lectures on evolution-第4章

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And　again；　if　it　be　true　that　all　varieties　of　fishes　and　the

great　whales；　and　the　like；　made　their　appearance　on　the　fifth

day；　we　ought　to　find　the　remains　of　these　animals　in　the　older

rocksin　those　which　were　deposited　before　the　Carboniferous

epoch。　Fishes　we　do　find；　in　considerable　number　and　variety；

but　the　great　whales　are　absent；　and　the　fishes　are　not　such　as

now　live。　Not　one　solitary　species　of　fish　now　in　existence　is

to　be　found　in　the　Devonian　or　Silurian　formations。　Hence　we　are

introduced　afresh　to　the　dilemma　which　I　have　already　placed

before　you：　either　the　animals　which　came　into　existence　on　the

fifth　day　were　not　such　as　those　which　are　found　at　present；　are

not　the　direct　and　immediate　ancestors　of　those　which　now　exist；

in　which　case；　either　fresh　creations　of　which　nothing　is　said；

or　a　process　of　evolution；　must　have　occurred；　or　else　the　whole

story　must　be　given　up；　as　not　only　devoid　of　any　circumstantial

evidence；　but　contrary　to　such　evidence　as　exists。



I　placed　before　you　in　a　few　words；　some　little　time　ago；　a

statement　of　the　sum　and　substance　of　Milton's　hypothesis。

Let　me　now　try　to　state　as　briefly；　the　effect　of　the

circumstantial　evidence　bearing　upon　the　past　history　of　the

earth　which　is　furnished；　without　the　possibility　of　mistake；

with　no　chance　of　error　as　to　its　chief　features；　by　the

stratified　rocks。　What　we　find　is；　that　the　great　series　of

formations　represents　a　period　of　time　of　which　our　human

chronologies　hardly　afford　us　a　unit　of　measure。　I　will　not

pretend　to　say　how　we　ought　to　estimate　this　time；　in　millions

or　in　billions　of　years。　For　my　purpose；　the　determination　of

its　absolute　duration　is　wholly　unessential。　But　that　the　time

was　enormous　there　can　be　no　question。



It　results　from　the　simplest　methods　of　interpretation；　that

leaving　out　of　view　certain　patches　of　metamorphosed　rocks；　and

certain　volcanic　products；　all　that　is　now　dry　land　has　once

been　at　the　bottom　of　the　waters。　It　is　perfectly　certain　that；

at　a　comparatively　recent　period　of　the　world's　historythe

Cretaceous　epochnone　of　the　great　physical　features　which　at

present　mark　the　surface　of　the　globe　existed。　It　is　certain

that　the　Rocky　Mountains　were　not。　It　is　certain　that　the

Himalaya　Mountains　were　not。　It　is　certain　that　the　Alps　and　the

Pyrenees　had　no　existence。　The　evidence　is　of　the　plainest

possible　character　and　is　simply　this：We　find　raised　up　on　the

flanks　of　these　mountains；　elevated　by　the　forces　of　upheaval

which　have　given　rise　to　them；　masses　of　Cretaceous　rock　which

formed　the　bottom　of　the　sea　before　those　mountains　existed。

It　is　therefore　clear　that　the　elevatory　forces　which　gave　rise

to　the　mountains　operated　subsequently　to　the　Cretaceous　epoch；

and　that　the　mountains　themselves　are　largely　made　up　of　the

materials　deposited　in　the　sea　which　once　occupied　their　place。

As　we　go　back　in　time；　we　meet　with　constant　alternations　of　sea

and　land；　of　estuary　and　open　ocean；　and；　in　correspondence　with

these　alternations；　we　observe　the　changes　in　the　fauna　and

flora　to　which　I　have　referred。



But　the　inspection　of　these　changes　gives　us　no　right　to　believe

that　there　has　been　any　discontinuity　in　natural　processes。

There　is　no　trace　of　general　cataclysms；　of　universal　deluges；

or　sudden　destructions　of　a　whole　fauna　or　flora。

The　appearances　which　were　formerly　interpreted　in　that　way　have

all　been　shown　to　be　delusive；　as　our　knowledge　has　increased

and　as　the　blanks　which　formerly　appeared　to　exist　between　the

different　formations　have　been　filled　up。　That　there　is　no

absolute　break　between　formation　and　formation；　that　there　has

been　no　sudden　disappearance　of　all　the　forms　of　life　and

replacement　of　them　by　others；　but　that　changes　have　gone　on

slowly　and　gradually；　that　one　type　has　died　out　and　another　has

taken　its　place；　and　that　thus；　by　insensible　degrees；　one　fauna

has　been　replaced　by　another；　are　conclusions　strengthened　by

constantly　increasing　evidence。　So　that　within　the　whole　of　the

immense　period　indicated　by　the　fossiliferous　stratified　rocks；

there　is　assuredly　not　the　slightest　proof　of　any　break　in　the

uniformity　of　Nature's　operations；　no　indication　that　events

have　followed　other　than　a　clear　and　orderly　sequence。



That；　I　say；　is　the　natural　and　obvious　teaching　of　the

circumstantial　evidence　contained　in　the　stratified　rocks。　I

leave　you　to　consider　how　far；　by　any　ingenuity　of

interpretation；　by　any　stretching　of　the　meaning　of　language；　it

can　be　brought　into　harmony　with　the　Miltonic　hypothesis。



There　remains　the　third　hypothesis；　that　of　which　I　have　spoken

as　the　hypothesis　of　evolution；　and　I　purpose　that；　in　lectures

to　come；　we　should　discuss　it　as　carefully　as　we　have　considered

the　other　two　hypotheses。　I　need　not　say　that　it　is　quite

hopeless　to　look　for　testimonial　evidence　of　evolution。　The　very

nature　of　the　case　precludes　the　possibility　of　such　evidence；

for　the　human　race　can　no　more　be　expected　to　testify　to　its　own

origin；　than　a　child　can　be　tendered　as　a　witness　of　its　own

birth。　Our　sole　inquiry　is；　what　foundation　circumstantial

evidence　lends　to　the　hypothesis；　or　whether　it　lends　none；　or

whether　it　controverts　the　hypothesis。　I　shall　deal　with　the

matter　entirely　as　a　question　of　history。　I　shall　not　indulge　in

the　discussion　of　any　speculative　probabilities。　I　shall　not

attempt　to　show　that　Nature　is　unintelligible　unless　we　adopt

some　such　hypothesis。　For　anything　I　know　about　the　matter；　it

may　be　the　way　of　Nature　to　be　unintelligible；　she　is　often

puzzling；　and　I　have　no　reason　to　suppose　that　she　is　bound　to

fit　herself　to　our　notions。



I　shall　place　before　you　three　kinds　of　evidence　entirely　based

upon　what　is　known　of　the　forms　of　animal　life　which　are

contained　in　the　series　of　stratified　rocks。　I　shall　endeavour

to　show　you　that　there　is　one　kind　of　evidence　which　is　neutral；

which　neither　helps　evolution　nor　is　inconsistent　with　it。

I　shall　then　bring　forward　a　second　kind　of　evidence　which

indicates　a　strong　probability　in　favour　of　evolution；　but　does

not　prove　it；　and；　lastly；　I　shall　adduce　a　third　kind　of

evidence　which；　being　as　complete　as　any　evidence　which　we　can

hope　to　obtain　upon　such　a　subject；　and　being　wholly　and

strikingly　in　favour　of　evolution；　may　fairly　be　called

demonstrative　evidence　of　its　occurrence。







LECTURES　ON　EVOLUTION

II



THE　HYPOTHESIS　OF　EVOLUTION。　THE　NEUTRAL　AND

THE　FAVOURABLE　EVIDENCE。



In　the　preceding　lecture　I　pointed　out　that　there　are　three

hypotheses　which　may　be　entertained；　and　which　have　been

entertained；　respecting　the　past　history　of　life　upon　the　globe。

According　to　the　first　of　these　hypotheses；　living　beings；　such

as　now　exist；　have　existed　from　all　eternity　upon　this　earth。

We　tested　that　hypothesis　by　the　circumstantial　evidence；　as　I

called　it；　which　is　furnished　by　the　fossil　remains　contained　in

the　earth's　crust；　and　we　found　that　it　was　obviously　untenable。

I　then　proceeded　to　consider　the　second　hypothesis；　which　I

termed　the　Miltonic　hypothesis；　not　because　it　is　of　any

particular　consequence　whether　John　Milton　seriously　entertained

it　or　not；　but　because　it　is　stated　in　a　clear　and　unmistakable

manner　in　his　great　poem。　I　pointed　out　to　you　that　the　evidence

at　our　command　as　completely　and　fully　negatives　that　hypothesis

as　it　did　the　preceding　one。　And　I　confess　that　I　had　too　much

respect　for　your　intelligence　to　think　it　necessary　to　add　that

the　negation　was　equally　clear　and　equally　valid；　whatever　the

source　from　which　that　hypothesis　might　be　derived；　or　whatever

the　authority　by　which　it　might　be　supported。　I　further　stated

that；　according　to　the　third　hypothesis；　or　that　of　evolution；

the　existing　state　of　things　is　the　last　term　of　a　long　series

of　states；　which；　when　traced　back；　would　be　found　to　show　no

interruption　and　no　breach　in　the　continuity　of　natural

causation。　I　propose；　in　the　present　and　the　following　lecture；

to　test　this　hypothesis　rigorously　by　the　evidence　at　command；

and　to　inquire　how　far　that　evidence　can　be　said　to　be

indifferent　to　it；　how　far　it　can　be　said　to　be　favourable　to

it；　and；　finally；　how　far　it　can　be　said　to　be　demonstrative。



From　almost　the　origin　of　the　discussions　about　the　existing

condition　of　the　animal　and　vegetable　worlds　and　the　causes

which　have　determined　that　condition；　an　argument　has　been　put

forward　as　an　objection　to　evolution；　which　we　shall　have　to

consider　very　seriously。　It　is　an　argument　which　was　first

clearly　stated　by　Cuvier　in　his　criticism　of　the　doctrines

propounded　by　his　great　contemporary；　Lamarck。　The　French

expedition　to　Egypt　had　called　the　attention　of　learned　men　to

the　wonderful　store　of　antiquities　in　that　country；　and　there

had　been　brought　back　to　France　numerous　mummified　corpses　of

the　animals　which　the　ancient　Egyptians　revered　and　preserved；

and　which；　at　a　reasonable　computation；　must　have　lived　not　less

than　three　or　four　thousand　years　before　the　time　at　which　they

were　thus　brought　to　light。　Cuvier　endeavoured　to　test　the

hypothesis　that　animals　have　undergone　gradual　and　progressive

modifications　of　structure；　by　comparing　the　skeletons　and　such

other　parts　of　the　mummies　as　were　in　a　fitting　state　of

preservation；　with　the　corresponding　parts　of　the

representatives　of　the　same　species　now　living　in　Egypt。

He　arrived　at　the　conviction　that　no　appreciable　change　had

taken　place　in　these　animals　in　the　course　of　this　considerable

lapse　of　time；　and　the　justice　of　his　conclusion　is

not　disputed。



It　is　obvious　that；　if　it　can　be　proved　that　animals　have

endured；　without　undergoing　any　demonstrable　change　of

structure；　for　so　long　a　period　as　four　thousand　years；　no　form

of　the　hypothesis　of　evolution　which　assumes　that　animals

undergo　a　constant　and　necessary　progressive　change　can　be

tenable；　unless；　indeed；　it　be　further　assumed　that　four

thousand　years　is　too　short　a　time　for　the　production　of　a

change　sufficiently　great　to　be　detected。



But　it　is　no　less　plain　that　if　the　process　of　evolution　of

animals　is　not　independent　of　surrounding　conditions；　if　it　may

be　indefinitely　hastened　or　retarded　by　variations　in　these

conditions；　or　if　evolution　is　simply　a　process　of　accommodation

to　varying　conditions；　the　argument　against　the　hypothesis　of

evolution　based　on　the　unchanged　character　of　the　Egyptian　fauna

is　worthless。　For　the　monuments　which　are　coeval　with　the

mummies　testify　as　strongly　to　the　absence　of　change　in　the

physical　geography　and　the　general　conditions　of　the　land　of

Egypt；　for　the　time　in　question；　as　the　mummies　do　to　the

unvarying　characters　of　its　living　population。



The　progress　of　research　since　Cuvier's　time　has　supplied　far

more　striking　examples　of　the　long　duration　of　spec