lectures on evolution-第7部分

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adverted；　and　that　we　are　justified　in　believing　that　all　such

cases　are　examples　of　what　I　have　designated　negative　or

indifferent　evidencethat　is　to　say；　they　in　no　way　directly

advance　the　hypothesis　of　evolution；　but　they　are　not　to　be

regarded　as　obstacles　in　the　way　of　our　belief　in　that　doctrine。



I　now　pass　on　to　the　consideration　of　those　cases　which；　for

reasons　which　I　will　point　out　to　you　by　and　by；　are　not　to　be

regarded　as　demonstrative　of　the　truth　of　evolution；　but　which

are　such　as　must　exist　if　evolution　be　true；　and　which　therefore

are；　upon　the　whole；　evidence　in　favour　of　the　doctrine。　If　the

doctrine　of　evolution　be　true；　it　follows；　that；　however　diverse

the　different　groups　of　animals　and　of　plants　may　be；　they　must

all；　at　one　time　or　other；　have　been　connected　by　gradational

forms；　so　that；　from　the　highest　animals；　whatever　they　may　be；

down　to　the　lowest　speck　of　protoplasmic　matter　in　which　life

can　be　manifested；　a　series　of　gradations；　leading　from　one　end

of　the　series　to　the　other；　either　exists　or　has　existed。

Undoubtedly　that　is　a　necessary　postulate　of　the　doctrine　of

evolution。　But　when　we　look　upon　living　Nature　as　it　is；　we　find

a　totally　different　state　of　things。　We　find　that　animals　and

plants　fall　into　groups；　the　different　members　of　which　are

pretty　closely　allied　together；　but　which　are　separated　by

definite；　larger　or　smaller；　breaks；　from　other　groups。　In　other

words；　no　intermediate　forms　which　bridge　over　these　gaps　or

intervals　are；　at　present；　to　be　met　with。



To　illustrate　what　I　mean：　Let　me　call　your　attention　to　those

vertebrate　animals　which　are　most　familiar　to　you；　such　as

mammals；　birds；　and　reptiles。　At　the　present　day；　these　groups

of　animals　are　perfectly　well…defined　from　one　another。　We　know

of　no　animal　now　living　which；　in　any　sense；　is　intermediate

between　the　mammal　and　the　bird；　or　between　the　bird　and　the

reptile；　but；　on　the　contrary；　there　are　many　very　distinct

anatomical　peculiarities；　well…defined　marks；　by　which　the

mammal　is　separated　from　the　bird；　and　the　bird　from　the

reptile。　The　distinctions　are　obvious　and　striking　if　you

compare　the　definitions　of　these　great　groups　as　they　now　exist。



The　same　may　be　said　of　many　of　the　subordinate　groups；　or

orders；　into　which　these　great　classes　are　divided。　At　the

present　time；　for　example；　there　are　numerous　forms　of　non…

ruminant　pachyderms；　or　what　we　may　call　broadly；　the　pig　tribe；

and　many　varieties　of　ruminants。　These　latter　have　their

definite　characteristics；　and　the　former　have　their

distinguishing　peculiarities。　But　there　is　nothing　that　fills　up

the　gap　between　the　ruminants　and　the　pig　tribe。　The　two　are

distinct。　Such　also　is　the　case　in　respect　of　the　minor　groups

of　the　class　of　reptiles。　The　existing　fauna　shows　us

crocodiles；　lizards；　snakes；　and　tortoises；　but　no　connecting

link　between　the　crocodile　and　lizard；　nor　between　the　lizard

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　tw