lectures on evolution-第6部分

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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　between　them　and　modern

lizards；　with　the　prodigious　lapse　of　time　between　the　Permian

epoch　and　the　present　day；　it　may　be　said　that　the　amount　of

change　is　insignificant。　But；　when　we　carry　our　researches

farther　back　in　time；　we　find　no　trace　of　lizards；　nor　of　any

true　reptile　whatever；　in　the　whole　mass　of　formations　beneath

the　Permian。



Now；　it　is　perfectly　clear　that　if　our　palaeontological

collections　are　to　be　taken；　even　approximately；　as　an　adequate

representation　of　all　the　forms　of　animals　and　plants　that　have

ever　lived；　and　if　the　record　furnished　by　the　known　series　of

beds　of　stratified　rock　covers　the　whole　series　of　events　which

constitute　the　history　of　life　on　the　globe；　such　a　fact　as　this

directly　contravenes　the　hypothesis　of　evolution；　because　this

hypothesis　postulates　that　the　existence　of　every　form　must　have

been　preceded　by　that　of　some　form　little　different　from　it。

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