lectures on evolution-第7章

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have　occasion　to　advert　incidentally　as　I　go　on；　and　which　are

not　met　with；　even　approximately；　in　any　existing　forms　of

reptiles。　On　the　other　hand；　existing　reptiles　have　no　feathers。

They　may　have　naked　skins；　or　be　covered　with　horny　scales；　or

bony　plates；　or　with　both。　They　possess　no　wings；　they　neither

fly　by　means　of　their　fore…limbs；　nor　habitually　walk　upright

upon　their　hind…limbs；　and　the　bones　of　their　legs　present　no

such　modifications　as　we　find　in　birds。　It　is　impossible　to

imagine　any　two　groups　more　definitely　and　distinctly　separated；

notwithstanding　certain　characters　which　they　possess　in　common。



As　we　trace　the　history　of　birds　back　in　time；　we　find　their

remains；　sometimes　in　great　abundance；　throughout　the　whole

extent　of　the　tertiary　rocks；　but；　so　far　as　our　present

knowledge　goes；　the　birds　of　the　tertiary　rocks　retain　the　same

essential　characters　as　the　birds　of　the　present　day。　In　other

words；　the　tertiary　birds　come　within　the　definition　of　the

class　constituted　by　existing　birds；　and　are　as　much　separated

from　reptiles　as　existing　birds　are。　Not　very　long　ago　no

remains　of　birds　had　been　found　below　the　tertiary　rocks；　and　I

am　not　sure　but　that　some　persons　were　prepared　to　demonstrate

that　they　could　not　have　existed　at　an　earlier　period。　But；　in

the　course　of　the　last　few　years；　such　remains　have　been

discovered　in　England；　though；　unfortunately；　in　so　imperfect

and　fragmentary　a　condition；　that　it　is　impossible　to　say

whether　they　differed　from　existing　birds　in　any　essential

character　or　not。　In　your　country　the　development　of　the

cretaceous　series　of　rocks　is　enormous；　the　conditions　under

which　the　later　cretaceous　strata　have　been　deposited　are　highly

favourable　to　the　preservation　of　organic　remains；　and　the

researches；　full　of　labour　and　risk；　which　have　been　carried　on

by　Professor　Marsh　in　these　cretaceous　rocks　of　Western　America；

have　rewarded　him　with　the　discovery　of　forms　of　birds　of　which

we　had　hitherto　no　conception。　By　his　kindness；　I　am　enabled　to

place　before　you　a　restoration　of　one　of　these　extraordinary

birds；　every　part　of　which　can　be　thoroughly　justified　by　the

more　or　less　complete　skeletons；　in　a　very　perfect　state　of

preservation；　which　he　has　discovered。　This　Hesperornis

（Fig。　3）；　which　measured　between　five　and　six　feet　in　length；　is

astonishingly　like　our　existing　divers　or　grebes　in　a　great　many

respects；　so　like　them　indeed　that；　had　the　skeleton　of

Hesperornis　been　found　in　a　museum　without　its　skull；　it

probably　would　have　been　placed　in　the　same　group　of　birds　as

the　divers　and　grebes　of　the　present　day。

But　Hesperornis　differs　from　all　existing　birds；　and　so

far　resembles　reptiles；　in　one　important　particularit　is

provided　with　teeth。　The　long　jaws　are　armed　with　teeth　which

have　curved　crowns　and　thick　roots　（Fig。　4）；　and　are　not　set　in

distinct　sockets；　but　are　lodged　in　a　groove。　In　possessing　true

teeth；　the　Hesperornis　differs　from　every　existing　bird；

and　from　every　bird　yet　discovered　in　the　tertiary　formations；

the　tooth…like　serrations　of　the　jaws　in　the　Odontopteryx

of　the　London　clay　being　mere　processes　of　the　bony　substance　of

the　jaws；　and　not　teeth　in　the　proper　sense　of　the　word。　In　view

of　the　characteristics　of　this　bird　we　are　therefore　obliged　to

modify　the　definitions　of　the　classes　of　birds　and　reptiles。

Before　the　discovery　of　Hesperornis；　the　definition　of

the　class　Aves　based　upon　our　knowledge　of　existing　birds　might

have　been　extended　to　all　birds；　it　might　have　been　said　that

the　absence　of　teeth　was　characteristic　of　the　class　of　birds；

but　the　discovery　of　an　animal　which；　in　every　part　of　its

skeleton；　closely　agrees　with　existing　birds；　and　yet　possesses

teeth；　shows　that　there　were　ancient　birds　which；　in　respect　of

possessing　teeth；　approached　reptiles　more　nearly　than　any

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　short