darwin and modern science-第85章

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ica。　　（Not　a　few　of　those　who　are　fascinated　by；　and　satisfied　with；　the　statistical　aspect　of　distribution　still　have　a　strong　dislike　to　the　use　of　〃bridges〃　if　these　lead　over　deep　seas；　and　they　get　over　present　discontinuous　occurrences　by　a　former　〃universal　or　sub…universal　distribution〃　of　their　groups。　　This　is　indeed　an　easy　method　of　cutting　the　knot；　but　in　reality　they　shunt　the　question　only　a　stage　or　two　back；　never　troubling　to　explain　how　their　groups　managed　to　attain　to　that　sub…universal　range；　or　do　they　still　suppose　that　the　whole　world　was　originally　one　paradise　where　everything　lived　side　by　side；　until　sin　and　strife　and　glacial　epochs　left　nothing　but　scattered　survivors？

The　permanence　of　the　great　ocean…basins　had　become　a　dogma　since　it　was　found　that　a　universal　elevation　of　the　land　to　the　extent　of　100　fathoms　would　produce　but　little　changes；　and　when　it　was　shown　that　even　the　1000　fathom…line　followed　the　great　masses　of　land　rather　closely；　and　still　leaving　the　great　basins　（although　transgression　of　the　sea　to　the　same　extent　would　change　the　map　of　the　world　beyond　recognition）；　by　general　consent　one　mile　was　allowed　as　the　utmost　speculative　limit　of　subsidence。　Naturally　two　or　three　miles；　the　average　depth　of　the　oceans；　seems　enormous；　and　yet　such　a　difference　in　level　is　as　nothing　in　comparison　with　the　size　of　the　Earth。　　On　a　clay　model　globe　ten　feet　in　diameter　an　ocean　bed　three　miles　deep　would　scarcely　be　detected；　and　the　highest　mountains　would　be　smaller　than　the　unavoidable　grains　in　the　glazed　surface　of　our　model。　　There　are　but　few　countries　which　have　not　be　submerged　at　some　time　or　other。）

CONNECTION　OF　SOUTH　EASTERN　ASIA　WITH　AUSTRALIA。　　Neumayr's　Sino…Australian　continent　during　mid…Mesozoic　times　was　probably　a　much　changing　Archipelago；　with　final　separations　subsequent　to　the　Cretaceous　period。　　Henceforth　Australasia　was　left　to　its　own　fate；　but　for　a　possible　connection　with　the　antarctic　continent。

AFRICA；　MADAGASCAR；　INDIA。　　The　〃Lemuria〃　of　Sclater　and　Haeckel　cannot　have　been　more　than　a　broad　bridge　in　Jurassic　times；　whether　it　was　ever　available　for　the　Lemurs　themselves　must　depend　upon　the　time　of　its　duration；　the　more　recent　the　better；　but　it　is　difficult　to　show　that　it　lasted　into　the　Miocene。

AFRICA　AND　SOUTH　AMERICA。　　Since　the　opposite　coasts　show　an　entire　absence　of　marine　fossils　and　deposits　during　the　Mesozoic　period；　whilst　further　north　and　south　such　are　known　to　exist　and　are　mostly　identical　on　either　side；　Neumayr　suggested　the　existence　of　a　great　Afro…Son　American　mass　of　land　during　the　Jurassic　epoch。　　Such　land　is　almost　a　necessity　and　is　supported　by　many　facts；　it　would　easily　explain　the　distribution　of　numerous　groups　of　terrestrial　creatures。　　Moreover　to　the　north　of　this　hypothetical　land；　somewhere　across　from　the　Antilles　and　Guiana　to　North　Africa　and　South　Western　Europe；　existed　an　almost　identical　fauna　of　Corals　and　Molluscs；　indicating　either　a　coast…line　or　a　series　of　islands　interrupted　by　shallow　seas；　just　as　one　would　expect　if；　and　when；　a　Brazil…Ethiopian　mass　of　land　were　breaking　up。　　Lastly　from　Central　America　to　the　Mediterranean　stretches　one　of　the　Tertiary　tectonic　lines　of　the　geologists。　　Here　also　the　great　question　is　how　long　this　continent　lasted。　　Apparently　the　South　Atlantic　began　to　encroach　from　the　south　so　that　by　the　later　Cretaceous　epoch　the　land　was　reduced　to　a　comparatively　narrow　Brazil…West　Africa；　remnants　of　which　persisted　certainly　into　the　early　Tertiary；　until　the　South　Atlantic　joined　across　the　equator　with　the　Atlantic　portion　of　the　〃Thetys；〃　leaving　what　remained　of　South　America　isolated　from　the　rest　of　the　world。

ANTARCTIC　CONNECTIONS。　　Patagonia　and　Argentina　seem　to　have　joined　Antartica　during　the　Cretaceous　epoch；　and　this　South　Georgian　bridge　had　broken　down　again　by　mid…Tertiary　times　when　South　America　became　consolidated。　　The　Antarctic　continent；　presuming　that　it　existed；　seems　also　to　have　been　joined；　by　way　of　Tasmania；　with　Australia；　also　during　the　Cretaceous　epoch；　and　it　is　assumed　that　the　great　Australia…Antarctic…　Patagonian　land　was　severed　first　to　the　south　of　Tasmania　and　then　at　the　South　Georgian　bridge。　　No　connection；　and　this　is　important；　is　indicated　between　Antarctica　and　either　Africa　or　Madagascar。

So　far　we　have　followed　what　may　be　called　the　vicissitudes　of　the　great　Permo…Carboniferous　Gondwana　land　in　its　fullest　imaginary　extent；　an　enormous　equatorial　and　south　temperate　belt　from　South　America　to　Africa；　South　India　and　Australia；　which　seems　to　have　provided　the　foundation　of　the　present　Southern　continents；　two　of　which　temporarily　joined　Antarctica；　of　which　however　we　know　nothing　except　that　it　exists　now。

Let　us　next　consider　the　Arctic　and　periarctic　lands。　　Unfortunately　very　little　is　known　about　the　region　within　the　arctic　circle。　　If　it　was　all　land；　or　more　likely　great　changing　archipelagoes；　faunistic　exchange　between　North　America；　Europe　and　Siberia　would　present　no　difficulties；　but　there　is　one　connection　which　engages　much　attention；　namely　a　land　where　now　lies　the　North　temperate　and　Northern　part　of　the　Atlantic　ocean。　　How　far　south　did　it　ever　extend　and　what　is　the　latest　date　of　a　direct　practicable　communication；　say　from　North　Western　Europe　to　Greenland？　　Connections；　perhaps　often　interrupted；　e。g。　between　Greenland　and　Labrador；　at　another　time　between　Greenland　and　Scandinavia；　seem　to　have　existed　at　least　since　the　Permo…Carboniferous　epoch。　　If　they　existed　also　in　late　Cretaceous　and　in　Tertiary　times；　they　would　of　course　easily　explain　exchanges　which　we　know　to　have　repeatedly　taken　place　between　America　and　Europe；　but　they　are　not　proved　thereby；　since　most　of　these　exchanges　can　almost　as　easily　have　occurred　across　the　polar　regions；　and　others　still　more　easily　by　repeated　junction　of　Siberia　with　Alaska。

Let　us　now　describe　a　hypothetical　case　based　on　the　supposition　of　connecting　bridges。　　Not　to　work　in　a　circle；　we　select　an　important　group　which　has　not　served　as　a　basis　for　the　reconstruction　of　bridges；　and　it　must　be　a　group　which　we　feel　justified　in　assuming　to　be　old　enough　to　have　availed　itself　of　ancient　land…connections。

The　occurrence　of　one　species　of　Peripatus　in　the　whole　of　Australia；　Tasmania　and　New　Zealand　（the　latter　being　joined　to　Australia　by　way　of　New　Britain　in　Cretaceous　times　but　not　later）　puts　the　genus　back　into　this　epoch；　no　unsatisfactory　assumption　to　the　morphologist。　　The　apparent　absence　of　Peripatus　in　Madagascar　indicates　that　it　did　not　come　from　the　east　into　Africa；　that　it　was　neither　Afro…Indian；　nor　Afro…Australian；　nor　can　it　have　started　in　South　America。　　We　therefore　assume　as　its　creative　centre　Australia　or　Malaya　in　the　Cretaceous　epoch；　whence　its　occurrence　in　Sumatra；　Malay　Peninsula；　New　Britain；　New　Zealand　and　Australia　is　easily　explained。　　Then　extension　across　Antarctica　to　Patagonia　and　Chile；　whence　it　could　spread　into　the　rest　of　South　America　as　this　became　consolidated　in　early　Tertiary　times。　　For　getting　to　the　Antilles　and　into　Mexico　it　would　have　to　wait　until　the　Miocene；　but　long　before　that　time　it　could　arrive　in　Africa；　there　surviving　as　a　Congolese　and　a　Cape　species。　　This　story　is　unsupported　by　a　single　fossil。　　Peripatus　may　have　been　〃sub…universal〃　all　over　greater　Gondwana　land　in　Carboniferous　times；　and　then　its　absence　from　Madagascar　would　be　difficult　to　explain；　but　the　migrations　suggested　above　amount　to　little　considering　that　the　distance　from　Tasmania　to　South　America　could　be　covered　in　far　less　time　than　that　represented　by　the　whole　of　the　Eocene　epoch　alone。

There　is　yet　another　field；　essentially　the　domain　of　geographical　distribution；　the　cultivation　of　which　promises　fair　to　throw　much　light　upon　Nature's　way　of　making　species。　　This　is　the　study　of　the　organisms　with　regard　to　their　environment。　　Instead　of　revealing　pedigrees　or　of　showing　how　and　when　the　creatures　got　to　a　certain　locality；　it　investigates　how　they　behaved　to　meet　the　ever　changing　conditions　of　their　habitats。　　There　is　a　facies；　characteristic　of；　and　often　peculiar　to；　the　fauna　of　tropical　moist　forests；　another　of　deserts；　of　high　mountains；　of　underground　life　and　so　forth；　these　same　facies　are　stamped　upon　whole　associations　of　animals　and　plants；　although　these　may　beand　in　widely　separated　countries　generally　aredrawn　from　totally　different　families　of　their　respective　orders。　　It　does　not　go　to　the　root　of　the　matter　to　say　that　these　facies　have　been　brought　about　by　the　extermination　of　all　the　others　which　did　not　happen　to　fit　into　their　particular　environment。　　One　might　almost　say　that　tropical　moist　forests　must　have　arboreal　frogs　and　that　these　are　made　out　of　whatever　suitable　material　happened　to　be　available；　in　Australia　and　South　America　Hylidae；　in　Africa　Ranidae；　since　there　Hylas　are　absent。　　The　deserts　must　have　lizards　capable　of　standing　the　glare；　the　great　changes　of　temperature；　of　running　over　or　burrowing　into　the　loose　sand。　　When　as　in　America　Iguanids　are　available；　some　of　these　are　thus　modified；　while　in　Africa　and　Asia　the　Agamids　are　drawn　upon。　　Both　in　the　Damara　and　in　the　Transcaspian　deserts；　a　Gecko　has　been　turned　into　a　runner　upon　sand！

We　cannot　assume　that　at　various　epochs　deserts；　and　at　others　moist　forests　were　continuous　all　over　the　world。　　The　different　facies　and　associations　were　developed　at　various　times　and　places。　　Are　we　to　suppose　that；　wherever　tropical　forests　came　into　existence；　amongst　the　stock　of　humivagous　lizards　were　always　some　which　presented　those　nascent　variations　which　made　them　keep　step　with　the　similarly　nascent　forests；　the　overwhelming　rest　being　eliminated？　　This　principle　would　imply　that　the　same　stratum　of　lizards　always　had　variations　ready　to　fit　any　changed　environment；　forests　and　deserts；　rocks　and　swamps。　　The　study　of　Ecology　indicates　a　different　procedure；　a　great；　almost　boundless　plasticity　of　the　organism；　not　in　the　sense　of　an　exuberant　moulding　force；　but　of　a　readiness　to　be　moulded；　and　of　this　the　〃variations〃　are　the　visible　outcome。　　In　most　cases　identical　facies　are　produced　by　heterogeneous　convergences　and　these　may　seem　to　be　but　superficial；　affecting　only　what　some　authors　are　pleased　to　call　the　physiological　characters；　but　environment　presumably　affects　first　those　parts　by　which　the　organism　comes　into　contact　with　it　most　directly；　and　if　the　internal　structures　remain　unchanged；　it　is　not　because　these　are　less　easily　modified　but　because　they　are　not　directly　affected。　　When　they　are　affected；　they　too　change　deeply　enough。

That　the　plasticity　should　react　so　quicklyindeed　this　very　quickness　seems　to　have　initiated　our　mistaking　the　variations　called　forth　for　something　performedand　to　the　point；　is　itself　the　outcome　of　the　long　training　which　protoplasm　has　undergone　since　its　creation。

In　Nature's　workshop　he　does　not　succeed　who　has　ready　an　arsenal　of　tools　for　every　conceivable　emergency；　but　he　who　can　make　a　tool　at　the　spur　of　the　moment。　　The　ordeal　of　the　practical　test　is　Charles　Darwin's　glorious　conception　of　Natural　Selection