lectures on evolution-第6章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！



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　specific　forms　of

life　than　those　which　are　furnished　by　the　mummified　Ibises　and

Crocodiles　of　Egypt。　A　remarkable　case　is　to　be　found　in　your

own　country；　in　the　neighbourhood　of　the　falls　of　Niagara。

In　the　immediate　vicinity　of　the　whirlpool；　and　again　upon　Goat

Island；　in　the　superficial　deposits　which　cover　the　surface　of

the　rocky　subsoil　in　those　regions；　there　are　found　remains　of

animals　in　perfect　preservation；　and　among　them；　shells

belonging　to　exactly　the　same　species　as　those　which　at　present

inhabit　the　still　waters　of　Lake　Erie。　It　is　evident；　from　the

structure　of　the　country；　that　these　animal　remains　were

deposited　in　the　beds　in　which　they　occur　at　a　time　when　the

lake　extended　over　the　region　in　which　they　are　found。　This

involves　the　conclusion　that　they　lived　and　died　before　the

falls　had　cut　their　way　back　through　the　gorge　of　Niagara；

and；　indeed；　it　has　been　determined　that；　when　these　animals

lived；　the　falls　of　Niagara　must　have　been　at　least　six　miles

further　down　the　river　than　they　are　at　present。

Many　computations　have　been　made　of　the　rate　at　which　the　falls

are　thus　cutting　their　way　back。　Those　computations　have　varied

greatly；　but　I　believe　I　am　speaking　within　the　bounds　of

prudence；　if　I　assume　that　the　falls　of　Niagara　have　not

retreated　at　a　greater　pace　than　about　a　foot　a　year。　Six　miles；

speaking　roughly；　are　30；000　feet；　30；000　feet；　at　a　foot　a

year；　gives　30；000　years；　and　thus　we　are　fairly　justified　in

concluding　that　no　less　a　period　than　this　has　passed　since　the

shell…fish；　whose　remains　are　left　in　the　beds　to　which　I　have

referred；　were　living　creatures。



But　there　is　still　stronger　evidence　of　the　long　duration　of

certain　types。　I　have　already　stated　that；　as　we　work　our　way

through　the　great　series　of　the　Tertiary　formations；　we　find

many　species　of　animals　identical　with　those　which　live　at　the

present　day；　diminishing　in　numbers；　it　is　true；　but　still

existing；　in　a　certain　proportion；　in　the　oldest　of　the　Tertiary

rocks。　Furthermore；　when　we　examine　the　rocks　of　the　Cretaceous

epoch；　we　find　the　remains　of　some　animals　which　the　closest

scrutiny　cannot　show　to　be；　in　any　important　respect；　different

from　those　which　live　at　the　present　time。　That　is　the　case　with

one　of　the　cretaceous　lamp…shells　（Terebratula）；　which

has　continued　to　exist　unchanged；　or　with　insignificant

variations；　down　to　the　present　day。　Such　is　the　case　with　the

Globigerin？；　the　skeletons　of　which；　aggregated　together；

form　a　large　proportion　of　our　English　chalk。

Those　Globigerinae　can　be　traced　down　to　the

Globigerinae　which　live　at　the　surface　of　the　present

great　oceans；　and　the　remains　of　which；　falling　to　the　bottom　of

the　sea；　give　rise　to　a　chalky　mud。　Hence　it　must　be　admitted

that　certain　existing　species　of　animals　show　no　distinct　sign

of　modification；　or　transformation；　in　the　course　of　a　lapse　of

time　as　great　as　that　which　carries　us　back　to　the　Cretaceous

period；　and　which；　whatever　its　absolute　measure；　is　certainly

vastly　greater　than　thirty　thousand　years。



There　are　groups　of　species　so　closely　allied　together；　that　it

needs　the　eye　of　a　naturalist　to　distinguish　them　one　from

another。　If　we　disregard　the　small　differences　which　separate

these　forms；　and　consider　all　the　species　of　such　groups　as

modifications　of　one　type；　we　shall　find　that；　even　among　the

higher　animals；　some　types　have　had　a　marvellous　duration。

In　the　chalk；　for　example；　there　is　found　a　fish　belonging　to

the　highest　and　the　most　differentiated　group　of　osseous　fishes；

which　goes　by　the　name　of　Beryx。　The　remains　of　that　fish

are　among　the　most　beautiful　and　well…preserved　of　the　fossils

found　in　our　English　chalk。　It　can　be　studied　anatomically；　so

far　as　the　hard　parts　are　concerned；　almost　as　well　as　if　it

were　a　recent　fish。　But　the　genus　Beryx