lectures on evolution- 第6部分

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　is　represented；

at　the　present　day；　by　very　closely　allied　species　which　are

living　in　the　Pacific　and　Atlantic　Oceans。　We　may　go　still

farther　back。　I　have　already　referred　to　the　fact　that　the

Carboniferous　formations；　in　Europe　and　in　America；　contain　the

remains　of　scorpions　in　an　admirable　state　of　preservation；　and

that　those　scorpions　are　hardly　distinguishable　from　such　as　now

live。　I　do　not　mean　to　say　that　they　are　not　different；　but

close　scrutiny　is　needed　in　order　to　distinguish　them　from

modern　scorpions。



More　than　this。　At　the　very　bottom　of　the　Silurian　series；　in

beds　which　are　by　some　authorities　referred　to　the　Cambrian

formation；　where　the　signs　of　life　begin　to　fail　useven　there；

among　the　few　and　scanty　animal　remains　which　are　discoverable；

we　find　species　of　molluscous　animals　which　are　so　closely

allied　to　existing　forms　that；　at　one　time；　they　were　grouped

under　the　same　generic　name。　I　refer　to　the　well…known　

Lingula　of　the　Lingula　flags；　lately；　in

consequence　of　some　slight　differences；　placed　in　the　new　genus

Lingulella。　Practically；　it　belongs　to　the　same　great

generic　group　as　the　Lingula；　which　is　to　be　found　at　the

present　day　upon　your　own　shores　and　those　of　many　other　parts

of　the　world。



The　same　truth　is　exemplified　if　we　turn　to　certain　great

periods　of　the　earth's　historyas；　for　example；　the　Mesozoic

epoch。　There　are　groups　of　reptiles；　such　as　the

Ichthyosauria　and　the　Plesiosauria；　which　appear

shortly　after　the　commencement　of　this　epoch；　and　they　occur　in

vast　numbers。　They　disappear　with　the　chalk　and；　throughout　the

whole　of　the　great　series　of　Mesozoic　rocks；　they　present　no

such　modifications　as　can　safely　be　considered　evidence　of

progressive　modification。



Facts　of　this　kind　are　undoubtedly　fatal　to　any　form　of　the

doctrine　of　evolution　which　postulates　the　supposition　that

there　is　an　intrinsic　necessity；　on　the　part　of　animal　forms

which　have　once　come　into　existence；　to　undergo　continual

modification；　and　they　are　as　distinctly　opposed　to　any　view

which　involves　the　belief；　that　such　modification　may　occur；

must　take　place；　at　the　same　rate；　in　all　the　different　types　of

animal　or　vegetable　life。　The　facts；　as　I　have　placed　them

before　you；　obviously　directly　contradict　any　form　of　the

hypothesis　of　evolution　which　stands　in　need　of　these

two　postulates。



But；　one　great　service　that　has　been　rendered　by　Mr。　Darwin　to

the　doctrine　of　evolution　in　general　is　this：　he　has　shown　that

there　are　two　chief　factors　in　the　process　of　evolution：　one　of

them　is　the　tendency　to　vary；　the　existence　of　which　in　all

living　forms　may　be　proved　by　observation；　the　other　is　the

influence　of　surrounding　conditions　upon　what　I　may　call　the

parent　form　and　the　variations　which　are　thus　evolved　from　it。

The　cause　of　the　production　of　variations　is　a　matter　not　at　all

properly　understood　at　present。　Whether　variation　depends　upon

some　intricate　machineryif　I　may　use　the　phraseof　the　living

organism　itself；　or　whether　it　arises　through　the　influence　of

conditions　upon　that　form；　is　not　certain；　and　the　question　may；

for　the　present；　be　left　open。　But　the　important　point　is　that；

granting　the　existence　of　the　tendency　to　the　production　of

variations；　then；　whether　the　variations　which　are　produced

shall　survive　and　supplant　the　parent；　or　whether　the　parent

form　shall　survive　and　supplant　the　variations；　is　a　matter

which　depends　entirely　on　those　conditions　which　give　rise　to

the　struggle　for　existence。　If　the　surrounding　conditions　are

such　that　the　parent　form　is　more　competent　to　deal　with　them；

and　flourish　in　them　than　the　derived　forms；　then；　in　the

struggle　for　existence；　the　parent　form　will　maintain　itself　and

the　derived　forms　will　be　exterminated。　But　if；　on　the　contrary；

the　conditions　are　such　as　to　be　more　favourable　to　a　derived

than　to　the　parent　form；　the　parent　form　will　be　extirpated　and

the　derived　form　will　take　its　place。　In　the　first　case；　there

will　be　no　progression；　no　change　of　structure；　through　any

imaginable　series　of　ages；　in　the　second　place　there　will　be

modification　of　change　and　form。



Thus　the　existence　of　these　persistent　types；　as　I　have　termed

them；　is　no　real　obstacle　in　the　way　of　the　theory　of　evolution。

Take　the　case　of　the　scorpions　to　which　I　have　just　referred。

No　doubt；　since　the　Carboniferous　epoch；　conditions　have　always

obtained；　such　as　existed　when　the　scorpions　of　that　epoch

flourished；　conditions　in　which　scorpions　find　themselves　better

off；　more　competent　to　deal　with　the　difficulties　in　their　way；

than　any　variation　from　the　scorpion　type　which　they　may　have

produced；　and；　for　that　reason；　the　scorpion　type　has　persisted；

and　has　not　been　supplanted　by　any　other　form。　And　there　is　no

reason；　in　the　nature　of　things；　why；　as　long　as　this　world

exists；　if　there　be　conditions　more　favourable　to　scorpions　than

to　any　variation　which　may　arise　from　them；　these　forms　of　life

should　not　persist。



Therefore；　the　stock　objection　to　the　hypothesis　of　evolution；

based　on　the　long　duration　of　certain　animal　and　vegetable

types；　is　no　objection　at　all。　The　facts　of　this　characterand

they　are　numerousbelong　to　that　class　of　evidence　which　I　have

called　indifferent。　That　is　to　say；　they　may　afford　no　direct

support　to　the　doctrine　of　evolution；　but　they　are　capable　of

being　interpreted　in　perfect　consistency　with　it。



There　is　another　order　of　facts　belonging　to　the　class　of

negative　or　indifferent　evidence。　The　great　group　of　Lizards；

which　abound　in　the　present　world；　extends　through　the　whole

series　of　formations　as　far　back　as　the　Permian；　or　latest

Palaeozoic；　epoch。　These　Permian　lizards　differ　astonishingly

little　from　the　lizards　which　exist　at　the　present　day。

Comparing　the　amount　of　the　differences　bet