lectures on evolution-第6章

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Here；　however；　we　have　to　take　into　consideration　that　important

truth　so　well　insisted　upon　by　Lyell　and　by　Darwinthe

imperfection　of　the　geological　record。　It　can　be　demonstrated

that　the　geological　record　must　be　incomplete；　that　it　can　only

preserve　remains　found　in　certain　favourable　localities　and

under　particular　conditions；　that　it　must　be　destroyed　by

processes　of　denudation；　and　obliterated　by　processes　of

metamorphosis。　Beds　of　rock　of　any　thickness　crammed　full　of

organic　remains；　may　yet；　either　by　the　percolation　of　water

through　them；　or　by　the　influence　of　subterranean　heat；　lose　all

trace　of　these　remains；　and　present　the　appearance　of　beds　of

rock　formed　under　conditions　in　which　living　forms　were　absent。

Such　metamorphic　rocks　occur　in　formations　of　all　ages；　and；　in

various　cases；　there　are　very　good　grounds　for　the　belief　that

they　have　contained　organic　remains；　and　that　those　remains　have

been　absolutely　obliterated。



I　insist　upon　the　defects　of　the　geological　record　the　more

because　those　who　have　not　attended　to　these　matters　are　apt　to

say；　〃It　is　all　very　well；　but；　when　you　get　into　a　difficulty

with　your　theory　of　evolution；　you　appeal　to　the　incompleteness

and　the　imperfection　of　the　geological　record；〃　and　I　want　to

make　it　perfectly　clear　to　you　that　this　imperfection　is　a　great

fact；　which　must　be　taken　into　account　in　all　our　speculations；

or　we　shall　constantly　be　going　wrong。



You　see　the　singular　series　of　footmarks；　drawn　of　its　natural

size　in　the　large　diagram　hanging　up　here　（Fig。　2）；　which　I　owe

to　the　kindness　of　my　friend　Professor　Marsh；　with　whom　I　had

the　opportunity　recently　of　visiting　the　precise　locality　in

Massachusetts　in　which　these　tracks　occur。　I　am；　therefore；　able

to　give　you　my　own　testimony；　if　needed；　that　the　diagram

accurately　represents　what　we　saw。　The　valley　of　the　Connecticut

is　classical　ground　for　the　geologist。　It　contains　great　beds　of

sandstone；　covering　many　square　miles；　which　have　evidently

formed　a　part　of　an　ancient　sea…shore；　or；　it　may　be；　lake…

shore。　For　a　certain　period　of　time　after　their　deposition；

these　beds　have　remained　sufficiently　soft　to　receive　the

impressions　of　the　feet　of　whatever　animals　walked　over　them；

and　to　preserve　them　afterwards；　in　exactly　the　same　way　as　such

impressions　are　at　this　hour　preserved　on　the　shores　of　the　Bay

of　Fundy　and　elsewhere。　The　diagram　represents　the　track　of　some

gigantic　animal；　which　walked　on　its　hind　legs。　You　see　the

series　of　marks　made　alternately　by　the　right　and　by　the　left

foot；　so　that；　from　one　impression　to　the　other　of　the　three…

toed　foot　on　the　same　side；　is　one　stride；　and　that　stride；　as

we　measured　it；　is　six　feet　nine　inches。　I　leave　you；　therefore；

to　form　an　impression　of　the　magnitude　of　the　creature　which；　as

it　walked　along　the　ancient　shore；　made　these　impressions。





Fig。　2。Tracks　of　Brontozoum。





Of　such　impressions　there　are　untold　thousands　upon　these

sandstones。　Fifty　or　sixty　different　kinds　have　been　discovered；

and　they　cover　vast　areas。　But；　up　to　this　present　time；　not　a

bone；　not　a　fragment；　of　any　one　of　the　animals　which　left　these

great　footmarks　has　been　found；　in　fact；　the　only　animal　remains

which　have　been　met　with　in　all　these　deposits；　from　the　time　of

their　discovery　to　the　present　daythough　they　have　been

carefully　hunted　overis　a　fragmentary　skeleton　of　one　of　the

smaller　forms。　What　has　become　of　the　bones　of　all　these

animals？　You　see　we　are　not　dealing　with　little　creatures；　but

with　animals　that　make　a　step　of　six　feet　nine　inches；　and　their

remains　must　have　been　left　somewhere。　The　probability　is；　that

they　have　been　dissolved　away；　and　completely　lost。



I　have　had　occasion　to　work　out　the　nature　of　fossil　remains；　of

which　there　was　nothing　left　except　casts　of　the　bones；　the

solid　material　of　the　skeleton　having　been　dissolved　out　by

percolating　water。　It　was　a　chance；　in　this　case；　that　the

sandstone　happened　to　be　of　such　a　constitution　as　to　set；　and

to　allow　the　bones　to　be　afterward　dissolved　out；　leaving

cavities　of　the　exact　shape　of　the　bones。　Had　that　constitution

been　other　than　what　it　was；　the　bones　would　have　been

dissolved；　the　layers　of　sandstone　would　have　fallen　together

into　one　mass；　and　not　the　slightest　indication　that　the　animal

had　existed　would　have　been　discoverable。



I　know　of　no　more　striking　evidence　than　these　facts　afford；　of

the　caution　which　should　be　used　in　drawing　the　conclusion；　from

the　absence　of　organic　remains　in　a　deposit；　that　animals　or

plants　did　not　exist　at　the　time　it　was　formed。　I　believe　that；

with　a　right　understanding　of　the　doctrine　of　evolution　on　the

one　hand；　and　a　just　estimation　of　the　importance　of　the

imperfection　of　the　geological　record　on　the　other；　all

difficulty　is　removed　from　the　kind　of　evidence　to　which　I　have

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　a