lectures on evolution-第1章

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Lectures　on　Evolution


by　Thomas　Henry　Huxley









I

THE　THREE　HYPOTHESES　RESPECTING　THE　HISTORY　OF　NATURE



We　live　in　and　form　part　of　a　system　of　things　of　immense

diversity　and　perplexity；　which　we　call　Nature；　and　it　is　a

matter　of　the　deepest　interest　to　all　of　us　that　we　should　form

just　conceptions　of　the　constitution　of　that　system　and　of　its

past　history。　With　relation　to　this　universe；　man　is；　in　extent；

little　more　than　a　mathematical　point；　in　duration　but　a

fleeting　shadow；　he　is　a　mere　reed　shaken　in　the　winds　of　force。

But　as　Pascal　long　ago　remarked；　although　a　mere　reed；　he　is　a

thinking　reed；　and　in　virtue　of　that　wonderful　capacity　of

thought；　he　has　the　power　of　framing　for　himself　a　symbolic

conception　of　the　universe；　which；　although　doubtless　highly

imperfect　and　inadequate　as　a　picture　of　the　great　whole；　is　yet

sufficient　to　serve　him　as　a　chart　for　the　guidance　of　his

practical　affairs。　It　has　taken　long　ages　of　toilsome　and　often

fruitless　labour　to　enable　man　to　look　steadily　at　the　shifting

scenes　of　the　phantasmagoria　of　Nature；　to　notice　what　is　fixed

among　her　fluctuations；　and　what　is　regular　among　her　apparent

irregularities；　and　it　is　only　comparatively　lately；　within　the

last　few　centuries；　that　the　conception　of　a　universal　order　and

of　a　definite　course　of　things；　which　we　term　the　course　of

Nature；　has　emerged。



But；　once　originated；　the　conception　of　the　constancy　of　the

order　of　Nature　has　become　the　dominant　idea　of　modern　thought。

To　any　person　who　is　familiar　with　the　facts　upon　which　that

conception　is　based；　and　is　competent　to　estimate　their

significance；　it　has　ceased　to　be　conceivable　that　chance　should

have　any　place　in　the　universe；　or　that　events　should　depend

upon　any　but　the　natural　sequence　of　cause　and　effect。　We　have

come　to　look　upon　the　present　as　the　child　of　the　past　and　as

the　parent　of　the　future；　and；　as　we　have　excluded　chance　from　a

place　in　the　universe；　so　we　ignore；　even　as　a　possibility；　the

notion　of　any　interference　with　the　order　of　Nature。

Whatever　may　be　men's　speculative　doctrines；　it　is　quite　certain

that　every　intelligent　person　guides　his　life　and　risks　his

fortune　upon　the　belief　that　the　orderof　Nature　is　constant；　and

that　the　chain　of　natural　causation　is　never　broken。



In　fact；　no　belief　which　we　entertain　has　so　complete　a　logical

basis　as　that　to　which　I　have　just　referred。　It　tacitly

underlies　every　process　of　reasoning；　it　is　the　foundation　of

every　act　of　the　will。　It　is　based　upon　the　broadest　induction；

and　it　is　verified　by　the　most　constant；　regular；　and　universal

of　deductive　processes。　But　we　must　recollect　that　any　human

belief；　however　broad　its　basis；　however　defensible　it　may　seem；

is；　after　all；　only　a　probable　belief；　and　that　our　widest　and

safest　generalisations　are　simply　statements　of　the　highest

degree　of　probability。　Though　we　are　quite　clear　about　the

constancy　of　the　order　of　Nature；　at　the　present　time；　and　in

the　present　state　of　things；　it　by　no　means　necessarily　follows

that　we　are　justified　in　expanding　this　generalisation　into　the

infinite　past；　and　in　denying；　absolutely；　that　there　may　have

been　a　time　when　Nature　did　not　follow　a　fixed　order；　when　the

relations　of　cause　and　effect　were　not　definite；　and　when　extra…

natural　agencies　interfered　with　the　general　course　of　Nature。

Cautious　men　will　allow　that　a　universe　so　different　from　that

which　we　know　may　have　existed；　just　as　a　very　candid　thinker

may　admit　that　a　world　in　which　two　and　two　do　not　make　four；

and　in　which　two　straight　lines　do　inclose　a　space；　may　exist。

But　the　same　caution　which　forces　the　admission　of　such

possibilities　demands　a　great　deal　of　evidence　before　it

recognises　them　to　be　anything　more　substantial。　And　when　it　is

asserted　that；　so　many　thousand　years　ago；　events　occurred　in　a

manner　utterly　foreign　to　and　inconsistent　with　the　existing

laws　of　Nature；　men；　who　without　being　particularly　cautious；

are　simply　honest　thinkers；　unwilling　to　deceive　themselves　or

delude　others；　ask　for　trustworthy　evidence　of　the　fact。



Did　things　so　happen　or　did　they　not？　This　is　a　historical

question；　and　one　the　answer　to　which　must　be　sought　in　the　same

way　as　the　solution　of　any　other　historical　problem。



So　far　as　I　know；　there　are　only　three　hypotheses　which　ever

have　been　entertained；　or　which　well　can　be　entertained；

respecting　the　past　history　of　Nature。　I　will；　in　the　first

place；　state　the　hypotheses；　and　then　I　will　consider　what

evidence　bearing　upon　them　is　in　our　possession；　and　by　what

light　of　criticism　that　evidence　is　to　be　interpreted。



Upon　the　first　hypothesis；　the　assumption　is；　that　phenomena　of

Nature　similar　to　those　exhibited　by　the　present　world　have

always　existed；　in　other　words；　that　the　universe　has　existed；

from　all　eternity；　in　what　may　be　broadly　termed　its

present　condition。



The　second　hypothesis　is　that　the　present　state　of　things　has

had　only　a　limited　duration；　and　that；　at　some　period　in　the

past；　a　condition　of　the　world；　essentially　similar　to　that

which　we　now　know；　came　into　existence；　without　any　precedent

condition　from　which　it　could　have　naturally　proceeded。

The　assumption　that　successive　states　of　Nature　have　arisen；

each　without　any　relation　of　natural　causation　to　an　antecedent

state；　is　a　mere　modification　of　this　second　hypothesis。



The　third　hypothesis　also　assumes　that　the　present　state　of

things　has　had　but　a　limited　duration；　but　it　supposes　that　this

state　has　been　evolved　by　a　natural　process　from　an　antecedent

state；　and　that　from　another；　and　so　on；　and；　on　this

hypothesis；　the　attempt　to　assign　any　limit　to　the　series　of

past　changes　is；　usually；　given　up。



It　is　so　needful　to　form　clear　and　distinct　notions　of　what　is

really　meant　by　each　of　these　hypotheses　that　I　will　ask　you　to

imagine　what；　according　to　each；　would　have　been　visible　to　a

spectator　of　the　events　which　constitute　the　history　of　the

earth。　On　the　first　hypothesis；　however　far　back　in　time　that

spectator　might　be　placed；　he　would　see　a　world　essentially；

though　perhaps　not　in　all　its　details；　similar　to　that　which　now

exists。　The　animals　which　existed　would　be　the　ancestors　of

those　which　now　live；　and　similar　to　them；　the　plants；　in　like

manner；　would　be　such　as　we　know；　and　the　mountains；　plains；　and

waters　would　foreshadow　the　salient　features　of　our　present　land

and　water。　This　view　was　held　more　or　less　distinctly；　sometimes

combined　with　the　notion　of　recurrent　cycles　of　change；　in

ancient　times；　and　its　influence　has　been　felt　down　to　the

present　day。　It　is　worthy　of　remark　that　it　is　a　hypothesis

which　is　not　inconsistent　with　the　doctrine　of

Uniformitarianism；　with　which　geologists　are　familiar。

That　doctrine　was　held　by　Hutton；　and　in　his　earlier　days　by

Lyell。　Hutton　was　struck　by　the　demonstration　of　astronomers

that　the　perturbations　of　the　planetary　bodies；　however　great

they　may　be；　yet　sooner　or　later　right　themselves；　and　that　the

solar　system　possesses　a　self…adjusting　power　by　which　these

aberrations　are　all　brought　back　to　a　mean　condition。

Hutton　imagined　that　the　like　might　be　true　of　terrestrial

changes；　although　no　one　recognised　more　clearly　than　he　the

fact　that　the　dry　land　is　being　constantly　washed　down　by　rain

and　rivers　and　deposited　in　the　sea；　and　that　thus；　in　a　longer

or　shorter　time；　the　inequalities　of　the　earth's　surface　must　be

levelled；　and　its　high　lands　brought　down　to　the　ocean。

But；　taking　into　account　the　internal　forces　of　the　earth；

which；　upheaving　the　sea…bottom　give　rise　to　new　land；　he

thought　that　these　operations　of　degradation　and　elevation　might

compensate　each　other；　and　that　thus；　for　any　assignable　time；

the　general　features　of　our　planet　might　remain　what　they　are。

And　inasmuch　as；　under　these　circumstances；　there　need　be　no

limit　to　the　propagation　of　animals　and　plants；　it　is　clear　that

the　consistent　working　out　of　the　uniformitarian　idea　might　lead

to　the　conception　of　the　eternity　of　the　world。　Not　that　I　mean

to　say　that　either　Hutton　or　Lyell　held　this　conception

assuredly　not；　they　would　have　been　the　first　to　repudiate　it。

Nevertheless；　the　logical　development　of　some　of　their　arguments

tends　directly　towards　this　hypothesis。



The　second　hypothesis　supposes　that　the　present　order　of　things；

at　some　no　very　remote　time；　had　a　sudden　origin；　and　that　the

world；　such　as　it　now　is；　had　chaos　for　its　phenomenal

antecedent。　That　is　the　doctrine　which　you　will　find　stated　most

fully　and　clearly　in　the　immortal　poem　of　John　Miltonthe

English　Divina　Commedia　〃Paradise　Lost。〃　I　believe　it

is　largely　to　the　influence　of　that　remarkable　work；　combined

with　the　daily　teachings　to　which　we　have　all　listened　in　our

childhood；　that　this　hypothesis　owes　its　general　wide　diffusion

as　one　of　the　current　beliefs　of　English…speaking　people。　If　you

turn　to　the　seventh　book　of　〃Paradise　Lost；〃　you　will　find　there

stated　the　hypothesis　to　which　I　refer；　which　is　briefly　this：

That　this　visible　universe　of　ours　came　into　existence　at　no

great　distance　of　time　from　the　present；　and　that　the　parts　of

which　it　is　composed　made　their　appearance；　in　a　certain

definite　order；　in　the　space　of　six　natural　days；　in　such　a

manner　that；　on　the　first　of　these　days；　light　appeared；　that；

on　the　second；　the　firmament；　or　sky；　separated　the　waters

above；　from　the　waters　beneath　the　firmament；　that；　on　the　third

day；　the　waters　drew　away　from　the　dry　land；　and　upon　it　a

varied　vegetable　life；　similar　to　that　which　now　exists；　made

its　appearance；　that　the　fourth　day　was　signalised　by　the

apparition　of　the　sun；　the　stars；　the　moon；　and　the　planets；

that；　on　the　fifth　day；　aquatic　animals　originated　within　the

waters；　that；　on　the　sixth　day；　the　earth　gave　rise　to　our　four…

footed　terrestrial　creatures；　and　to　all　varieties　of

terrestrial　animals　except　birds；　which　had　appeared　on　the

preceding　day；　and；　finally；　that　man　appeared　upon　the　earth；

and　the　emergence　of　the　universe　from　chaos　was　finished。

Milton　tells　us；　without　the　least　ambiguity；　what　a　spectator

of　these　marvellous　occurrences　would　have　witnessed。　I　doubt

not　that　his　poem　is　familiar　to　all　of　you；　but　I　should　like

to　recall　one　passage　to　your　minds；　in　order　that　I　may　be

justified　in　what　I　have　said　regarding　the　perfectly　concrete；

definite；　picture　of　the　origin　of　the　animal　world　which　Milton

draws。　He　says：



〃The　sixth；　and　of　creation　last；　arose

With　evening　harp　and　matin；　when　God　said；

'Let　the　earth　bring　forth　soul　living　in　her　kind；

Cattle　and　creeping　things；　and　beast　of　the　earth。

Each　in　their　kind！'　The　earth　obeyed；　and；　straight

Opening　her　fertile　womb；　teemed　at　a　birth

Innumerous　living　creatures；　perfect　forms；

Limbe