracks；　we　should　not　find　in　them　the

actual　steps　of　the　evolutional　process　by　which　reptiles　gave

rise　to　birds。



The　evidential　value　of　the　facts　I　have　brought　forward　in　this

Lecture　must　be　neither　over　nor　under　estimated。　It　is　not

historical　proof　of　the　occurrence　of　the　evolution　of　birds

from　reptiles；　for　we　have　no　safe　ground　for　assuming　that　true

birds　had　not　made　their　appearance　at　the　commencement　of　the

Mesozoic　epoch。　It　is；　in　fact；　quite　possible　that　all　these

more　or　less　avi…form　reptiles　of　the　Mesozoic　epochs　are　not

terms　in　the　series　of　progression　from　birds　to　reptiles　at

all；　but　simply　the　more　or　less　modified　descendants　of

Palaeozoic　forms　through　which　that　transition　was

actually　effected。



We　are　not　in　a　position　to　say　that　the　known

Ornithoscelida　are　intermediate　in　the　order　of　their

appearance　on　the　earth　between　reptiles　and　birds。　All　that　can

be　said　is　that；　if　independent　evidence　of　the　actual

occurrence　of　evolution　is　producible；　then　these　intercalary

forms　remove　every　difficulty　in　the　way　of　understanding　what

the　actual　steps　of　the　process；　in　the　case　of　birds；　may

have　been。



That　intercalary　forms　should　have　existed　in　ancient　times　is　a

necessary　consequence　of　the　truth　of　the　hypothesis　of

evolution；　and；　hence；　the　evidence　I　have　laid　before　you　in

proof　of　the　existence　of　such　forms；　is；　so　far　as　it　goes；　in

favour　of　that　hypothesis。



There　is　another　series　of　extinct　reptiles　which　may　be　said　to

be　intercalary　between　reptiles　and　birds；　in　so　far　as　they

combine　some　of　the　characters　of　both　these　groups；　and　which；

as　they　possessed　the　power　of　flight；　may　seem；　at　first　sight；

to　be　nearer　representatives　of　the　forms　by　which　the

transition　from　the　reptile　to　the　bird　was　effected；　than　the

Ornithoscelida。



These　are　the　Pterosauria；　or　Pterodactyles；　the　remains

of　which　are　met　with　throughout　the　series　of　Mesozoic　rocks；

from　the　lias　to　the　chalk；　and　some　of　which　attained　a　great

size；　their　wings　having　a　span　of　eighteen　or　twenty　feet。

These　animals；　in　the　form　and　proportions　of　the　head　and　neck

relatively　to　the　body；　and　in　the　fact　that　the　ends　of　the

jaws　were　often；　if　not　always；　more　or　less　extensively

ensheathed　in　horny　beaks；　remind　us　of　birds。　Moreover；　their

bones　contained　air　cavities；　rendering　them　specifically

lighter；　as　is　the　case　in　most　birds。　The　breast　bone　was　large

and　keeled；　as　in　most　birds　and　in　bats；　and　the　shoulder

girdle　is　strikingly　similar　to　that　of　ordinary　birds。　But；　it

seems　to　me；　that　the　special　resemblance　of　pterodactyles　to

birds　ends　here；　unless　I　may　add　the　entire　absence　of　teeth

which　characterises　the　great　pterodactyles　（Pteranodon）

discovered　by　Professor　Marsh。　All　other　known　pterodactyles

have　teeth　lodged　in　sockets。　In　the　vertebral　column　and　the

hind　limbs　there　are　no　special　resemblances　to　birds；　and　when

we　turn　to　the　wings　they　are　found　to　be　constructed　on　a

totally　different　principle　from　those　of　birds。





Fig。　8。Pterodactylus　Spectabilis　（Von　Meyer）。





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。