第9章

Agassizinsiststhatancientanimalsresembletoacertainextenttheembryosofrecentanimalsofthesameclasses。orthatthegeologicalsuccessionofextinctformsisinsomedegreeparalleltotheembryologicaldevelopmentofrecentforms。ImustfollowPictetandHuxleyinthinkingthatthetruthofthisdoctrineisveryfarfromproved。YetIfullyexpecttoseeithereafterconfirmed，atleastinregardtosubordinategroups，whichhavebranchedofffromeachotherwithincomparativelyrecenttimes。ForthisdoctrineofAgassizaccordswellwiththetheoryofnaturalselection。

InafuturechapterIshallattempttoshowthattheadultdiffersfromitsembryo，owingtovariationssuperveningatanotearlyage，andbeinginheritedatacorrespondingage。Thisprocess，whilstitleavestheembryoalmostunaltered，continuallyadds，inthecourseofsuccessivegenerations，moreandmoredifferencetotheadult。

Thustheembryocomestobeleftasasortofpicture，preservedbynature，oftheancientandlessmodifiedconditionofeachanimal。Thisviewmaybetrue，andyetitmayneverbecapableoffullproof。Seeing，forinstance，thattheoldestknownmammals，reptiles，andfishstrictlybelongtotheirownproperclasses，thoughsomeoftheseoldformsareinaslightdegreelessdistinctfromeachotherthanarethetypicalmembersofthesamegroupsatthepresentday，itwouldbevaintolookforanimalshavingthecommonembryologicalcharacteroftheVertebrata，untilbedsfarbeneaththelowestSilurianstrataarediscoveredadiscoveryofwhichthechanceisverysmall。OntheSuccessionofthesameTypeswithinthesameareas，duringthelatertertiaryperiodsMrCliftmanyyearsagoshowedthatthefossilmammalsfromtheAustraliancaveswerecloselyalliedtothelivingmarsupialsofthatcontinent。InSouthAmerica，asimilarrelationshipismanifest，eventoanuneducatedeye，inthegiganticpiecesofarmourlikethoseofthearmadillo，foundinseveralpartsofLaPlata。andProfessorOwenhasshowninthemoststrikingmannerthatmostofthefossilmammals，buriedthereinsuchnumbers，arerelatedtoSouthAmericantypes。ThisrelationshipisevenmoreclearlyseeninthewonderfulcollectionoffossilbonesmadebyMM。LundandClauseninthecavesofBrazil。IwassomuchimpressedwiththesefactsthatI

stronglyinsisted，in1839and1845，onthis`lawofthesuccessionoftypes，’on`thiswonderfulrelationshipinthesamecontinentbetweenthedeadandtheliving。’ProfessorOwenhassubsequentlyextendedthesamegeneralisationtothemammalsoftheOldWorld。Weseethesamelawinthisauthor’srestorationsoftheextinctandgiganticbirdsofNewZealand。

WeseeitalsointhebirdsofthecavesofBrazil。MrWoodwardhasshownthatthesamelawholdsgoodwithsea-shells，butfromthewidedistributionofmostgeneraofmolluscs，itisnotwelldisplayedbythem。Othercasescouldbeadded，astherelationbetweentheextinctandlivingland-shellsofMadeira。andbetweentheextinctandlivingbrackish-watershellsoftheAralo-CaspianSea。

Nowwhatdoesthisremarkablelawofthesuccessionofthesametypeswithinthesameareasmean？Hewouldbeaboldman，whoaftercomparingthepresentclimateofAustraliaandofpartsofSouthAmericaunderthesamelatitude，wouldattempttoaccount，ontheonehand，bydissimilarphysicalconditionsforthedissimilarityoftheinhabitantsofthesetwocontinents，and，ontheotherhand，bysimilarityofconditions，fortheuniformityofthesametypesineachduringthelatertertiaryperiods。

NorcanitbepretendedthatitisanimmutablelawthatmarsupialsshouldhavebeenchieflyorsolelyproducedinAustralia。orthatEdentataandotherAmericantypesshouldhavebeensolelyproducedinSouthAmerica。

ForweknowthatEuropeinancienttimeswaspeopledbynumerousmarsupials。

andIhaveshowninthepublicationsabovealludedto，thatinAmericathelawofdistributionofterrestrialmammalswasformerlydifferentfromwhatitnowis。NorthAmericaformerlypartookstronglyofthepresentcharacterofthesouthernhalfofthecontinent。andthesouthernhalfwasformerlymorecloselyallied，thanitisatpresent，tothenorthernhalf。InasimilarmannerweknowfromFalconerandCautley’sdiscoveries，thatnorthernIndiawasformerlymorecloselyrelatedinitsmammalstoAfricathanitisatthepresenttime。Analogousfactscouldbegiveninrelationtothedistributionofmarineanimals。

Onthetheoryofdescentwithmodification，thegreatlawofthelongenduring，butnotimmutable，successionofthesametypeswithinthesameareas，isatonceexplained。fortheinhabitantsofeachquarteroftheworldwillobviouslytendtoleaveinthatquarter，duringthenextsucceedingperiodoftime，closelyalliedthoughinsomedegreemodifieddescendants。

Iftheinhabitantsofonecontinentformerlydifferedgreatlyfromthoseofanothercontinent，sowilltheirmodifieddescendantsstilldifferinnearlythesamemanneranddegree。Butafterverylongintervalsoftimeandaftergreatgeographicalchanges，permittingmuchinter-migration，thefeeblerwillyieldtothemoredominantforms，andtherewillbenothingimmutableinthelawsofpastandpresentdistribution。

Itmaybeaskedinridicule，whetherIsupposethatthemegatheriumandotheralliedhugemonstershaveleftbehindtheminSouthAmericathesloth，armadillo，andanteater，astheirdegeneratedescendants。Thiscannotforaninstantbeadmitted。Thesehugeanimalshavebecomewhollyextinct，andhaveleftnoprogeny。ButinthecavesofBrazil，therearemanyextinctspecieswhicharecloselyalliedinsizeandinothercharacterstothespeciesstilllivinginSouthAmerica。andsomeofthesefossilsmaybetheactualprogenitorsoflivingspecies。Itmustnotbeforgottenthat，onmytheory，allthespeciesofthesamegenushavedescendedfromsomeonespecies。sothatifsixgenera，eachhavingeightspecies，befoundinonegeologicalformation，andinthenextsucceedingformationtherebesixotheralliedorrepresentativegenerawiththesamenumberofspecies，thenwemayconcludethatonlyonespeciesofeachofthesixoldergenerahasleftmodifieddescendants，constitutingthesixnewgenera。Theothersevenspeciesoftheoldgenerahavealldiedoutandhaveleftnoprogeny。

Or，whichwouldprobablybeafarcommonercase，twoorthreespeciesoftwoorthreealoneofthesixoldergenerawillhavebeentheparentsofthesixnewgenera。theotheroldspeciesandtheotherwholegenerahavingbecomeutterlyextinct。Infailingorders，withthegeneraandspeciesdecreasinginnumbers，asapparentlyisthecaseoftheEdentataofSouthAmerica，stillfewergeneraandspecieswillhaveleftmodifiedblood-descendants。SummaryoftheprecedingandpresentChaptersIhaveattemptedtoshowthatthegeologicalrecordisextremelyimperfect。

thatonlyasmallportionoftheglobehasbeengeologicallyexploredwithcare。thatonlycertainclassesoforganicbeingshavebeenlargelypreservedinafossilstate。thatthenumberbothofspecimensandofspecies，preservedinourmuseums，isabsolutelyasnothingcomparedwiththeincalculablenumberofgenerationswhichmusthavepassedawayevenduringasingleformation。that，owingtosubsidencebeingnecessaryfortheaccumulationoffossiliferousdepositsthickenoughtoresistfuturedegradation，enormousintervalsoftimehaveelapsedbetweenthesuccessiveformations。thattherehasprobablybeenmoreextinctionduringtheperiodsofsubsidence，andmorevariationduringtheperiodsofelevation，andduringthelattertherecordwillhavebeenleastperfectlykept。thateachsingleformationhasnotbeencontinuouslydeposited。thatthedurationofeachformationis，perhaps，shortcomparedwiththeaveragedurationofspecificforms。

thatmigrationhasplayedanimportantpartinthefirstappearanceofnewformsinanyoneareaandformation。thatwidelyrangingspeciesarethosewhichhavevariedmost，andhaveoftenestgivenrisetonewspecies。

andthatvarietieshaveatfirstoftenbeenlocal。Allthesecausestakenconjointly，musthavetendedtomakethegeologicalrecordextremelyimperfect，andwilltoalargeextentexplainwhywedonotfindinterminablevarieties，connectingtogetheralltheextinctandexistingformsoflifebythefinestgraduatedsteps。

Hewhorejectstheseviewsonthenatureofthegeologicalrecord，willrightlyrejectmywholetheory。Forhemayaskinvainwherearethenumberlesstransitionallinkswhichmustformerlyhaveconnectedthecloselyalliedorrepresentativespecies，foundintheseveralstagesofthesamegreatformation。Hemaydisbelieveintheenormousintervalsoftimewhichhaveelapsedbetweenourconsecutiveformations。hemayoverlookhowimportantapartmigrationmusthaveplayed，whentheformationsofanyonegreatregionalone，asthatofEurope，areconsidered。hemayurgetheapparent，butoftenfalselyapparent，suddencominginofwholegroupsofspecies。

HemayaskwherearetheremainsofthoseinfinitelynumerousorganismswhichmusthaveexistedlongbeforethefirstbedoftheSiluriansystemwasdeposited：Icananswerthislatterquestiononlyhypothetically，bysayingthatasfaraswecansee，whereouroceansnowextendtheyhaveforanenormousperiodextended，andwhereouroscillatingcontinentsnowstandtheyhavestoodeversincetheSilurianepoch。butthatlongbeforethatperiod，theworldmayhavepresentedawhollydifferentaspect。andthattheoldercontinents，formedofformationsolderthananyknowntous，maynowallbeinametamorphosedcondition，ormaylieburiedundertheocean。

Passingfromthesedifficulties，alltheothergreatleadingfactsinpalaeontologyseemtomesimplytofollowonthetheoryofdescentwithmodificationthroughnaturalselection。Wecanthusunderstandhowitisthatnewspeciescomeinslowlyandsuccessively。howspeciesofdifferentclassesdonotnecessarilychangetogether，oratthesamerate，orinthesamedegree。yetinthelongrunthatallundergomodificationtosomeextent。Theextinctionofoldformsisthealmostinevitableconsequenceoftheproductionofnewforms。Wecanunderstandwhywhenaspecieshasoncedisappeareditneverreappears。Groupsofspeciesincreaseinnumbersslowly，andendureforunequalperiodsoftime。fortheprocessofmodificationisnecessarilyslow，anddependsonmanycomplexcontingencies。Thedominantspeciesofthelargerdominantgroupstendtoleavemanymodifieddescendants，andthusnewsub-groupsandgroupsareformed。Astheseareformed，thespeciesofthelessvigorousgroups，fromtheirinferiorityinheritedfromacommonprogenitor，tendtobecomeextincttogether，andtoleavenomodifiedoffspringonthefaceoftheearth。Buttheutterextinctionofawholegroupofspeciesmayoftenbeaveryslowprocess，fromthesurvivalofafewdescendants，lingeringinprotectedandisolatedsituations。Whenagrouphasoncewhollydisappeared，itdoesnotreappear。forthelinkofgenerationhasbeenbroken。

Wecanunderstandhowthespreadingofthedominantformsoflife，whicharethosethatoftenestvary，willinthelongruntendtopeopletheworldwithallied，butmodified，descendants。andthesewillgenerallysucceedintakingtheplacesofthosegroupsofspecieswhicharetheirinferiorsinthestruggleforexistence。Hence，afterlongintervalsoftime，theproductionsoftheworldwillappeartohavechangedsimultaneously。

Wecanunderstandhowitisthatalltheformsoflife，ancientandrecent，maketogetheronegrandsystem。forallareconnectedbygeneration。

Wecanunderstand，fromthecontinuedtendencytodivergenceofcharacter，whythemoreancientaformis，themoreitgenerallydiffersfromthosenowliving。Whyancientandextinctformsoftentendtofillupgapsbetweenexistingforms，sometimesblendingtwogroupspreviouslyclassedasdistinctintoone。butmorecommonlyonlybringingthemalittleclosertogether。

Themoreancientaformis，themoreoften，apparently，itdisplayscharactersinsomedegreeintermediatebetweengroupsnowdistinct。forthemoreancientaformis，themorenearlyitwillberelatedto，andconsequentlyresemble，thecommonprogenitorofgroups，sincebecomewidelydivergent。Extinctformsareseldomdirectlyintermediatebetweenexistingforms。butareintermediateonlybyalongandcircuitouscoursethroughmanyextinctandverydifferentforms。Wecanclearlyseewhytheorganicremainsofcloselyconsecutiveformationsaremorecloselyalliedtoeachother，thanarethoseofremoteformations。fortheformsaremorecloselylinkedtogetherbygeneration：wecanclearlyseewhytheremainsofanintermediateformationareintermediateincharacter。

Theinhabitantsofeachsuccessiveperiodintheworld’shistoryhavebeatentheirpredecessorsintheraceforlife，andare，insofar，higherinthescaleofnature。andthismayaccountforthatvagueyetill-definedsentiment，feltbymanypalaeontologists，thatorganisationonthewholehasprogressed。Ifitshouldhereafterbeprovedthatancientanimalsresembletoacertainextenttheembryosofmorerecentanimalsofthesameclass，thefactwillbeintelligible。Thesuccessionofthesametypesofstructurewithinthesameareasduringthelatergeologicalperiodsceasestobemysterious，andissimplyexplainedbyinheritance。

IfthenthegeologicalrecordbeasimperfectasIbelieveittobe，anditmayatleastbeassertedthattherecordcannotbeprovedtobemuchmoreperfect，themainobjectionstothetheoryofnaturalselectionaregreatlydiminishedordisappear。Ontheotherhand，allthechieflawsofpalaeontologyplainlyproclaim，asitseemstome，thatspecieshavebeenproducedbyordinarygeneration：oldformshavingbeensupplantedbynewandimprovedformsoflife，producedbythelawsofvariationstillactingroundus，andpreservedbyNaturalSelection。PreviousChapterNextChapterTheOriginofSpecies-Chapter11TheOriginofSpeciesbyCharlesDarwinPreviousChapterNextChapterChapter11-GeographicalDistributionInconsideringthedistributionoforganicbeingsoverthefaceoftheglobe，thefirstgreatfactwhichstrikesusis，thatneitherthesimilaritynorthedissimilarityoftheinhabitantsofvariousregionscanbeaccountedforbytheirclimatalandotherphysicalconditions。Oflate，almosteveryauthorwhohasstudiedthesubjecthascometothisconclusion。ThecaseofAmericaalonewouldalmostsufficetoproveitstruth：forifweexcludethenorthernpartswherethecircumpolarlandisalmostcontinuous，allauthorsagreethatoneofthemostfundamentaldivisionsingeographicaldistributionisthatbetweentheNewandOldWorlds。yetifwetraveloverthevastAmericancontinent，fromthecentralpartsoftheUnitedStatestoitsextremesouthernpoint，wemeetwiththemostdiversifiedconditions。

themosthumiddistricts，ariddeserts，loftymountains，grassyplains，forests，marshes，lakes，andgreatrivers，underalmosteverytemperature。

ThereishardlyaclimateorconditionintheOldWorldwhichcannotbeparalleledintheNewatleastascloselyasthesamespeciesgenerallyrequire。foritisamostrarecasetofindagroupoforganismsconfinedtoanysmallspot，havingconditionspeculiarinonlyaslightdegree。

forinstance，smallareasintheOldWorldcouldbepointedouthotterthananyintheNewWorld，yetthesearenotinhabitedbyapeculiarfaunaorflora。NotwithstandingthisparallelismintheconditionsoftheOldandNewWorlds，howwidelydifferentaretheirlivingproductions！

Inthesouthernhemisphere，ifwecomparelargetractsoflandinAustralia，SouthAfrica，andwesternSouthAmerica，betweenlatitudes25°。and35°。，weshallfindpartsextremelysimilarinalltheirconditions，yetitwouldnotbepossibletopointoutthreefaunasandflorasmoreutterlydissimilar。OragainwemaycomparetheproductionsofSouthAmericasouthoflat。35°。withthosenorthof25°。，whichconsequentlyinhabitaconsiderablydifferentclimate，andtheywillbefoundincomparablymorecloselyrelatedtoeachother，thantheyaretotheproductionsofAustraliaorAfricaundernearlythesameclimate。Analogousfactscouldbegivenwithrespecttotheinhabitantsofthesea。

Asecondgreatfactwhichstrikesusinourgeneralreviewis，thatbarriersofanykind，orobstaclestofreemigration，arerelatedinacloseandimportantmannertothedifferencesbetweentheproductionsofvariousregions。WeseethisinthegreatdifferenceofnearlyalltheterrestrialproductionsoftheNewandOldWorlds，exceptinginthenorthernparts，wherethelandalmostjoins，andwhere，underaslightlydifferentclimate，theremighthavebeenfreemigrationforthenortherntemperateforms，astherenowisforthestrictlyarcticproductions。WeseethesamefactinthegreatdifferencebetweentheinhabitantsofAustralia，Africa，andSouthAmericaunderthesamelatitude：forthesecountriesarealmostasmuchisolatedfromeachotherasispossible。Oneachcontinent，also，weseethesamefact。forontheoppositesidesofloftyandcontinuousmountain-ranges，andofgreatdeserts，andsometimesevenoflargerivers，wefinddifferentproductions。thoughasmountainchains，deserts，&。c。，arenotasimpassable，orlikelytohaveenduredsolongastheoceansseparatingcontinents，thedifferencesareveryinferiorindegreetothosecharacteristicofdistinctcontinents。

Turningtothesea，wefindthesamelaw。Notwomarinefaunasaremoredistinct，withhardlyafish，shell，orcrabincommon，thanthoseoftheeasternandwesternshoresofSouthandCentralAmerica。yetthesegreatfaunasareseparatedonlybythenarrow，butimpassable，isthmusofpanama。

WestwardoftheshoresofAmerica，awidespaceofopenoceanextends，withnotanislandasahalting-placeforemigrants。herewehaveabarrierofanotherkind，andassoonasthisispassedwemeetintheeasternislandsofthePacific，withanotherandtotallydistinctfauna。Sothatherethreemarinefaunasrangefarnorthwardandsouthward，inparallellinesnotfarfromeachother，undercorrespondingclimates。butfrombeingseparatedfromeachotherbyimpassablebarriers，eitheroflandoropensea，theyarewhollydistinct。Ontheotherhand，proceedingstillfurtherwestwardfromtheeasternislandsofthetropicalpartsofthePacific，weencounternoimpassablebarriers，andwehaveinnumerableislandsashalting-places，untilaftertravellingoverahemispherewecometotheshoresofAfrica。

andoverthisvastspacewemeetwithnowell-definedanddistinctmarinefaunas。Althoughhardlyoneshell，craborfishiscommontotheabove-namedthreeapproximatefaunasofEasternandWesternAmericaandtheeasternPacificislands，yetmanyfishrangefromthePacificintotheIndianOcean，andmanyshellsarecommontotheeasternislandsofthePacificandtheeasternshoresofAfrica，onalmostexactlyoppositemeridiansoflongitude。

Athirdgreatfact，partlyincludedintheforegoingstatements，istheaffinityoftheproductionsofthesamecontinentorsea，thoughthespeciesthemselvesaredistinctatdifferentpointsandstations。Itisalawofthewidestgenerality，andeverycontinentoffersinnumerableinstances。Neverthelessthenaturalistintravelling，forinstance，fromnorthtosouthneverfailstobestruckbythemannerinwhichsuccessivegroupsofbeings，specificallydistinct，yetclearlyrelated，replaceeachother。Hehearsfromcloselyallied，yetdistinctkindsofbirds，notesnearlysimilar，andseestheirnestssimilarlyconstructed，butnotquitealike，witheggscolouredinnearlythesamemanner。TheplainsneartheStraitsofMagellanareinhabitedbyonespeciesofRhea(Americanostrich)，andnorthwardtheplainsofLaPlatabyanotherspeciesofthesamegenus。

andnotbyatrueostrichoremeu，likethosefoundinAfricaandAustraliaunderthesamelatitude。OnthesesameplainsofLaPlata，weseetheagoutiandbizcacha，animalshavingnearlythesamehabitsasourharesandrabbitsandbelongingtothesameorderofRodents，buttheyplainlydisplayanAmericantypeofstructure。WeascendtheloftypeaksoftheCordilleraandwefindanalpinespeciesofbizcacha。welooktothewaters，andwedonotfindthebeaverormusk-rat，butthecoypuandcapybara，rodentsoftheAmericantype。Innumerableotherinstancescouldbegiven。IfwelooktotheislandsofftheAmericanshore，howevermuchtheymaydifferingeologicalstructure，theinhabitants，thoughtheymaybeallpeculiarspecies，areessentiallyAmerican。Wemaylookbacktopastages，asshowninthelastchapter，andwefindAmericantypesthenprevalentontheAmericancontinentandintheAmericanseas。Weseeinthesefactssomedeeporganicbond，prevailingthroughoutspaceandtime，overthesameareasoflandandwater，andindependentoftheirphysicalconditions。Thenaturalistmustfeellittlecuriosity，whoisnotledtoinquirewhatthisbondis。

Thisbond，onmytheory，issimplyinheritance，thatcausewhichalone，asfaraswepositivelyknow，producesorganismsquitelike，or，asweseeinthecaseofvarietiesnearlylikeeachother。Thedissimilarityoftheinhabitantsofdifferentregionsmaybeattributedtomodificationthroughnaturalselection，andinaquitesubordinatedegreetothedirectinfluenceofdifferentphysicalconditions。Thedegreeofdissimilaritywilldependonthemigrationofthemoredominantformsoflifefromoneregionintoanotherhavingbeeneffectedwithmoreorlessease，atperiodsmoreorlessremote。onthenatureandnumberoftheformerimmigrants——

andontheiractionandreaction，intheirmutualstrugglesforlife。

therelationoforganismtoorganismbeing，asIhavealreadyoftenremarked，themostimportantofallrelations。Thusthehighimportanceofbarrierscomesintoplaybycheckingmigration。asdoestimefortheslowprocessofmodificationthroughnaturalselection。Widely-rangingspecies，aboundinginindividuals，whichhavealreadytriumphedovermanycompetitorsintheirownwidely-extendedhomeswillhavethebestchanceofseizingonnewplaces，whentheyspreadintonewcountries。Intheirnewhomestheywillbeexposedtonewconditions，andwillfrequentlyundergofurthermodificationandimprovement。andthustheywillbecomestillfurthervictorious，andwillproducegroupsofmodifieddescendants。Onthisprincipleofinheritancewithmodification，wecanunderstandhowitisthatsectionsofgenera，wholegenera，andevenfamiliesareconfinedtothesameareas，asissocommonlyandnotoriouslythecase。

Ibelieve，aswasremarkedinthelastchapter，innolawofnecessarydevelopment。Asthevariabilityofeachspeciesisanindependentproperty，andwillbetakenadvantageofbynaturalselection，onlysofarasitprofitstheindividualinitscomplexstruggleforlife，sothedegreeofmodificationindifferentspecieswillbenouniformquantity。If，forinstance，anumberofspecies，whichstandindirectcompetitionwitheachother，migrateinabodyintoanewandafterwardsisolatedcountry，theywillbelittleliabletomodification。forneithermigrationnorisolationinthemselvescandoanything。Theseprinciplescomeintoplayonlybybringingorganismsintonewrelationswitheachother，andinalesserdegreewiththesurroundingphysicalconditions。Aswehaveseeninthelastchapterthatsomeformshaveretainednearlythesamecharacterfromanenormouslyremotegeologicalperiod，socertainspecieshavemigratedovervastspaces，andhavenotbecomegreatlymodified。

Ontheseviews，itisobvious，thattheseveralspeciesofthesamegenus，thoughinhabitingthemostdistantquartersoftheworld，mustoriginallyhaveproceededfromthesamesource，astheyhavedescendedfromthesameprogenitor。Inthecaseofthosespecies，whichhaveundergoneduringwholegeologicalperiodsbutlittlemodification，thereisnotmuchdifficultyinbelievingthattheymayhavemigratedfromthesameregion。forduringthevastgeographicalandclimatalchangeswhichwillhavesupervenedsinceancienttimes，almostanyamountofmigrationispossible。Butinmanyothercases，inwhichwehavereasontobelievethatthespeciesofagenushavebeenproducedwithincomparativelyrecenttimes，thereisgreatdifficultyonthishead。Itisalsoobviousthattheindividualsofthesamespecies，thoughnowinhabitingdistantandisolatedregions，musthaveproceededfromonespot，wheretheirparentswerefirstproduced：for，asexplainedinthelastchapter，itisincrediblethatindividualsidenticallythesameshouldeverhavebeenproducedthroughnaturalselectionfromparentsspecificallydistinct。

Wearethusbroughttothequestionwhichhasbeenlargelydiscussedbynaturalists，namely，whetherspecieshavebeencreatedatoneormorepointsoftheearth’ssurface。Undoubtedlythereareverymanycasesofextremedifficulty，inunderstandinghowthesamespeciescouldpossiblyhavemigratedfromsomeonepointtotheseveraldistantandisolatedpoints，wherenowfound。Neverthelessthesimplicityoftheviewthateachspecieswasfirstproducedwithinasingleregioncaptivatesthemind。Hewhorejectsit，rejectstheveracausaofordinarygenerationwithsubsequentmigration，andcallsintheagencyofamiracle。Itisuniversallyadmitted，thatinmostcasestheareainhabitedbyaspeciesiscontinuous。andwhenaplantoranimalinhabitstwopointssodistantfromeachother，orwithanintervalofsuchanature，thatthespacecouldnotbeeasilypassedoverbymigration，thefactisgivenassomethingremarkableandexceptional。

Thecapacityofmigratingacrosstheseaismoredistinctlylimitedinterrestrialmammals，thanperhapsinanyotherorganicbeings。and，accordingly，wefindnoinexplicablecasesofthesamemammalinhabitingdistantpointsoftheworld。NogeologistwillfeelanydifficultyinsuchcasesasGreatBritainhavingbeenformerlyunitedtoEurope，andconsequentlypossessingthesamequadrupeds。Butifthesamespeciescanbeproducedattwoseparatepoints，whydowenotfindasinglemammalcommontoEuropeandAustraliaorSouthAmerica？Theconditionsoflifearenearlythesame，sothatamultitudeofEuropeananimalsandplantshavebecomenaturalisedinAmericaandAustralia。andsomeoftheaboriginalplantsareidenticallythesameatthesedistantpointsofthenorthernandsouthernhemispheres？Theanswer，asIbelieve，is，thatmammalshavenotbeenabletomigrate，whereassomeplants，fromtheirvariedmeansofdispersal，havemigratedacrossthevastandbrokeninterspace。Thegreatandstrikinginfluencewhichbarriersofeverykindhavehadondistribution，isintelligibleonlyontheviewthatthegreatmajorityofspecieshavebeenproducedononesidealone，andhavenotbeenabletomigratetotheotherside。Somefewfamilies，manysub-families，verymanygenera，andastillgreaternumberofsectionsofgeneraareconfinedtoasingleregion。andithasbeenobservedbyseveralnaturalists，thatthemostnaturalgenera，orthosegenerainwhichthespeciesaremostcloselyrelatedtoeachother，aregenerallylocal，orconfinedtoonearea。Whatastrangeanomalyitwouldbe，if，whencomingonesteplowerintheseries，totheindividualsofthesamespecies，adirectlyoppositeruleprevailed。andspecieswerenotlocal，buthadbeenproducedintwoormoredistinctareas！

Henceitseemstome，asithastomanyothernaturalists，thattheviewofeachspecieshavingbeenproducedinoneareaalone，andhavingsubsequentlymigratedfromthatareaasfarasitspowersofmigrationandsubsistenceunderpastandpresentconditionspermitted，isthemostprobable。Undoubtedlymanycasesoccur，inwhichwecannotexplainhowthesamespeciescouldhavepassedfromonepointtotheother。Butthegeographicalandclimatalchanges，whichhavecertainlyoccurredwithinrecentgeologicaltimes，musthaveinterruptedorrendereddiscontinuoustheformerlycontinuousrangeofmanyspecies。Sothatwearereducedtoconsiderwhethertheexceptionstocontinuityofrangearesonumerousandofsograveanature，thatweoughttogiveupthebelief，renderedprobablebygeneralconsiderations，thateachspecieshasbeenproducedwithinonearea，andhasmigratedthenceasfarasitcould。Itwouldbehopelesslytedioustodiscussalltheexceptionalcasesofthesamespecies，nowlivingatdistantandseparatedpoints。nordoIforamomentpretendthatanyexplanationcouldbeofferedofmanysuchcases。Butaftersomepreliminaryremarks，Iwilldiscussafewofthemoststrikingclassesoffacts。namely，theexistenceofthesamespeciesonthesummitsofdistantmountain-ranges，andatdistantpointsinthearcticandantarcticregions。

andsecondly(inthefollowingchapter)，thewidedistributionoffreshwaterproductions。andthirdly，theoccurrenceofthesameterrestrialspeciesonislandsandonthemainland，thoughseparatedbyhundredsofmilesofopensea。Iftheexistenceofthesamespeciesatdistantandisolatedpointsoftheearth’ssurface，caninmanyinstancesbeexplainedontheviewofeachspecieshavingmigratedfromasinglebirthplace。then，consideringourignorancewithrespecttoformerclimatalandgeographicalchangesandvariousoccasionalmeansoftransport，thebeliefthatthishasbeentheuniversallaw，seemstomeincomparablythesafest。

Indiscussingthissubject，weshallbeenabledatthesametimetoconsiderapointequallyimportantforus，namely，whethertheseveraldistinctspeciesofagenus，whichonmytheoryhavealldescendedfromacommonprogenitor，canhavemigrated(undergoingmodificationduringsomepartoftheirmigration)fromtheareainhabitedbytheirprogenitor。

Ifitcanbeshowntobealmostinvariablythecase，thataregion，ofwhichmostofitsinhabitantsarecloselyrelatedto，orbelongtothesamegenerawiththespeciesofasecondregion，hasprobablyreceivedatsomeformerperiodimmigrantsfromthisotherregion，mytheorywillbestrengthened。forwecanclearlyunderstand，ontheprincipleofmodification，whytheinhabitantsofaregionshouldberelatedtothoseofanotherregion，whenceithasbeenstocked。Avolcanicisland，forinstance，upheavedandformedatthedistanceofafewhundredsofmilesfromacontinent，wouldprobablyreceivefromitinthecourseoftimeafewcolonists，andtheirdescendants，thoughmodified，wouldstillbeplainlyrelatedbyinheritancetotheinhabitantsofthecontinent。Casesofthisnaturearecommon，andare，asweshallhereaftermorefullysee，inexplicableonthetheoryofindependentcreation。Thisviewoftherelationofspeciesinoneregiontothoseinanother，doesnotdiffermuch(bysubstitutingthewordvarietyforspecies)fromthatlatelyadvancedinaningeniouspaperbyMrWallace，inwhichheconcludes，that`everyspecieshascomeintoexistencecoincidentbothinspaceandtimewithapre-existingcloselyalliedspecies。’AndInowknowfromcorrespondence，thatthiscoincidenceheattributestogenerationwithmodification。

Thepreviousremarkson`singleandmultiplecentresofcreation’donotdirectlybearonanotheralliedquestion，namelywhetheralltheindividualsofthesamespecieshavedescendedfromasinglepair，orsinglehermaphrodite，orwhether，assomeauthorssuppose，frommanyindividualssimultaneouslycreated。Withthoseorganicbeingswhichneverintercross(ifsuchexist)，thespecies，onmytheory，musthavedescendedfromasuccessionofimprovedvarieties，whichwillneverhaveblendedwithotherindividualsorvarieties，butwillhavesupplantedeachother。sothat，ateachsuccessivestageofmodificationandimprovement，alltheindividualsofeachvarietywillhavedescendedfromasingleparent。Butinthemajorityofcases，namely，withallorganismswhichhabituallyuniteforeachbirth，orwhichoftenintercross，Ibelievethatduringtheslowprocessofmodificationtheindividualsofthespecieswillhavebeenkeptnearlyuniformbyintercrossing。

sothatmanyindividualswillhavegoneonsimultaneouslychanging，andthewholeamountofmodificationwillnothavebeendue，ateachstage，todescentfromasingleparent。ToillustratewhatImean：ourEnglishracehorsesdifferslightlyfromthehorsesofeveryotherbreed。buttheydonotowetheirdifferenceandsuperioritytodescentfromanysinglepair，buttocontinuedcareinselectingandtrainingmanyindividualsduringmanygenerations。

Beforediscussingthethreeclassesoffacts，whichIhaveselectedaspresentingthegreatestamountofdifficultyonthetheoryof`singlecentresofcreation，’Imustsayafewwordsonthemeansofdispersal。MeansofDispersalSirC。Lyellandotherauthorshaveablytreatedthissubject。Icangivehereonlythebriefestabstractofthemoreimportantfacts。Changeofclimatemusthavehadapowerfulinfluenceonmigration：aregionwhenitsclimatewasdifferentmayhavebeenahighroadformigration，butnowbeimpassable。Ishall，however，presentlyhavetodiscussthisbranchofthesubjectinsomedetail。Changesoflevelinthelandmustalsohavebeenhighlyinfluential：anarrowisthmusnowseparatestwomarinefaunas。

submergeit，orletitformerlyhavebeensubmerged，andthetwofaunaswillnowblendormayformerlyhaveblended：wheretheseanowextends，landmayataformerperiodhaveconnectedislandsorpossiblyevencontinentstogether，andthushaveallowedterrestrialproductionstopassfromonetotheother。Nogeologistwilldisputethatgreatmutationsoflevelhaveoccurredwithintheperiodofexistingorganisms。EdwardForbesinsistedthatalltheislandsintheAtlanticmustrecentlyhavebeenconnectedwithEuropeorAfrica，andEuropelikewisewithAmerica。Otherauthorshavethushypotheticallybridgedovereveryocean，andhaveunitedalmosteveryislandtosomemainland。IfindeedtheargumentsusedbyForbesaretobetrusted，itmustbeadmittedthatscarcelyasingleislandexistswhichhasnotrecentlybeenunitedtosomecontinent。ThisviewcutstheGordianknotofthedispersalofthesamespeciestothemostdistantpoints，andremovesmanyadifficulty：buttothebestofanyjudgementwearenotauthorisedinadmittingsuchenormousgeographicalchangeswithintheperiodofexistingspecies。Itseemstomethatwehaveabundantevidenceofgreatoscillationsoflevelinourcontinents。butnotofsuchvastchangesintheirpositionandextension，astohaveunitedthemwithintherecentperiodtoeachotherandtotheseveralinterveningoceanicislands。Ifreelyadmittheformerexistenceofmanyislands，nowburiedbeneaththesea，whichmayhaveservedashaltingplacesforplantsandformanyanimalsduringtheirmigration。Inthecoral-producingoceanssuchsunkenislandsarenowmarked，asIbelieve，byringsofcoraloratollsstandingoverthem。Wheneveritisfullyadmitted，asIbelieveitwillsomedaybe，thateachspecieshasproceededfromasinglebirthplace，andwheninthecourseoftimeweknowsomethingdefiniteaboutthemeansofdistribution，weshallbeenabledtospeculatewithsecurityontheformerextensionoftheland。ButIdonotbelievethatitwilleverbeprovedthatwithintherecentperiodcontinentswhicharenowquiteseparate，havebeencontinuously，oralmostcontinuously，unitedwitheachother，andwiththemanyexistingoceanicislands。Severalfactsindistribution，suchasthegreatdifferenceinthemarinefaunasontheoppositesidesofalmosteverycontinent，thecloserelationofthetertiaryinhabitantsofseverallandsandevenseastotheirpresentinhabitants，acertaindegreeofrelation(asweshallhereaftersee)betweenthedistributionofmammalsandthedepthofthesea，theseandothersuchfactsseemtomeopposedtotheadmissionofsuchprodigiousgeographicalrevolutionswithintherecentperiod，asarenecessitatedintheviewadvancedbyForbesandadmittedbyhismanyfollowers。Thenatureandrelativeproportionsoftheinhabitantsofoceanicislandslikewiseseemtomeopposedtothebeliefoftheirformercontinuitywithcontinents。Nordoestheiralmostuniversallyvolcaniccompositionfavourtheadmissionthattheyarethewrecksofsunkencontinents。iftheyhadoriginallyexistedasmountain-rangesontheland，someatleastoftheislandswouldhavebeenformed，likeothermountain-summits，ofgranite，metamorphicschists，oldfossiliferousorothersuchrocks，insteadofconsistingofmerepilesofvolcanicmatter。

Imustnowsayafewwordsonwhatarecalledaccidentalmeans，butwhichmoreproperlymightbecalledoccasionalmeansofdistribution。I

shallhereconfinemyselftoplants。Inbotanicalworks，thisorthatplantisstatedtobeilladaptedforwidedissemination。butfortransportacrossthesea，thegreaterorlessfacilitiesmaybesaidtobealmostwhollyunknown。UntilItried，withMrBerkeley’said，afewexperiments，itwasnotevenknownhowfarseedscouldresisttheinjuriousactionofsea-water。

TomysurpriseIfoundthatoutof87kinds，64germinatedafteranimmersionof28days，andafewsurvivedanimmersionof137days。ForconveniencesakeIchieflytriedsmallseeds，withoutthecapsuleorfruit。andasallofthesesankinafewdays，theycouldnotbefloatedacrosswidespacesofthesea，whetherornottheywereinjuredbythesalt-water。

AfterwardsItriedsomelargerfruits，capsules，&。c。，andsomeofthesefloatedforalongtime。Itiswellknownwhatadifferencethereisinthebuoyancyofgreenandseasonedtimber。anditoccurredtomethatfloodsmightwashdownplantsorbranches，andthatthesemightbedriedonthebanks，andthenbyafreshriseinthestreambewashedintothesea。HenceIwasledtodrystemsandbranchesof94plantswithripefruit，andtoplacethemonseawater。Themajoritysankquickly，butsomewhichwhilstgreenfloatedforaveryshorttime，whendriedfloatedmuchlonger。forinstance，ripehazel-nutssankimmediately，butwhendried，theyfloatedfor90daysandafterwardswhenplantedtheygerminated。anasparagusplantwithripeberriesfloatedfor23days，whendrieditfloatedfor85days，andtheseedsafterwardsgerminated：theripeseedsofHelosciadiumsankintwodays，whendriedtheyfloatedforabove90days，andafterwardsgerminated。Altogetheroutofthe94driedplants，18floatedforabove28days，andsomeofthe18floatedforaverymuchlongerperiod。Sothatas64/87seedsgerminatedafteranimmersionof28days。andas18/94plantswithripefruit(butnotallthesamespeciesasintheforegoingexperiment)

floated，afterbeingdried，forabove28days，asfaraswemayinferanythingfromthesescantyfacts，wemayconcludethattheseedsof14/100plantsofanycountrymightbefloatedbysea-currentsduring28days，andwouldretaintheirpowerofgermination。InJohnston’sphysicalAtlas，theaveragerateoftheseveralAtlanticcurrentsis33milesperdiem(somecurrentsrunningattherateof60milesperdiem)。onthisaverage，theseedsof14/100plantsbelongingtoonecountrymightbefloatedacross924milesofseatoanothercountry。andwhenstranded，ifblowntoafavourablespotbyaninlandgale，theywouldgerminate。

Subsequentlytomyexperiments，M。Martenstriedsimilarones，butinamuchbettermanner，forheplacedtheseedsinaboxintheactualsea，sothattheywerealternatelywetandexposedtotheairlikereallyfloatingplants。Hetried98seeds，mostlydifferentfrommine。buthechosemanylargefruitsandlikewiseseedsfromplantswhichlivenearthesea。andthiswouldhavefavouredtheaveragelengthoftheirflotationandoftheirresistancetotheinjuriousactionofthesalt-water。Ontheotherhandhedidnotpreviouslydrytheplantsorbrancheswiththefruit。andthis，aswehaveseen，wouldhavecausedsomeofthemtohavefloatedmuchlonger。

Theresultwasthat18/98ofhisseedsfloatedfor42days，andwerethencapableofgermination。ButIdonotdoubtthatplantsexposedtothewaveswouldfloatforalesstimethanthoseprotectedfromviolentmovementasinourexperiments。Thereforeitwouldperhapsbesafertoassumethattheseedsofabout10/100plantsofaflora，afterhavingbeendried，couldbefloatedacrossaspaceofsea900milesinwidth，andwouldthengerminate。

Thefactofthelargerfruitsoftenfloatinglongerthanthesmall，isinteresting。asplantswithlargeseedsorfruitcouldhardlybetransportedbyanyothermeans。andAlph。deCandollehasshownthatsuchplantsgenerallyhaverestrictedranges。

Butseedsmaybeoccasionallytransportedinanothermanner。Drifttimberisthrownuponmostislands，evenonthoseinthemidstofthewidestoceans。andthenativesofthecoral-islandsinthePacific，procurestonesfortheirtools，solelyfromtherootsofdriftedtrees，thesestonesbeingavaluableroyaltax。Ifindonexamination，thatwhenirregularlyshapedstonesareembeddedintherootsoftrees，smallparcelsofearthareveryfrequentlyenclosedintheirintersticesandbehindthem，soperfectlythatnotaparticlecouldbewashedawayinthelongesttransport：outofonesmallportionofearththuscompletelyenclosedbywoodinanoakabout50yearsold，threedicotyledonousplantsgerminated：Iamcertainoftheaccuracyofthisobservation。Again，Icanshowthatthecarcassesofbirds，whenfloatingonthesea，sometimesescapebeingimmediatelydevoured。andseedsofmanykindsinthecropsoffloatingbirdslongretaintheirvitality：peasandvetches，forinstance，arekilledbyevenafewdays’immersioninsea-water。butsometakenoutofthecropofapigeon，whichhadfloatedonartificialsalt-waterfor30days，tomysurprisenearlyallgerminated。

Livingbirdscanhardlyfailtobehighlyeffectiveagentsinthetransportationofseeds。Icouldgivemanyfactsshowinghowfrequentlybirdsofmanykindsareblownbygalestovastdistancesacrosstheocean。WemayIthinksafelyassumethatundersuchcircumstancestheirrateofflightwouldoftenbe35milesanhour。andsomeauthorshavegivenafarhigherestimate。

Ihaveneverseenaninstanceofnutritiousseedspassingthroughtheintestinesofabird。buthardseedsoffruitwillpassuninjuredthrougheventhedigestiveorgansofaturkey。Inthecourseoftwomonths，Ipickedupinmygarden12kindsofseeds，outoftheexcrementofsmallbirds，andtheseseemedperfect，andsomeofthem，whichItried，germinated。Butthefollowingfactismoreimportant：thecropsofbirdsdonotsecretegastricjuice，anddonotintheleastinjure，asIknowbytrial，thegerminationofseeds。nowafterabirdhasfoundanddevouredalargesupplyoffood，itispositivelyassertedthatallthegrainsdonotpassintothegizzardfor12oreven18hours。Abirdinthisintervalmighteasilybeblowntothedistanceof500miles，andhawksareknowntolookoutfortiredbirds，andthecontentsoftheirtorncropsmightthusreadilygetscattered。MrBrentinformsmethatafriendofhishadtogiveupflyingcarrier-pigeonsfromFrancetoEngland，asthehawksontheEnglishcoastdestroyedsomanyontheirarrival。Somehawksandowlsbolttheirpreywhole，andafteranintervaloffromtwelvetotwentyhours，disgorgepellets，which，asIknowfromexperimentsmadeintheZoologicalGardens，includeseedscapableofgermination。Someseedsoftheoat，wheat，millet，canary，hemp，clover，andbeetgerminatedafterhavingbeenfromtwelvetotwenty-onehoursinthestomachsofdifferentbirdsofprey。andtwoseedsofbeetgrewafterhavingbeenthusretainedfortwodaysandfourteenhours。Freshwaterfish，Ifind，eatseedsofmanylandandwaterplants：

fisharefrequentlydevouredbybirds，andthustheseedsmightbetransportedfromplacetoplace。Iforcedmanykindsofseedsintothestomachsofdeadfish，andthengavetheirbodiestofishing-eagles，storks，andpelicans。

thesebirdsafteranintervalofmanyhours，eitherrejectedtheseedsinpelletsorpassedthemintheirexcrement。andseveraloftheseseedsretainedtheirpowerofgermination。Certainseeds，however，werealwayskilledbythisprocess。

Althoughthebeaksandfeetofbirdsaregenerallyquiteclean，Icanshowthatearthsometimesadherestothem：inoneinstanceIremovedtwenty-twograinsofdryargillaceousearthfromonefootofapartridge，andinthisearththerewasapebblequiteaslargeastheseedofavetch。Thusseedsmightoccasionallybetransportedtogreatdistances。formanyfactscouldbegivenshowingthatsoilalmosteverywhereischargedwithseeds。ReflectforamomentonthemillionsofquailswhichannuallycrosstheMediterranean。

andcanwedoubtthattheearthadheringtotheirfeetwouldsometimesincludeafewminuteseeds？ButIshallpresentlyhavetorecurtothissubject。

Asicebergsareknowntobesometimesloadedwithearthandstones，andhaveevencarriedbrushwood，bones，andthenestofaland-bird，I

canhardlydoubtthattheymustoccasionallyhavetransportedseedsfromoneparttoanotherofthearcticandantarcticregions，assuggestedbyLyell。andduringtheGlacialperiodfromonepartofthenowtemperateregionstoanother。IntheAzores，fromthelargenumberofthespeciesofplantscommontoEurope，incomparisonwiththeplantsofotheroceanicislandsnearertothemainland，and(asremarkedbyMrH。C。Watson)fromthesomewhatnortherncharacterofthefloraincomparisonwiththelatitude，Isuspectedthattheseislandshadbeenpartlystockedbyice-borneseeds，duringtheGlacialepoch。AtmyrequestSirC。LyellwrotetoM。Hartungtoinquirewhetherhehadobservederraticbouldersontheseislands，andheansweredthathehadfoundlargefragmentsofgraniteandotherrocks，whichdonotoccurinthearchipelago。Hencewemaysafelyinferthaticebergsformerlylandedtheirrockyburthensontheshoresofthesemid-oceanislands，anditisatleastpossiblethattheymayhavebroughtthithertheseedsofnorthernplants。

Consideringthattheseveralabovemeansoftransport，andthatseveralothermeans，whichwithoutdoubtremaintobediscovered，havebeeninactionyearafteryear，forcenturiesandtensofthousandsofyears，itwouldIthinkbeamarvellousfactifmanyplantshadnotthusbecomewidelytransported。Thesemeansoftransportaresometimescalledaccidental，butthisisnotstrictlycorrect：thecurrentsoftheseaarenotaccidental，noristhedirectionofprevalentgalesofwind。Itshouldbeobservedthatscarcelyanymeansoftransportwouldcarryseedsforverygreatdistances。

forseedsdonotretaintheirvitalitywhenexposedforagreatlengthoftimetotheactionofseawater。norcouldtheybelongcarriedinthecropsorintestinesofbirds。Thesemeans，however，wouldsufficeforoccasionaltransportacrosstractsofseasomehundredmilesinbreadth，orfromislandtoisland，orfromacontinenttoaneighbouringisland，butnotfromonedistantcontinenttoanother。Theflorasofdistantcontinentswouldnotbysuchmeansbecomemingledinanygreatdegree。butwouldremainasdistinctaswenowseethemtobe。Thecurrents，fromtheircourse，wouldneverbringseedsfromNorthAmericatoBritain，thoughtheymightanddobringseedsfromtheWestIndiestoourwesternshores，where，ifnotkilledbysolonganimmersioninsalt-water，theycouldnotendureourclimate。

Almosteveryyear，oneortwoland-birdsareblownacrossthewholeAtlanticOcean，fromNorthAmericatothewesternshoresofIrelandandEngland。

butseedscouldbetransportedbythesewanderersonlybyonemeans，namely，indirtstickingtotheirfeet，whichisinitselfarareaccident。Eveninthiscase，howsmallwouldthechancebeofaseedfallingonfavourablesoil，andcomingtomaturity！Butitwouldbeagreaterrortoarguethatbecauseawell-stockedisland，likeGreatBritain，hasnot，asfarasisknown(anditwouldbeverydifficulttoprovethis)，receivedwithinthelastfewcenturies，throughoccasionalmeansoftransport，immigrantsfromEuropeoranyothercontinent，thatapoorly-stockedisland，thoughstandingmoreremotefromthemainland，wouldnotreceivecolonistsbysimilarmeans。

Idonotdoubtthatoutoftwentyseedsoranimalstransportedtoanisland，eveniffarlesswell-stockedthanBritain，scarcelymorethanonewouldbesowellfittedtoitsnewhome，astobecomenaturalised。Butthis，asitseemstome，isnovalidargumentagainstwhatwouldbeeffectedbyoccasionalmeansoftransport，duringthelonglapseofgeologicaltime，whilstanislandwasbeingupheavedandformed，andbeforeithadbecomefullystockedwithinhabitants。Onalmostbareland，withfewornodestructiveinsectsorbirdslivingthere，nearlyeveryseed，whichchancedtoarrive，wouldbesuretogerminateandsurvive。DispersalduringtheGlacialperiodTheidentityofmanyplantsandanimals，onmountain-summits，separatedfromeachotherbyhundredsofmilesoflowlands，wheretheAlpinespeciescouldnotpossiblyexist，isoneofthemoststrikingcasesknownofthesamespecieslivingatdistantpoints，withouttheapparentpossibilityoftheirhavingmigratedfromonetotheother。ItisindeedaremarkablefacttoseesomanyofthesameplantslivingonthesnowyregionsoftheAlpsorPyrenees，andintheextremenorthernpartsofEurope。butitisfarmoreremarkable，thattheplantsontheWhiteMountains，intheUnitedStatesofAmerica，areallthesamewiththoseofLabrador，andnearlyallthesame，aswehearfromAsaGray，withthoseontheloftiestmountainsofEurope。Evenaslongagoas1747，suchfactsledGmelintoconcludethatthesamespeciesmusthavebeenindependentlycreatedatseveraldistinctpoints。andwemighthaveremainedinthissamebelief，hadnotAgassizandotherscalledvividattentiontotheGlacialperiod，which，asweshallimmediatelysee，affordsasimpleexplanationofthesefacts。Wehaveevidenceofalmosteveryconceivablekind，organicandinorganic，thatwithinaveryrecentgeologicalperiod，centralEuropeandNorthAmericasufferedunderanArcticclimate。Theruinsofahouseburntbyfiredonottelltheirtalemoreplainly，thandothemountainsofScotlandandWales，withtheirscoredflanks，polishedsurfaces，andperchedboulders，oftheicystreamswithwhichtheirvalleyswerelatelyfilled。SogreatlyhastheclimateofEuropechanged，thatinNorthernItaly，giganticmoraines，leftbyoldglaciers，arenowclothedbythevineandmaize。ThroughoutalargepartoftheUnitedStates，erraticboulders，androcksscoredbydriftedicebergsandcoast-ice，plainlyrevealaformercoldperiod。

TheformerinfluenceoftheglacialclimateonthedistributionoftheinhabitantsofEurope，asexplainedwithremarkableclearnessbyEdwardForbes，issubstantiallyasfollows。Butweshallfollowthechangesmorereadily，bysupposinganewglacialperiodtocomeslowlyon，andthenpassaway，asformerlyoccurred。Asthecoldcameon，andaseachmoresouthernzonebecamefittedforarcticbeingsandill-fittedfortheirformermoretemperateinhabitants，thelatterwouldbesupplantedandarcticproductionswouldtaketheirplaces。Theinhabitantsofthemoretemperateregionswouldatthesametimetravelsouthward，unlesstheywerestoppedbybarriers，inwhichcasetheywouldperish。Themountainswouldbecomecoveredwithsnowandice，andtheirformerAlpineinhabitantswoulddescendtotheplains。Bythetimethatthecoldhadreacheditsmaximum，weshouldhaveauniformarcticfaunaandflora，coveringthecentralpartsofEurope，asfarsouthastheAlpsandPyrenees，andevenstretchingintoSpain。

ThenowtemperateregionsoftheUnitedStateswouldlikewisebecoveredbyarcticplantsandanimals，andthesewouldbenearlythesamewiththoseofEurope。forthepresentcircumpolarinhabitants，whichwesupposetohaveeverywheretravelledsouthward，areremarkablyuniformroundtheworld。

WemaysupposethattheGlacialperiodcameonalittleearlierorlaterinNorthAmericathaninEurope，sowillthesouthernmigrationtherehavebeenalittleearlierorlater。butthiswillmakenodifferenceinthefinalresult。

Asthewarmthreturned，thearcticformswouldretreatnorthward，closelyfollowedupintheirretreatbytheproductionsofthemoretemperateregions。

Andasthesnowmeltedfromthebasesofthemountains，thearcticformswouldseizeontheclearedandthawedground，alwaysascendinghigherandhigher，asthewarmthincreased，whilsttheirbrethrenwerepursuingtheirnorthernjourney。Hence，whenthewarmthhadfullyreturned，thesamearcticspecies，whichhadlatelylivedinabodytogetheronthelowlandsoftheOldandNewWorlds，wouldbeleftisolatedondistantmountain-summits(havingbeenexterminatedonalllesserheights)andinthearcticregionsofbothhemispheres。

ThuswecanunderstandtheidentityofmanyplantsatpointssoimmenselyremoteasonthemountainsoftheUnitedStatesandofEurope。WecanthusalsounderstandthefactthattheAlpineplantsofeachmountain-rangearemoreespeciallyrelatedtothearcticformslivingduenorthornearlyduenorthofthem：forthemigrationasthecoldcameon，andthere-migrationonthereturningwarmth，willgenerallyhavebeenduesouthandnorth。

TheAlpineplants，forexample，ofScotland，asremarkedbyMrH。C。Watson，andthoseofthePyrenees，asremarkedbyRamond，aremoreespeciallyalliedtotheplantsofnorthernScandinavia。thoseoftheUnitedStatestoLabrador。

thoseofthemountainsofSiberiatothearcticregionsofthatcountry。

Theseviews，groundedastheyareontheperfectlywell-ascertainedoccurrenceofaformerGlacialperiod，seemtometoexplaininsosatisfactoryamannerthepresentdistributionoftheAlpineandArcticproductionsofEuropeandAmerica，thatwheninotherregionswefindthesamespeciesondistantmountain-summits，wemayalmostconcludewithoutotherevidence，thatacolderclimatepermittedtheirformermigrationacrossthelowinterveningtracts，sincebecometoowarmfortheirexistence。

Iftheclimate，sincetheGlacialperiod，haseverbeeninanydegreewarmerthanatpresent(assomegeologistsintheUnitedStatesbelievetohavebeenthecase，chieflyfromthedistributionofthefossilGnathodon)，thenthearcticandtemperateproductionswillataverylateperiodhavemarchedalittlefurthernorth，andsubsequentlyhaveretreatedtotheirpresenthomes。butIhavemetwithnosatisfactoryevidencewithrespecttothisintercalatedslightlywarmerperiod，sincetheGlacialperiod。

Thearcticforms，duringtheirlongsouthernmigrationandre-migrationnorthward，willhavebeenexposedtonearlythesameclimate，and，asisespeciallytobenoticed，theywillhavekeptinabodytogether。consequentlytheirmutualrelationswillnothavebeenmuchdisturbed，and，inaccordancewiththeprinciplesinculcatedinthisvolume，theywillnothavebeenliabletomuchmodification。ButwithourAlpineproductions，leftisolatedfromthemomentofthereturningwarmth，firstatthebasesandultimatelyonthesummitsofthemountains，thecasewillhavebeensomewhatdifferent。

foritisnotlikelythatallthesamearcticspecieswillhavebeenleftonmountainrangesdistantfromeachother，andhavesurvivedthereeversince。theywill，also，inallprobabilityhavebecomemingledwithancientAlpinespecies，whichmusthaveexistedonthemountainsbeforethecommencementoftheGlacialepoch，andwhichduringitscoldestperiodwillhavebeentemporarilydrivendowntotheplains。theywill，also，havebeenexposedtosomewhatdifferentclimatalinfluences。Theirmutualrelationswillthushavebeeninsomedegreedisturbed。consequentlytheywillhavebeenliabletomodification。andthiswefindhasbeenthecase。forifwecomparethepresentAlpineplantsandanimalsoftheseveralgreatEuropeanmountain-ranges，thoughverymanyofthespeciesareidenticallythesame，somepresentvarieties，somearerankedasdoubtfulforms，andsomefewaredistinctyetcloselyalliedorrepresentativespecies。

Inillustratingwhat，asIbelieve，actuallytookplaceduringtheGlacialperiod，Iassumedthatatitscommencementthearcticproductionswereasuniformroundthepolarregionsastheyareatthepresentday。Buttheforegoingremarksondistributionapplynotonlytostrictlyarcticforms，butalsotomanysub-arcticandtosomefewnortherntemperateforms，forsomeofthesearethesameonthelowermountainsandontheplainsofNorthAmericaandEurope。anditmaybereasonablyaskedhowIaccountforthenecessarydegreeofuniformityofthesub-arcticandnortherntemperateformsroundtheworld，atthecommencementoftheGlacialperiod。Atthepresentday，thesub-arcticandnortherntemperateproductionsoftheOldandNewWorldsareseparatedfromeachotherbytheAtlanticOceanandbytheextremenorthernpartofthePacific。DuringtheGlacialperiod，whentheinhabitantsoftheOldandNewWorldslivedfurthersouthwardsthanatpresent，theymusthavebeenstillmorecompletelyseparatedbywiderspacesofocean。Ibelievetheabovedifficultymaybesurmountedbylookingtostillearlierchangesofclimateofanoppositenature。WehavegoodreasontobelievethatduringthenewerPlioceneperiod，beforetheGlacialepoch，andwhilstthemajorityoftheinhabitantsoftheworldwerespecificallythesameasnow，theclimatewaswarmerthanatthepresentday。Hencewemaysupposethattheorganismsnowlivingundertheclimateoflatitude60°。，duringthePlioceneperiodlivedfurthernorthunderthePolarCircle，inlatitude66°-67°……andthatthestrictlyarcticproductionsthenlivedonthebrokenlandstillnearertothepole。Nowifwelookataglobe，weshallseethatunderthePolarCirclethereisalmostcontinuouslandfromwesternEurope，throughSiberia，toeasternAmerica。Andtothiscontinuityofthecircumpolarland，andtotheconsequentfreedomforintermigrationunderamorefavourableclimate，Iattributethenecessaryamountofuniformityinthesub-arcticandnortherntemperateproductionsoftheOldandNewWorlds，ataperiodanteriortotheGlacialepoch。

Believing，fromreasonsbeforealludedto，thatourcontinentshavelongremainedinnearlythesamerelativeposition，thoughsubjectedtolarge，butpartialoscillationsoflevel，Iamstronglyinclinedtoextendtheaboveview，andtoinferthatduringsomeearlierandstillwarmerperiod，suchastheolderPlioceneperiod，alargenumberofthesameplantsandanimalsinhabitedthealmostcontinuouscircumpolarland。andthattheseplantsandanimals，bothintheOldandNewWorlds，beganslowlytomigratesouthwardsastheclimatebecamelesswarm，longbeforethecommencementoftheGlacialperiod。Wenowsee，asIbelieve，theirdescendants，mostlyinamodifiedcondition，inthecentralpartsofEuropeandtheUnitedStates。Onthisviewwecanunderstandtherelationship，withverylittleidentity，betweentheproductionsofNorthAmericaandEurope，arelationshipwhichismostremarkable，consideringthedistanceofthetwoareas，andtheirseparationbytheAtlanticOcean。Wecanfurtherunderstandthesingularfactremarkedonbyseveralobservers，thattheproductionsofEuropeandAmericaduringthelatertertiarystagesweremorecloselyrelatedtoeachotherthantheyareatthepresenttime。forduringthesewarmerperiodsthenorthernpartsoftheOldandNewWorldswillhavebeenalmostcontinuouslyunitedbyland，servingasabridge，sincerenderedimpassablebycold，fortheinter-migrationoftheirinhabitants。

DuringtheslowlydecreasingwarmthofthePlioceneperiod，assoonasthespeciesincommon，whichinhabitedtheNewandOldWorlds，migratedsouthofthePolarCircle，theymusthavebeencompletelycutofffromeachother。Thisseparation，asfarasthemoretemperateproductionsareconcerned，tookplacelongagesago。Andastheplantsandanimalsmigratedsouthward，theywillhavebecomemingledintheonegreatregionwiththenativeAmericanproductions，andhavehadtocompetewiththem。andintheothergreatregion，withthoseoftheOldWorld。Consequentlywehavehereeverythingfavourableformuchmodification，forfarmoremodificationthanwiththeAlpineproductions，leftisolated，withinamuchmorerecentperiod，ontheseveralmountain-rangesandonthearcticlandsofthetwoWorlds。Henceithascome，thatwhenwecomparethenowlivingproductionsofthetemperateregionsoftheNewandOldWorlds，wefindveryfewidenticalspecies(thoughAsaGrayhaslatelyshownthatmoreplantsareidenticalthanwasformerlysupposed)，butwefindineverygreatclassmanyforms，whichsomenaturalistsrankasgeographicalraces，andothersasdistinctspecies。andahostofcloselyalliedorrepresentativeformswhicharerankedbyallnaturalistsasspecificallydistinct。

Asontheland，sointhewatersofthesea，aslowsouthernmigrationofamarinefauna，whichduringthePlioceneorevenasomewhatearlierperiod，wasnearlyuniformalongthecontinuousshoresofthePolarCircle，willaccount，onthetheoryofmodification，formanycloselyalliedformsnowlivinginareascompletelysundered。Thus，Ithink，wecanunderstandthepresenceofmanyexistingandtertiaryrepresentativeformsontheeasternandwesternshoresoftemperateNorthAmerica。andthestillmorestrikingcaseofmanycloselyalliedcrustaceans(asdescribedinDana’sadmirablework)，ofsomefishandothermarineanimals，intheMediterraneanandintheseasofJapan，areasnowseparatedbyacontinentandbynearlyahemisphereofequatorialocean。

Thesecasesofrelationship，withoutidentity，oftheinhabitantsofseasnowdisjoined，andlikewiseofthepastandpresentinhabitantsofthetemperatelandsofNorthAmericaandEurope，areinexplicableonthetheoryofcreation。Wecannotsaythattheyhavebeencreatedalike，incorrespondencewiththenearlysimilarphysicalconditionsoftheareas。

forifwecompare，forinstance，certainpartsofSouthAmericawiththesoutherncontinentsoftheOldWorld，weseecountriescloselycorrespondinginalltheirphysicalconditions，butwiththeirinhabitantsutterlydissimilar。

Butwemustreturntoourmoreimmediatesubject，theGlacialperiod。

IamconvincedthatForbes’sviewmaybelargelyextended。InEuropewehavetheplainestevidenceofthecoldperiod，fromthewesternshoresofBritaintotheOuralrange，andsouthwardtothePyrenees。Wemayinfer，fromthefrozenmammalsandnatureofthemountainvegetation，thatSiberiawassimilarlyaffected。AlongtheHimalaya，atpoints900milesapart，glaciershaveleftthemarksoftheirformerlowdescent。andinSikkim，DrHookersawmaizegrowingongiganticancientmoraines。Southoftheequator，wehavesomedirectevidenceofformerglacialactioninNewZealand。

andthesameplants，foundonwidelyseparatedmountainsinthisisland，tellthesamestory。Ifoneaccountwhichhasbeenpublishedcanbetrusted，wehavedirectevidenceofglacialactioninthesoutheasterncornerofAustralia。

LookingtoAmerica。inthenorthernhalf，ice-bornefragmentsofrockhavebeenobservedontheeasternsideasfarsouthaslat。36°-37°。，andontheshoresofthePacific，wheretheclimateisnowsodifferent，asfarsouthaslat。46°……erraticbouldershave，also，beennoticedontheRockyMountains。IntheCordilleraofEquatorialSouthAmerica，glaciersonceextendedfarbelowtheirpresentlevel。IncentralChileIwasastonishedatthestructureofavastmoundofdetritus，about800

feetinheight，crossingavalleyoftheAndes。andthisInowfeelconvincedwasagiganticmoraine，leftfarbelowanyexistingglacier。Furthersouthonbothsidesofthecontinent，fromlat。41°。tothesouthernmostextremity，wehavetheclearestevidenceofformerglacialaction，inhugeboulderstransportedfarfromtheirparentsource。

WedonotknowthattheGlacialepochwasstrictlysimultaneousattheseseveralfardistantpointsonoppositesidesoftheworld。Butwehavegoodevidenceinalmosteverycase，thattheepochwasincludedwithinthelatestgeologicalperiod。Wehave，also，excellentevidence，thatitenduredforanenormoustime，asmeasuredbyyears，ateachpoint。Thecoldmayhavecomeon，orhaveceased，earlieratonepointoftheglobethanatanother，butseeingthatitenduredforlongateach，andthatitwascontemporaneousinageologicalsense，itseemstomeprobablethatitwas，duringapartatleastoftheperiod，actuallysimultaneousthroughouttheworld。Withoutsomedistinctevidencetothecontrary，wemayatleastadmitasprobablethattheglacialactionwassimultaneousontheeasternandwesternsidesofNorthAmerica，intheCordilleraundertheequatorandunderthewarmertemperatezones，andonbothsidesofthesouthernextremityofthecontinent。Ifthisbeadmitted，itisdifficulttoavoidbelievingthatthetemperatureofthewholeworldwasatthisperiodsimultaneouslycooler。Butitwouldsufficeformypurpose，ifthetemperaturewasatthesametimeloweralongcertainbroadbeltsoflongitude。

Onthisviewofthewholeworld，oratleastofbroadlongitudinalbelts，havingbeensimultaneouslycolderfrompoletopole，muchlightcanbethrownonthepresentdistributionofidenticalandalliedspecies。InAmerica，DrHookerhasshownthatbetweenfortyandfiftyofthefloweringplantsofTierradelFuego，formingnoinconsiderablepartofitsscantyflora，arecommontoEurope，enormouslyremoteasthesetwopointsare。

andtherearemanycloselyalliedspecies。OntheloftymountainsofequatorialAmericaahostofpeculiarspeciesbelongingtoEuropeangeneraoccur。

OnthehighestmountainsofBrazil，somefewEuropeangenerawerefoundbyGardner，whichdonotexistinthewideinterveninghotcountries。SoontheSillaofCaraccastheillustriousHumboldtlongagofoundspeciesbelongingtogeneracharacteristicoftheCordillera。OnthemountainsofAbyssinia，severalEuropeanformsandsomefewrepresentativesofthepeculiarfloraoftheCapeofGoodHopeoccur。AttheCapeofGoodHopeaveryfewEuropeanspecies，believednottohavebeenintroducedbyman，andonthemountains，somefewrepresentativeEuropeanformsarefound，whichhavenotbeendiscoveredintheintertropicalpartsofAfrica。OntheHimalaya，andontheisolatedmountain-rangesofthepeninsulaofIndia，ontheheightsofCeylon，andonthevolcanicconesofJava，manyplantsoccur，eitheridenticallythesameorrepresentingeachother，andatthesametimerepresentingplantsofEurope，notfoundintheinterveninghotlowlands。AlistofthegeneracollectedontheloftierpeaksofJavaraisesapictureofacollectionmadeonahillinEurope！StillmorestrikingisthefactthatsouthernAustralianformsareclearlyrepresentedbyplantsgrowingonthesummitsofthemountainsofBorneo。SomeoftheseAustralianforms，asIhearfromDr。Hooker，extendalongtheheightsofthepeninsulaofMalacca，andarethinlyscattered，ontheonehandoverIndiaandontheotherasfarasJapan。

OnthesouthernmountainsofAustralia，Dr。F。Mü。llerhasdiscoveredseveralEuropeanspecies。otherspecies，notintroducedbyman，occuronthelowlands。andalonglistcanbegiven，asIaminformedbyDr。Hooker，ofEuropeangenera，foundinAustralia，butnotintheintermediatetorridregions。Intheadmirable`IntroductiontotheFloraofNewZealand，’byDr。Hooker，analogousandstrikingfactsaregiveninregardtotheplantsofthatlargeisland。Henceweseethatthroughouttheworld，theplantsgrowingonthemoreloftymountains，andonthetemperatelowlandsofthenorthernandsouthernhemispheres，aresometimesidenticallythesame。

buttheyaremuchoftenerspecificallydistinct，thoughrelatedtoeachotherinamostremarkablemanner。

Thisbriefabstractappliestoplantsalone：somestrictlyanalogousfactscouldbegivenonthedistributionofterrestrialanimals。Inmarineproductions，similarcasesoccur。asanexample，Imayquotearemarkbythehighestauthority，Prof。Dana，that`itiscertainlyawonderfulfactthatNewZealandshouldhaveacloserresemblanceinitscrustaceatoGreatBritain，itsantipode，thantoanyotherpartoftheworld。’SirJ。Richardson，also，speaksofthereappearanceontheshoresofNewZealand，Tasmania，&。c。，ofnorthernformsoffish。DrHookerinformsmethattwenty-fivespeciesofAlgaearecommontoNewZealandandtoEurope，buthavenotbeenfoundintheintermediatetropicalseas。

Itshouldbeobservedthatthenorthernspeciesandformsfoundinthesouthernpartsofthesouthernhemisphere，andonthemountain-rangesoftheintertropicalregions，arenotarctic，butbelongtothenortherntemperatezones。AsMr。H。C。Watsonhasrecentlyremarked，`Inrecedingfrompolartowardsequatoriallatitudes，theAlpineormountainflorasreallybecomelessandlessarctic。’Manyoftheformslivingonthemountainsofthewarmerregionsoftheearthandinthesouthernhemisphereareofdoubtfulvalue，beingrankedbysomenaturalistsasspecificallydistinct，byothersasvarieties。butsomearecertainlyidentical，andmany，thoughcloselyrelatedtonorthernforms，mustberankedasdistinctspecies。

Nowletusseewhatlightcanbethrownontheforegoingfacts，onthebelief，supportedasitisbyalargebodyofgeologicalevidence，thatthewholeworld，oralargepartofit，wasduringtheGlacialperiodsimultaneouslymuchcolderthanatpresent。TheGlacialperiod，asmeasuredbyyears，musthavebeenverylong。andwhenwerememberoverwhatvastspacessomenaturalisedplantsandanimalshavespreadwithinafewcenturies，thisperiodwillhavebeenampleforanyamountofmigration。Asthecoldcameslowlyon，allthetropicalplantsandotherproductionswillhaveretreatedfrombothsidestowardstheequator，followedintherearbythetemperateproductions，andthesebythearctic。butwiththelatterwearenotnowconcerned。Thetropicalplantsprobablysufferedmuchextinction。howmuchnoonecansay。perhapsformerlythetropicssupportedasmanyspeciesasweseeatthepresentdaycrowdedtogetherattheCapeofGoodHope，andinpartsoftemperateAustralia。Asweknowthatmanytropicalplantsandanimalscanwithstandaconsiderableamountofcold，manymighthaveescapedexterminationduringamoderatefalloftemperature，moreespeciallybyescapingintothewarmestspots。Butthegreatfacttobearinmindis，thatalltropicalproductionswillhavesufferedtoacertainextent。

Ontheotherhand，thetemperateproductions，aftermigratingnearertotheequator，thoughtheywillhavebeenplacedundersomewhatnewconditions，willhavesufferedless。Anditiscertainthatmanytemperateplants，ifprotectedfromtheinroadsofcompetitors，canwithstandamuchwarmerclimatethantheirown。Hence，itseemstomepossible，bearinginmindthatthetropicalproductionswereinasufferingstateandcouldnothavepresentedafirmfrontagainstintruders，thatacertainnumberofthemorevigorousanddominanttemperateformsmighthavepenetratedthenativeranksandhavereachedorevencrossedtheequator。Theinvasionwould，ofcourse，havebeengreatlyfavouredbyhighland，andperhapsbyadryclimate。forDr。Falconerinformsmethatitisthedampwiththeheatofthetropicswhichissodestructivetoperennialplantsfromatemperateclimate。Ontheotherhand，themosthumidandhottestdistrictswillhaveaffordedanasylumtothetropicalnatives。Themountain-rangesnorth-westoftheHimalaya，andthelonglineoftheCordillera，seemtohaveaffordedtwogreatlinesofinvasion：anditisastrikingfact，latelycommunicatedtomebyDr。Hooker，thatallthefloweringplants，aboutforty-sixinnumber，commontoTierradelFuegoandtoEuropestillexistinNorthAmerica，whichmusthavelainonthelineofmarch。ButIdonotdoubtthatsometemperateproductionsenteredandcrossedeventhelowlandsofthetropicsattheperiodwhenthecoldwasmostintense，whenarcticformshadmigratedsometwenty-fivedegreesoflatitudefromtheirnativecountryandcoveredthelandatthefootofthePyrenees。Atthisperiodofextremecold，Ibelievethattheclimateundertheequatorattheleveloftheseawasaboutthesamewiththatnowfeltthereattheheightofsixorseventhousandfeet。Duringthisthecoldestperiod，Isupposethatlargespacesofthetropicallowlandswereclothedwithamingledtropicalandtemperatevegetation，likethatnowgrowingwithstrangeluxurianceatthebaseoftheHimalaya，asgraphicallydescribedbyHooker。

Thus，asIbelieve，aconsiderablenumberofplants，afewterrestrialanimals，andsomemarineproductions，migratedduringtheGlacialperiodfromthenorthernandsoutherntemperatezonesintotheintertropicalregions，andsomeevencrossedtheequator。Asthewarmthreturned，thesetemperateformswouldnaturallyascendthehighermountains，beingexterminatedonthelowlands。thosewhichhadnotreachedtheequator，wouldre-migratenorthwardorsouthwardtowardstheirformerhomes。buttheforms，chieflynorthern，whichhadcrossedtheequator，wouldtravelstillfurtherfromtheirhomesintothemoretemperatelatitudesoftheoppositehemisphere。

Althoughwehavereasontobelievefromgeologicalevidencethatthewholebodyofarcticshellsunderwentscarcelyanymodificationduringtheirlongsouthernmigrationandre-migrationnorthward，thecasemayhavebeenwhollydifferentwiththoseintrudingformswhichsettledthemselvesontheintertropicalmountains，andinthesouthernhemisphere。Thesebeingsurroundedbystrangerswillhavehadtocompetewithmanynewformsoflife。anditisprobablethatselectedmodificationsintheirstructure，habits，andconstitutionswillhaveprofitedthem。Thusmanyofthesewanderers，thoughstillplainlyrelatedbyinheritancetotheirbrethrenofthenorthernorsouthernhemispheres，nowexistintheirnewhomesaswell-markedvarietiesorasdistinctspecies。

Itisaremarkablefact，stronglyinsistedonbyHookerinregardtoAmerica，andbyAlph。deCandolleinregardtoAustralia，thatmanymoreidenticalplantsandalliedformshaveapparentlymigratedfromthenorthtothesouth，thaninareverseddirection。Wesee，however，afewsouthernvegetableformsonthemountainsofBorneoandAbyssinia。Isuspectthatthispreponderantmigrationfromnorthtosouthisduetothegreaterextentoflandinthenorth，andtothenorthernformshavingexistedintheirownhomesingreaternumbers，andhavingconsequentlybeenadvancedthroughnaturalselectionandcompetitiontoahigherstageofperfectionordominatingpower，thanthesouthernforms。Andthus，whentheybecamecommingledduringtheGlacialperiod，thenorthernformswereenabledtobeatthelesspowerfulsouthernforms。Justinthesamemannerasweseeatthepresentday，thatverymanyEuropeanproductionscoverthegroundinLaPlata，andinalesserdegreeinAustralia，andhavetoacertainextentbeatenthenatives。whereasextremelyfewsouthernformshavebecomenaturalisedinanypartofEurope，thoughhides，wool，andotherobjectslikelytocarryseedshavebeenlargelyimportedintoEuropeduringthelasttwoorthreecenturiesfromLaPlata，andduringthelastthirtyorfortyyearsfromAustralia。Somethingofthesamekindmusthaveoccurredontheintertropicalmountains：nodoubtbeforetheGlacialperiodtheywerestockedwithendemicAlpineforms。

butthesehavealmosteverywherelargelyyieldedtothemoredominantforms，generatedinthelargerareasandmoreefficientworkshopsofthenorth。

Inmanyislandsthenativeproductionsarenearlyequalledorevenoutnumberedbythenaturalised。andifthenativeshavenotbeenactuallyexterminated，theirnumbershavebeengreatlyreduced，andthisisthefirststagetowardsextinction。Amountainisanislandontheland。andtheintertropicalmountainsbeforetheGlacialperiodmusthavebeencompletelyisolated。

andIbelievethattheproductionsoftheseislandsonthelandyieldedtothoseproducedwithinthelargerareasofthenorth，justinthesamewayastheproductionsofrealislandshaveeverywherelatelyyieldedtocontinentalforms，naturalisedbyman’sagency。

Iamfarfromsupposingthatalldifficultiesareremovedontheviewheregiveninregardtotherangeandaffinitiesofthealliedspecieswhichliveinthenorthernandsoutherntemperatezonesandonthemountainsoftheintertropicalregions。Verymanydifficultiesremaintobesolved。

Idonotpretendtoindicatetheexactlinesandmeansofmigration，orthereasonwhycertainspeciesandnotothershavemigrated。whycertainspecieshavebeenmodifiedandhavegivenrisetonewgroupsofforms，andothershaveremainedunaltered。Wecannothopetoexplainsuchfacts，untilwecansaywhyonespeciesandnotanotherbecomesnaturalisedbyman’sagencyinaforeignland。whyonerangestwiceorthriceasfar，andistwiceorthriceascommon，asanotherspecieswithintheirownhomes。

Ihavesaidthatmanydifficultiesremaintobesolved：someofthemostremarkablearestatedwithadmirableclearnessbyDr。Hookerinhisbotanicalworksontheantarcticregions。Thesecannotbeherediscussed。

IwillonlysaythatasfarasregardstheoccurrenceofidenticalspeciesatpointssoenormouslyremoteasKerguelenLand，NewZealand，andFuegia，IbelievethattowardsthecloseoftheGlacialperiod，icebergs，assuggestedbyLyell，havebeenlargelyconcernedintheirdispersal。Buttheexistenceofseveralquitedistinctspecies，belongingtogeneraexclusivelyconfinedtothesouth，attheseandotherdistantpointsofthesouthernhemisphere，is，onmytheoryofdescentwithmodification，afarmoreremarkablecaseofdifficulty。Forsomeofthesespeciesaresodistinct，thatwecannotsupposethattherehasbeentimesincethecommencementoftheGlacialperiodfortheirmigration，andfortheirsubsequentmodificationtothenecessarydegree。Thefactsseemtometoindicatethatpeculiarandverydistinctspecieshavemigratedinradiatinglinesfromsomecommoncentre。

andIaminclinedtolookinthesouthern，asinthenorthernhemisphere，toaformerandwarmerperiod，beforethecommencementoftheGlacialperiod，whentheantarcticlands，nowcoveredwithice，supportedahighlypeculiarandisolatedflora。IsuspectthatbeforethisflorawasexterminatedbytheGlacialepoch，afewformswerewidelydispersedtovariouspointsofthesouthernhemispherebyoccasionalmeansoftransport，andbytheaid，ashalting-places，ofexistingandnowsunkenislands，andperhapsatthecommencementoftheGlacialperiod，byicebergs。Bythesemeans，asIbelieve，thesouthernshoresofAmerica，Australia，NewZealandhavebecomeslightlytintedbythesamepeculiarformsofvegetablelife。