lectures on evolution- 第8部分

and　snake；　nor　between　the　snake　and　the　crocodile；　nor　between

any　two　of　these　groups。　They　are　separated　by　absolute　breaks。

If；　then；　it　could　be　shown　that　this　state　of　things　had　always

existed；　the　fact　would　be　fatal　to　the　doctrine　of　evolution。

If　the　intermediate　gradations；　which　the　doctrine　of　evolution

requires　to　have　existed　between　these　groups；　are　not　to　be

found　anywhere　in　the　records　of　the　past　history　of　the　globe；

their　absence　is　a　strong　and　weighty　negative　argument　against

evolution；　while；　on　the　other　hand；　if　such　intermediate　forms

are　to　be　found；　that　is　so　much　to　the　good　of　evolution；

although；　for　reasons　which　I　will　lay　before　you　by　and　by；　we

must　be　cautious　in　our　estimate　of　the　evidential　cogency　of

facts　of　this　kind。



It　is　a　very　remarkable　circumstance　that；　from　the　commencement

of　the　serious　study　of　fossil　remains；　in　fact；　from　the　time

when　Cuvier　began　his　brilliant　researches　upon　those　found　in

the　quarries　of　Montmartre；　palaeontology　has　shown　what　she　was

going　to　do　in　this　matter；　and　what　kind　of　evidence　it　lay　in

her　power　to　produce。



I　said　just　now　that；　in　the　existing　Fauna；　the　group　of　pig…

like　animals　and　the　group　of　ruminants　are　entirely　distinct；

but　one　of　the　first　of　Cuvier's　discoveries　was　an　animal　which

he　called　the　Anoplotherium；　and　which　proved　to　be；　in　a

great　many　important　respects；　intermediate　in　character　between

the　pigs；　on　the　one　hand；　and　the　ruminants　on　the　other。

Thus；　research　into　the　history　of　the　past　did；　to　a　certain

extent；　tend　to　fill　up　the　breach　between　the　group　of

ruminants　and　the　group　of　pigs。　Another　remarkable　animal

restored　by　the　great　French　palaeontologist；　the

Palaeotherium；　similarly　tended　to　connect　together

animals　to　all　appearance　so　different　as　the　rhinoceros；　the

horse；　and　the　tapir。　Subsequent　research　has　brought　to　light

multitudes　of　facts　of　the　same　order；　and　at　the　present　day；

the　investigations　of　such　anatomists　as　Rutimeyer　and　Gaudry

have　tended　to　fill　up；　more　and　more；　the　gaps　in　our　existing

series　of　mammals；　and　to　connect　groups　formerly　thought　to

be　distinct。



But　I　think　it　may　have　an　especial　interest　if；　instead　of

dealing　with　these　examples；　which　would　require　a　great　deal　of

tedious　osteological　detail；　I　take　the　case　of　birds　and

reptiles；　groups　which；　at　the　present　day；　are　so　clearly

distinguished　from　one　another　that　there　are　perhaps　no　classes

of　animals　which；　in　popular　apprehension；　are　more　completely

separated。　Existing　birds；　as　you　are　aware；　are　covered　with

feathers；　their　anterior　extremities；　specially　and　peculiarly

modified；　are　converted　into　wings　by　the　aid　of　which　most　of

them　are　able　to　fly；　they　walk　upright　upon　two　legs；　and　these

limbs；　when　they　are　considered　anatomically；　present　a　great

number　of　exceedingly　remarkable　peculiarities；　to　which　I　may

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；　s