lectures on evolution- 第8部分

existing　bird　does；　and；　to　that　extent；　diminishes　the

hiatus　between　the　two　classes。





Fig。　3Hesperornis　regalis　（Marsh）



Fig。　4Hesperornis　regalis　（Marsh）

（Side　and　upper　views　of　half　the　lower　jaw；　side　and　end　views

of　a　vertebra　and　a　separate　tooth。）





The　same　formation　has　yielded　another　bird；　Ichthyornis

（Fig。　5）；　which　also　possesses　teeth；　but　the　teeth　are　situated

in　distinct　sockets；　while　those　of　Hesperornis　are　not

so　lodged。　The　latter　also　has　such　very　small；　almost

rudimentary　wings；　that　it　must　have　been　chiefly　a　swimmer　and

a　diver　like　a　Penguin；　while　Ichthyornis　has　strong

wings　and　no　doubt　possessed　corresponding　powers　of　flight。

Ichthyornis　also　differed　in　the　fact　that　its　vertebrae

have　not　the　peculiar　characters　of　the　vertebrae　of　existing

and　of　all　known　tertiary　birds；　but　were　concave　at　each　end。

This　discovery　leads　us　to　make　a　further　modification　in　the

definition　of　the　group　of　birds；　and　to　part　with　another　of

the　characters　by　which　almost　all　existing　birds　are

distinguished　from　reptiles。





Figure。　5Ichthyornis　Dispar　（Marsh）。

Side　and　upper　views　of　half　the　lower　jaw；　and　side　and　end

views　of　a　vertebra。）





Apart　from　the　few　fragmentary　remains　from　the　English

greensand；　to　which　I　have　referred；　the　Mesozoic　rocks；　older

than　those　in　which　Hesperornis　and　Ichthyornis

have　been　discovered；　have　afforded　no　certain　evidence　of

birds；　with　the　remarkable　exception　of　the　Solenhofen　slates。

These　so…called　slates　are　composed　of　a　fine　grained　calcareous

mud　which　has　hardened　into　lithographic　stone；　and　in　which

organic　remains　are　almost　as　well　preserved　as　they　would　be　if

they　had　been　imbedded　in　so　much　plaster　of　Paris。　They　have

yielded　the　Archaeopteryx；　the　existence　of　which　was

first　made　known　by　the　finding　of　a　fossil　feather；　or　rather

of　the　impression　of　one。　It　is　wonderful　enough　that　such　a

perishable　thing　as　a　feather；　and　nothing　more；　should　be

discovered；　yet；　for　a　long　time；　nothing　was　known　of　this　bird

except　its　feather。　But　by　and　by　a　solitary　skeleton　was

discovered　which　is　now　in　the　British　Museum。　The　skull　of　this

solitary　specimen　is　unfortunately　wanting；　and　it　is　therefore

uncertain　whether　the　Archaeopteryx　possessed　teeth　or

not。　But　the　remainder　of　the　skeleton　is　so　well　preserved

as　to　leave　no　doubt　respecting　the　main　features　of　the　animal；

which　are　very　singular。　The　feet　are　not　only　altogether　bird…

like；　but　have　the　special　characters　of　the　feet　of　perching

birds；　while　the　body　had　a　clothing　of　true　feathers。

Nevertheless；　in　some　other　respects；　Archaeopteryx　is

unlike　a　bird　and　like　a　reptile。　There　is　a　long　tail　composed

of　many　vertebrae。　The　structure　of　the　wing　differs　in　some

very　remarkable　respects　from　that　which　it　presents　in　a　true

bird。　In　the　latter；　the　end　of　the　wing　answers　to　the　thumb

and　two　fingers　of　my　hand；　but　the　metacarpal　bones；　or　those

which　answer　to　the　bones　of　the　fingers　which　lie　in　the　palm

of　the　hand；　are　fused　together　into　one　mass；　and　the　whole

apparatus；　except　the　last　joints　of　the　thumb；　is　bound　up　in　a

sheath　of　integument；　while　the　edge　of　the　hand　carries　the

principal　quill…feathers。　In　the　Archaeopteryx；　the

upper…arm　bone　is　like　that　of　a　bird；　and　the　two　bones　of　the

forearm　are　more　or　less　like　those　of　a　bird；　but　the　fingers

are　not　bound　togetherthey　are　free。　What　their　number　may

have　been　is　uncertain；　but　several；　if　not　all；　of　them　were

terminated　by　strong　curved　claws；　not　like　such　as　are

sometimes　found　in　birds；　but　such　as　reptiles　possess；　so　that；

in　the　Archaeopteryx；　we　have　an　animal　which；　to　a

certain　extent；　occupies　a　midway　place　between　a　bird　and　a

reptile。　It　is　a　bird　so　far　as　its　foot　and　sundry　other　parts

of　its　skeleton　are　concerned；　it　is　essentially　and　thoroughly

a　bird　by　its　feathers；　but　it　is　much　more　properly　a　reptile

in　the　fact　that　the　region　which　represents　the　hand　has

separate　bones；　with　claws　resembling　those　which　terminate　the

forelimb　of　a　reptile。　Moreover；　it　has　a　long　reptile…like　tail

with　a　fringe　of　feathers　on　each　side；　while；　in　all　true　birds

hitherto　known；　the　tail　is　relatively　short；　and　the　vertebrae

which　constitute　its　skeleton　are　generally　peculiarly　modified。



Like　the　Anoplotherium　and　the　Palaeotherium；

therefore；　Archaeopteryx　tends　to　fill　up　the　interval

between　groups　which；　in　the　existing　world；　are　widely

separated；　and　to　destroy　the　value　of　the　definitions　of

zoological　groups　based　upon　our　knowledge　of　existing　forms。

And　such　cases　as　these　constitute　evidence　in　favour　of

evolution；　in　so　far　as　they　prove　that；　in　former　periods　of

the　world's　history；　there　were　animals　which　overstepped　the

bounds　of　existing　groups；　and　tended　to　merge　them　into　larger

assemblages。　They　show　that　animal　organisation　is　more　flexible

than　our　knowledge　of　recent　forms　might　have　led　us　to　believe；

and　that　many　structural　permutations　and　combinations；　of　which

the　present　world　gives　us　no　indication；　may　nevertheless

have　existed。



But　it　by　no　means　follows；　because　the　Palaeotherium　has

much　in　common　with　the　horse；　on　the　one　hand；　and　with　the

rhinoceros　on　the　other；　that　it　is　the　intermediate　form

through　which　rhinoceroses　have　passed　to　become　horses；　or

vice　versa；　on　the　contrary；　any　such　supposition　would

certainly　be　erroneous。　Nor　do　I　think　it　likely　that　the

transition　from　the　reptile　to　the　bird　has　been　effected　by

such　a　form　as　Archaeopteryx。　And　it　is　convenient　to

distinguish　these　intermediate　forms　between　two　groups；　which

do　not　represent　the　actual　passage　from　the　one　group　to　the

other；　as　intercalary　types；　from　those　linear

types　which；　more　or　less　approximately；　indicate　the　nature　of

the　steps　by　which　the　transition　from　one　group　to　the　other

was　effected。



I　conceive　that　such　linear　forms；　constituting　a　series　of

natural　gradations　between　the　reptile　and　the　bird；　and

enabling　us　to　understand　the　manner　in　which　the　reptilian　has

been　metamorphosed　into　the　bird　type；　are　really　to　be　found

among　a　group　of　ancient　and　extinct　terrestrial　reptiles　known

as　the　Ornithoscelida。　The　remains　of　these　animals　occur

throughout　the　series　of　mesozoic　formations；　from　the　Trias　to

the　Chalk；　and　there　are　indications　of　their　existence　even　in

the　later　Palaeozoic　strata。



Most　of　these　reptiles；　at　present　known；　are　of　great　size；

some　having　attained　a　length　of　forty　feet　or　perhaps　more。

The　majority　resembled　lizards　and　crocodiles　in　their　general

form；　and　many　of　them　were；　like　crocodiles；　protected　by　an

armour　of　heavy　bony　plates。　But；　in　others；　the　hind　limbs

elongate　and　the　fore　limbs　shorten；　until　their　relative

proportions　approach　those　which　are　observed　in　the　short…

winged；　flightless；　ostrich　tribe　among　birds。



The　skull　is　relatively　light；　and　in　some　cases　the　jaws；

though　bearing　teeth；　are　beak…like　at　their　extremities　and

appear　to　have　been　enveloped　in　a　horny　sheath。　In　the　part　of

the　vertebral　column　which　lies　between　the　haunch　bones　and　is

called　the　sacrum；　a　number　of　vertebrae　may　unite　together　into

one　whole；　and　in　this　respect；　as　in　some　details　of　its

structure；　the　sacrum　of　these　reptiles　approaches　that

of　birds。



But　it　is　in　the　structure　of　the　pelvis　and　of　the　hind　limb

that　some　of　these　ancient　reptiles　present　the　most　remarkable

approximation　to　birds；　and　clearly　indicate　the　way　by　which

the　most　specialised　and　characteristic　features　of　the　bird　may

have　been　evolved　from　the　corresponding　parts　of　the　reptile。



In　Fig。　6；　the　pelvis　and　hind　limbs　of　a　crocodile；　a　three…

toed　bird；　and　an　ornithoscelidan　are　represented　side　by　side；

and；　for　facility　of　comparison；　in　corresponding　positions；

but　it　must　be　recollected　that；　while　the　position　of　the

bird's　limb　is　natural；　that　of　the　crocodile　is　not　so。　In　the

bird；　the　thigh　bone　lies　close　to　the　body；　and　the　metatarsal

bones　of　the　foot　（ii。；　iii。；　iv。；　Fig。　6）　are；　ordinarily；

raised　into　a　more　or　less　vertical　position；　in　the　crocodile；

the　thigh　bone　stands　out　at　an　angle　from　the　body；　and　the

metatarsal　bones　（i。；　ii。；　iii。；　iv。；　Fig。　6）　lie　flat　on　the

ground。　Hence；　in　the　crocodile；　the　body　usually　lies　squat

between　the　legs；　while；　in　the　bird；　it　is　raised　upon　the　hind

legs；　as　upon　pillars。



In　the　crocodile；　the　pelvis　is　obviously　composed　of　three

bones　on　each　side：　the　ilium　（Il。）；　the　pubis

（Pb。）；　and　the　ischium　（Is。）。　In　the　adult　bird

there　appears　to　be　but　one　bone　on　each　side。　The　examination

of　the　pelvis　of　a　chick；　however；　shows　that　each　half　is　made

up　of　three　bones；　which　answer　to　those　which　remain　distinct

throughout　life　in　the　crocodile。　There　is；　therefore；　a

fundamental　identity　of　plan　in　the　construction　of　the　pelvis

of　both　bird　and　reptile；　though　the　difference　in　form；

relative　size；　and　direction　of　the　corresponding　bones　in　the

two　cases　are　very　great。



But　the　most　striking　contrast　between　the　two　lies　in　the　bones

of　the　leg　and　of　that　part　of　the　foot　termed　the　tarsus；　which

follows　upon　the　leg。　In　the　crocodile；　the　fibula　（F）　is

relatively　large　and　its　lower　end　is　complete。　The　tibia

（T）　has　no　marked　crest　at　its　upper　end；　and　its　lower

end　is　narrow　and　not　pulley…shaped。　There　are　two　rows　of

separate　tarsal　bones　（As。；　Ca。；　&c。）　and　four　distinct

metatarsal　bones；　with　a　rudiment　of　a　fifth。



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