THEsubjectuponwhichIwishtoaddressyouto—nightisthestructureandoriginofCoralandCoralReefs。Undertheheadof\"coral\"thereareincludedtwoverydifferentthings；oneofthemisthatsubstancewhichIimagineagreatnumberofushavechampedwhenwewereverymuchyoungerthanwearenow，——thecommonredcoral，whichisusedsomuch，asyouknow，fortheedificationandthedelectationofchildrenoftenderyears，andisalsoemployedforthepurposesofornamentforthosewhoaremucholder，andassomethinkmightknowbetter。Theotherkindofcoralisaverydifferentsubstance；itmayfordistinction’ssakebecalledthewhitecoral；itisamaterialwhichmostassuredlynotthehardest—heartedofbabyfarmerswouldgivetoababytochew，anditisasubstancewhichistobeseenonlyinthecabinetsofcuriouspersons，orinmuseums，or，maybe，overthemantelpiecesofsea—faringmen。Butalthoughtheredcoral，asIhavementionedtoyou，hasaccesstotheverybestsociety；andalthoughthewhitecoraliscomparativelyadespisedproduct，yetinthis，asinmanyothercases，thehumblerthingisinrealitythegreater；theamountofworkwhichisdoneintheworldbythewhitecoralbeingabsolutelyinfinitecomparedwiththateffectedbyitsdelicateandpamperednamesake。Eachofthesesubstances，thewhitecoralandthered，however，hasarelationshiptotheother。Theyare，inazoologicalsense，cousins，eachofthembeingformedbythesamekindofanimalsinwhatissubstantiallythesameway。Eachofthesebodiesis，infact，thehardskeletonofaverycuriousandaverysimpleanimal，morecomparabletothebonesofsuchanimalsasourselvesthantotheshellsofoystersorcreaturesofthatkind；foritisthehardeningoftheinternaltissueofthecreature，ofitsinternalsubstance，bythedepositinthebodyofamaterialwhichisexceedinglycommon，notonlyinfreshbutinseawater，andwhichisspeciallyabundantinthosewaterswhichweknowas\"hard，\"thosewaters，forexample，whichleavea\"fur\"uponthebottomofatea—kettle。This\"fur\"iscarbonateoflime，thesamesortofsubstanceaslimestoneandchalk。Thatmaterialiscontainedinsolutioninseawater，anditisoutoftheseawaterinwhichthesecoralcreatureslivethattheygetthelimewhichisneededfortheformingoftheirhardskeleton。
Butnowwhatmannerofcreaturesarethesewhichformthesehardskeletons？Idaresaythatinthesedaysofkeepingaquaria，oflocomotiontothesea—side，mostofthosewhomIamaddressingmayhaveseenoneofthosecreatureswhichusedtobeknownasthe\"seaanemone，\"receivingthatnameonaccountofitsgeneralresemblance，inaroughsortofway，totheflowerwhichisknownasthe\"anemone\"；butbeingathingwhichlivesinthesea，itwasqualifiedasthe\"seaanemone。\"Well，then，youmustsupposeabodyshapedlikeashortcylinder，thetopcutoff，andinthetopaholeratherovalthanround。
Allroundthisaperture，whichisthemouth，imaginethatthereareplacedanumberoffeelersformingacircle。Thecavityofthemouthleadsintoasortofstomach，whichisveryunlikethoseofthehigheranimals，inthecircumstancethatitopensatthelowerendintoacavityofthebody，andallthedigestedmatter，convertedintonourishment，isthusdistributedthroughtherestofthebody。Thatisthegeneralstructureofoneoftheseseaanemones。Ifyoutouchititcontractsimmediatelyintoaheap。Itlooksatfirstquitelikeaflowerinthesea，butifyoutouchityoufindthatitexhibitsallthepeculiaritiesofalivinganimal；andifanythingwhichcanserveasitspreycomesnearitstentacles，itclosesthemrounditandsucksthematerialintoitsstomachandtheredigestsitandturnsittotheaccountofitsownbody。
Thesecreaturesareveryvoracious，andnotatallparticularwhattheyseize；andsometimesitmaybethattheylayholdofashellfishwhichisfartoobigtobepackedintothatinteriorcavity，and，ofcourse，inanyordinaryanimalaproceedingofthiskindwouldgiverisetoaveryseverefitofindigestion。Butthisisbynomeansthecaseintheseaanemone，becausewhendigestivedifficultiesofthiskindarisehegetsoutofthembysplittinghimselfintwo；andtheneachhalfbuildsitselfupintoafreshcreature，andyouhavetwopolypeswheretherewaspreviouslyone，andthebonewhichstuckinthewaylyingbetweenthem！Notonlycanthesecreaturesmultiplyinthisfashion，buttheycanmultiplybybuds。Abudwillgrowoutofthesideofthebody（I
amnotspeakingofthecommonseaanemone，butofalliedcreatures）
justlikethebudofaplant，andthatwillfashionitselfintoacreaturejustliketheparent。Therearesomeoftheminwhichthesebudsremainconnectedtogether，andyouwillsoonseewhatwouldbetheresultofthat。IfImakeabudgrowouthere，andanotherontheoppositeside，andeachfashionsitselfintoanewpolype，thepracticaleffectwillbethatbeforelongyouwillseeasinglepolypeconvertedintoasortoftreeorbushofpolypes。Andthesewillallremainassociatedtogether，likeakindofco—operativestore，whichisathingIbelieveyouunderstandverywellhere，——eachmouthwillhelptofeedthebodyandeachpartofthebodyhelptosupportthemultifariousmouths。Ithinkthatisasgoodanexampleofazoologicalco—operativestoreasyoucanwellhave。Sucharethesewonderfulcreatures。Buttheyarecapablenotonlyofmultiplyinginthisway，butinotherways，byhavingamoreordinaryandregularkindofoffspring。Littleeggsarehatchedandtheyoungarepassedoutbythewayofthemouth，andtheygoswimmingaboutaslittleovalbodiescoveredwithaverycuriouskindofhairlikeprocesses。Eachoftheseprocessesiscapableofstrikingwaterlikeanoar；andtheconsequenceisthattheyoungcreatureispropelledthroughthewater。Sothatyouhavetheyoungpolypefloatingaboutinthisfashion，coveredbyits’vibratilecilia’，astheselongfilaments，whicharecapableofvibrationaretermed。Andthus，althoughthepolypeitselfmaybeafixedcreatureunabletomoveabout，itisabletospreaditsoffspringovergreatareas。Forthesecreaturesnotonlypropelthemselves，butwhileswimmingaboutintheseaformanyhours，orperhapsdays，itwillbeobviousthattheymustbecarriedhitherandthitherbythecurrentsofthesea，whichnotunfrequentlymoveattherateofoneortwomilesanhour。Thus，inthecourseofafewdays，theoffspringofthisstationarycreaturemaybecarriedtoaverygreatdistancefromitsparent；andhavingbeensocarrieditlosestheseorgansbywhichitispropelled，andsettlesdownuponthebottomoftheseaandgrowsupagainintotheformandconditionofitsparents。Sothatifyousupposeasinglepolypeofthiskindsettleduponthebottomofthesea，itmaybythesevariousmethods——thatistosay，bycuttingitselfintwo，whichwecall\"fission，\"orbybudding；orbysendingouttheseswimmingembryos，——multiplyitselftoanenormousextent，andgiverisetothousands，ormillions，ofprogenyinacomparativelyshorttime；
andthesethousands，ormillions，ofprogenymaycoveraverylargesurfaceoftheseabottom；infact，youwillreadilyperceivethat，givethemtime，andthereisnolimittothesurfacewhichtheymaycover。
Havingunderstoodthusfarthegeneralnatureofthesepolypes，whicharethefabricatorsbothoftheredandwhitecoral，letusconsideralittlemoreparticularlyhowtheskeletonsoftheredcoralandofthewhitecoralareformed。Theredcoralpolypeperchesupontheseabottom，itthengrowsupintoasortofstem，andoutofthatstemtheregrowbranches，eachofwhichhasitsownpolypes；andthusyouhaveakindoftreeformed，everybranchofthetreeterminatedbyitspolype。Itisatree，butattheendofthebranchesthereareopenmouthsofpolypesinsteadofflowers。Thusthereisacommonsoftbodyconnectingthewhole，andasitgrowsupthesoftbodydepositsinitsinterioraquantityofcarbonateoflime，whichacquiresabeautifulredorfleshcolour，andformsakindofstemrunningthroughthewhole，anditisthatstemwhichistheredcoral。TheredcoralgrowsprincipallyatthebottomoftheMediterraneanSea，atverygreatdepths，andthecoralfishers，whoareveryadventurousseamen，taketheirdragnets，ofapeculiarkind，roughlymade，butefficientfortheirpurpose，anddragthemalongthebottomoftheseatocatchthebranchesoftheredcoral，whichbecomeentangledandarethusbroughtuptothesurface。Theyarethenallowedtoputrefy，inordertogetridoftheanimalmatter，andtheredcoralistheskeletonthatisleft。
Inthecaseofthewhitecoral，theskeletonismorecomplete。Intheredcoral，theskeletonbelongstothewhole；inthewhitecoralthereisaspecialskeletonforeveryoneofthesepolypesinadditiontothatforthewholebody。Thereisaskeletonformedinthebodyofeachofthem，likeacupdividedbyanumberofradiatingpartitionstowardstheoutside；andthatcupisformedofcarbonateoflime，onlynotstainedred，asinthecaseoftheredcoral。Andallthesecupsarejoinedtogetherintoacommonbranch，theresultofwhichistheformationofabeautifulcoraltree。Thisisagreatmassofmadrepore，andinthelivingstateeveryoneoftheendsofthesebrancheswasterminatedbyabeautifullittlepolype，likeaseaanemone，andalltheskeletonwascoveredbyasoftbodywhichunitedthepolypestogether。Youmustunderstandthatallthisskeletonhasbeenformedintheinteriorofthebody，tosuitthebranchedbodyofthepolypemass，andthatitisasmuchitsskeletonasourownbonesareourskeleton。Inthisnextcoralthecreaturewhichhasformedtheskeletonhasdivideditselfasitgrew，andconsequentlyhasformedagreatexpansion；butscatteredalloverthissurfacetherewerepolypebodieslikethoseIpreviouslydescribed。Again，whenthisgreatcupwasalive，thewholesurfacewascoveredwithabeautifulbodyuponwhichweresetinnumerablesmallpolypeflowers，ifwemaysocallthem，oftenbrilliantlycoloured；andthewholecupwasbuiltupinthesamefashionbythedepositofcarbonateoflimeintheinteriorofthecombinedpolypebody，formedbybuddingandbyfissioninthewayI
described。Youwillperceivethatthereisnonecessarylimittothisprocess。Thereisnoreasonwhyweshouldnothavecoralthreeorfourtimesasbig；andtherearecertaincreaturesofthiskindthatdofabricateverylargemasses，orhalfspheresseveralfeetindiameter。
Thustheactivityoftheseanimalsinseparatingcarbonateoflimefromtheseaandbuildingitupintodefiniteshapesisveryconsiderableindeed。
NowIthinkIhavesaidsufficient——asmuchasIcanwithouttakingyouintotechnicaldetails，ofthegeneralnatureofthesecreatureswhichformcoral。Theanimalswhichformcoralarescatteredovertheseasofallcountriesintheworld。Theredcoraliscomparativelylimited，butthepolypeswhichformthewhitecoralarewidelyscattered。Therearesomeofthemwhichremainsingle，orwhichgiverisetoonlysmallaccumulations；andtheskeletonsofthese，astheydie，accumulateuponthebottomofthesea，buttheydonotcometomuch；theyarewashedaboutanddonotadheretogether，butbecomemixedupwiththemudofthesea。Buttherearecertainpartsoftheworldinwhichthecoralpolypeswhichliveandgrowareofakindwhichremain，adheretogether，andformgreatmasses。Theydifferfromtheordinarypolypesjustinthesamewayasthoseplantswhichformapeat—bogormeadow—turfdifferfromordinaryplants。Theyhaveahabitofgrowingtogetherinmassesinthesameplace；theyarewhatwecall\"gregarious\"things；andtheconsequenceofthisis，thatastheydieandleavetheirskeletons，thoseskeletonsformaconsiderablesolidaggregationatthebottomofthesea，andotherpolypesperchuponthem，andbeginbuildinguponthem，andsobydegreesagreatmassisformed。AndjustasweknowtherearesomeancientcitiesinwhichyouhaveaBritishcity，andoverthatthefoundationsofaRomancity；andoverthataSaxoncity，andoverthatagainamoderncity，sointheselocalitiesofwhichIamspeaking，youhavetheaccumulationsofthefoundationsofthehouses，ifImayusetheterm，ofnationafternationofthesecoralpolypes；andtheseaccumulationsmaycoveraveryconsiderablespace，andmayriseinthecourseoftimefromthebottomtothesurfaceofthesea。
Marinershaveanamewhichtheyapplytoallsortsofobstaclesconsistingofhardandrockymatterwhichcomesintheirwayinthecourseoftheirnavigation；theycallsuchobstacles\"reefs，\"andtheyhavelongbeeninthehabitofcallingtheparticularkindofreef，whichisformedbytheaccumulationoftheskeletonsofdeadcorals，bythenameof\"coralreefs，\"therefore，thosepartsoftheworldinwhichtheseaccumulationsoccurhavebeentermedbythem\"coralreefareas，\"
orregionsinwhichcoralreefsarefound。ThereisaverynotableexampleofasimplecoralreefabouttheislandofMauritius，whichI
daresayyouallknow，liesinthemiddleoftheIndianOcean。Itisaveryconsiderableandbeautifulisland，andissurroundedonallsidesbyamassofcoral，whichhasbeenformedinthewayIhavedescribed；
sothatifyoucouldgetuponthetopofoneofthepeaksoftheisland，andlookdownupontheIndianOcean，youwouldseethatthebeachroundtheIslandwascontinuedoutwardbyakindofshallowterrace，whichiscoveredbythesea，andwheretheseaisquiteshallow；andatadistancevaryingfromthree—quartersofamiletoamileandahalffromtheproperbeach，youwouldseealineoffoamorsurfwhichlooksmostbeautifulincontrastwiththebrightgreenwaterintheinside，andthedeepblueoftheseabeyond。Thatlineofsurfindicatesthepointatwhichthewatersoftheoceanarebreakinguponthecoralreefwhichsurroundstheisland。Youseeitsweeproundtheislanduponallsides，exceptwherearivermaychancetocomedown，andthatalwaysmakesagapintheshore。
TherearetwoorthreepointswhichIwishtobringclearlybeforeyournoticeaboutsuchareefasthis。Inthefirstplace，youperceiveitformsakindoffringeroundtheisland，andisthereforecalleda\"fringingreef。\"Inthenextplace，ifyougooutinaboat，andtakesoundingsattheedgeofthereef，youfindthatthedepthofthewaterisnotmorethanfrom20to25fathoms——thatisabout120to150feet。
Outsidethatpointyoucometothenaturalseabottom；butallinsidethatdepthiscoral，builtupfromthebottombytheaccumulationoftheskeletonsofinnumerablegenerationsofcoralpolypes。Sothatyouseethecoralformsaveryconsiderablerampartroundtheisland。WhattheexactcircumferencemaybeIdonotremember，butitcannotbelessthan100miles，andtheoutwardheightofthiswallofcoralrocknowhereamountstolessthanabout100or150feet。
Whentheoutwardfaceofthereefisexamined，youfindthattheupperedge，whichisexposedtothewashofthesea，andalltheseawardface，iscoveredwiththoselivingplant—likeflowerswhichIhavedescribedtoyou。Theyarethecoralpolypeswhichgrow，flourish，andaddtothemassofcalcareousmatterwhichalreadyformsthereef。Buttowardsthelowerpartofthereef，atadepthofabout120feet，thesecreaturesarelessactive，andfewerofthematwork；andatgreaterdepthsthanthatyoufindnolivingcoralpolypeatall；anditmaybelaiddownasarule，derivedfromveryextensiveobservation，thatthesereef—buildingcoralscannotliveinagreaterdepthofwaterthanabout120to150feet。Ibegyoutorecollectthatfact，becauseitisoneIshallhavetocomebacktobyandby，andtoshowtowhatverycuriousconsequencesthatruleleads。Wellthen，comingbacktothemarginofthereef，youfindthatpartofitwhichliesjustwithinthesurftobecoatedbyaverycuriousplant，asortofseaweed，whichcontainsinitssubstanceaverygreatdealofcarbonateoflime，andlooksalmostlikerock；thisiswhatiscalledthenullipore。Moretowardstheland，wecometotheshallowwaterupontheinsideofthereef，whichhasaparticularname，derivedfromtheSpanishorthePortuguese——itiscalleda\"lagoon，\"orlake。Inthislagoonthereiscomparativelylittlelivingcoral；thebottomofitisformedofcoralmud。Ifwepoundedthiscoralinwater，itwouldbeconvertedintocalcareousmud，andthewavesduringstormsdoforthecoralskeletonsexactlywhatwemightdoforthiscoralinamortar；thewavestearoffgreatfragmentsandcrushthemwithprodigiousforce，untiltheyaregroundintothemerestpowder，andthatpowderiswashedintotheinteriorofthelagoon，andformsamuddycoatingatthebottom。Besidethatthereareagreatmanyanimalsthatpreyuponthecoral——fishes，worms，andcreaturesofthatkind，andallthese，bytheirdigestiveprocesses，reducethecoraltothesamestate，andcontributeaveryimportantelementtothisfinemud。Thelivingcoralfoundinthelagoon，isnotthereefbuildingcoral；itdoesnotgiverisetothesamemassiveskeletons。Asyougoinaboatovertheseshallowpools，youseethesebeautifulthings，colouredred，blue，green，andallcolours，buildingtheirhouses；butthesearemeretenements，andnottobecomparedinmagnitudeandimportancetothemasseswhicharebuiltbythereef—buildersthemselves。Nowsuchastructureasthisiswhatistermeda\"fringingreef。\"YoumeetwithfringingreefsofthiskindnotonlyintheMauritius，butinanumberofotherpartsoftheworld。
Iftheseweretheonlyreefstobeseenanywhere，theproblemoftheformationofcoralreefswouldneverhavebeenadifficultone。Nothingcanbeeasierthantounderstandhowtheremusthavebeenatimewhenthecoralpolypescameandsettledontheshoresofthisisland，everywherewithinthe20to25fathomline，andhow，havingperchedthere，theygraduallygrewuntiltheybuiltupthereef。
Butthesearebynomeanstheonlysortofcoralreefsintheworld；onthecontrary，thereareverylargeareas，notonlyoftheIndianocean，butofthePacific，inwhichmanymanythousandsofsquaremilesarecoveredeitherwithapeculiarkindofreef，whichiscalledthe\"encirclingreef，\"orbyastillmorecuriousreefwhichgoesbythenameofthe\"atoll。\"Thereisaverygoodpicture，whichProfessorRoscoehasbeenkindenoughtoprepareforme，ofoneoftheseatolls，whichwillenableyoutoformanotionofitasalandscape。YouhaveintheforegroundthewatersofthePacific。Youmustfancyyourselfinthemiddleofthegreatocean，andyouwillperceivethatthereisanalmostcircularisland，withalowbeach，whichisformedentirelyofcoralsand；growinguponthatbeachyouhavevegetation，whichtakes，ofcourse，theshapeofthecircularland；andthen，intheinteriorofthecircle，thereisapoolofwater，whichisnotverydeep——probablyinthiscasenotmorethaneightorninefathoms——andwhichformsastrangeandbeautifulcontrasttothedeepbluewateroutside。Thiscircularisland，oratoll，withalagooninthemiddle，isnotacompletecircle；upononesideofitthereisabreak，exactlyliketheentranceintoadock；and，asamatterofcourse，thesecircularislets，oratolls，formmostefficientbreak—waters，forifyoucanonlygetinsideyourshipisinperfectsafety，withadmirableanchorageintheinterior。Iftheshipwerelyingwithinamileofthatbeach，thewaterwouldbeoneortwothousandfeetdeep；therefore，asectionofthatatoll，withthesoundingsasdeepasthisallround，wouldgiveyouthenotionofagreatcone，cutoffatthetop，andwithashallowcupinthemiddleofit。Now，whataverysingularfactthisis，thatweshouldhaverisingfromthebottomofthedeepoceanagreatpyramid，besidewhichallhumanpyramidssinkintothemostutterinsignificance！Thesesingularcorallimestonestructuresareverybeautiful，especiallywhencrownedwithcocoa—nuttrees。Thereyouseethelonglineofland，coveredwithvegetation——cocoa—nuttrees——andyouhavetheseaupontheinnerandoutersides，withavesselverycomfortablyridingatanchor。ThatisoneoftheremarkableformsofreefinthePacific。Anotherisasortofhalf—wayhouse，betweentheatollandthefringingreef；itiswhatiscalledan\"encirclingreef。\"
InthiscaseyouseeanIslandrisingoutofthesea，andattwoorthreemilesdistance，ormore，andseparatedbyadeepchannel，whichmaybeeighttotwelvefathomsdeep，thereisareef，whichencirclesitlikeagreatgirdle；andoutsidethatagainthewaterisoneortwothousandfeetdeep。Ispentthreeorfouryearsofmylifeincruisingaboutamodificationofoneoftheseencirclingreefs，calleda\"barrierreef，\"upontheeastcoastofAustralia——oneofthemostwonderfulaccumulationsofcoralrockintheworld。Itisabout1，100
mileslong，andvariesinwidthfromoneortwotomanymiles。ItisseparatedfromthecoastofAustraliabyachannelofabout25fathomsdeep；whileoutside，lookingtowardAmerica，thewateristwoorthreethousandfeetdeepatamilefromtheedgeofthereef。Thisisanaccumulationoflimestonerock，builtupbycorals，towhichwehavenoparallelanywhereelse。Imaginetoyourselfaheapofthismaterialmorethanonethousandmileslong，andseveralmileswide。Thatisabarrierreef；butabarrierreefismerelyasitwereafragmentofanencirclingreefrunningparalleltothecoastofagreatcontinent。
Itoldyouthatthepolypeswhichbuiltthesereefswerenotabletoliveatagreaterdepththan20to25fathomsofwater；andthatisthereasonwhythefringingreefgoesnofartherfromthelandthanitdoes。Andforthesamereason，ifthePacificcouldbelaidbareweshouldhaveamostsingularspectacle。Therewouldbeanumberofmountainswithtruncatedtopsscatteredoverit，andthosemountainswouldhaveanappearancejusttheveryreverseofthatpresentedbythemountainsweseeonshore。Youknowthatthemountainsonshorearecoveredwithvegetationattheirbases，whiletheirtopsarebarrenorcoveredwithsnow；butthesemountainswouldbeperfectlybareattheirbases，andallroundtheirtopstheywouldbecoveredwithabeautifulvegetationofcoralpolypes。Andnotonlywouldthisbethecase，butweshouldfindthatforaconsiderabledistancedown，allthematerialoftheseatollandencirclingreefswasbuiltupofpreciselythesamecoralrockasthefringingreef。Thatistosay，youhaveanenormousmassofcoralrockatadepthbelowthesurfaceofthewaterwhereweknowperfectlywellthatthecoralanimalscouldnothavelivedtoformit。Whenthosetwofactswerefirstputtogether，naturalistswerequiteasmuchpuzzledasIdaresayyouare，atpresent，tounderstandhowthesetwoseemingcontradictionscouldbereconciled；andallsortsofoddhypotheseswereresortedto。Itwassupposedthatthecoraldidnotextendsofardown，butthattherewasagreatchainofsubmarinemountainsstretchingthroughthePacific，andthatthecoralhadgrownuponthem。Butonlyfancywhatsuppositionthatwas，foryouwouldhavetoimaginethattherewasachainofmountainsathousandmilesormorelong，andthatthetopofeverymountaincamewithin20fathomsofthesurfaceofthesea，andneitherroseabovenorsunkbeneaththatlevel。Thatishighlyimprobable：suchachainofmountainswasneverknown。Thenhowcanyoupossiblyaccountforthecuriouscircularformoftheatollsbyanysuppositionofthiskind？Ibelievetherewassomeonewhoimaginedthatallthesemountainswerevolcanoes，andthatthereefshadgrownroundthetopsofthecraters，soweallstuckfast。Imaysay\"we，\"thoughitwasratherbeforemytime。Andwhenweallstickfast，itisjusttheuseofamanofgeniusthathecomesandshowsusthemeaningofthething。Hegenerallygivesanexplanationwhichissoridiculouslysimplethateverybodyisashamedthathedidnotfinditoutbefore；andthewaysuchadiscovererisoftenrewardedisbyfindingoutthatsomeonehadmadethediscoverybeforehim！I
donotmeantosaythatitwassointhisparticularinstance，becausethegreatmanwhoplayedthepartofColumbusandtheeggonthisoccasionhad，Ibelieve，alwayshadthefullcreditwhichhesowelldeserves。Thediscovererofthekeytotheseproblemswasamanwhosenameyouknowverywellinconnectionwithothermatters，andIshouldnotwonderifsomeofyouhavehearditsaidthathewasasuperficialkindofpersonwhodidnotknowmuchaboutthesubjectonwhichhewrites。HewasMr。Darwin，andthisbrilliantdiscoveryofhiswasmadepublicthirtyyearsago，longbeforehebecamethecelebratedmanhenowis；anditwasoneofthemostsingularinstancesofthatastonishingsagacitywhichhepossessesofdrawingconsequencesbywayofdeductionfromsimpleprinciplesofnaturalscience——apowerwhichhasservedhimingoodsteadonotheroccasions。Well，Mr。Darwin，lookingatthesecuriousdifficultiesandhavingthatsortofknowledgeofnaturalphenomenaingeneral，withoutwhichhecouldnothavemadeasteptowardsthesolutionoftheproblem，saidtohimself——\"Itisperfectlyclearthatthecoralwhichformsthebaseoftheatollsandfringingreefscouldnotpossiblyhavebeenformedthereiftheleveloftheseahasalwaysbeenexactlywhereitisnow，forweknowforcertainthatthesepolypescannotbuildatagreaterdepththan20to25fathoms，andherewefindthemat50to100fathoms。\"
Thatwasthefirstpointtomakeclear。Thesecondpointtodealwithwas——ifthepolypescannothavebuilttherewhiletheleveloftheseahasremainedstationary，thenoneoftwothingsmusthavehappened——eithertheseahasgoneup，orthelandhasgonedown。
Thereisnoescapefromoneofthesetwoalternatives。Nowtheobjectionstothenotionoftheseahavinggoneupareveryconsiderableindeed；foryouwillreadilyperceivethattheseacouldnotpossiblyhaverisenathousandfeetinthePacificwithoutrisingprettymuchthesamedistanceeverywhereelse；andifithadrisenthatheighteverywhereelsesincethereefsbegantobeformed，thegeographyoftheworldingeneralmusthavebeenverydifferentindeed，atthattime，fromwhatitisnow。Andwehaveverygoodmeansofknowingthatanysuchriseasthiscertainlyhasnottakenplaceintheleveloftheseasincethetimethatthecoralshavebeenbuildingtheirhouses。Andsotheonlyotheralternativewastosupposethatthelandhadgonedown，andatsoslowaratethatthecoralswereabletogrowupwardasfastasitwentdownward。Youwillseeatoncethatthisisthesolutionofthemystery，andnothingcanbesimplerormoreobviouswhenyoucometothinkaboutit。SupposewestartwithacoralseaandputinthemiddleofitanislandsuchastheMauritius。Nowletthecoralpolypescomeandperchontheshoreandbuildafringingreef，whichwillstopwhentheycometo20or25fathoms，andyouwillhaveafringingreeflikethatroundtheislandintheillustration。Solongasthelandremainsstationary，solongasitdoesnotdescendsolongwillthatreefbeunabletogetanyfurtherout，becausethemomentthepolypeembryostrytogetbelowtheydie。Butnowsupposethatthelandsinksverygraduallyindeed。Letitsubsidebyslowdegrees，untilthemountainpeak，whichwehaveinthemiddleofit，aloneprojectsbeyondthesealevel。Thefringingreefwouldbecarrieddownalso；butwesupposethatthesinkingissoslowthatthecoralpolypesareabletogrowupasfastasthelandiscarrieddown；
consequentlytheywilladdlayeruponlayeruntiltheyformadeepcup，becausetheinnerpartofthereefgrowsmuchmoreslowlythantheouterpart。Thusyouhavethereefformingabedthickerupontheflanksoftheisland；buttheedgeofthereefwillbeverymuchfurtheroutfromtheland，andthelagoonwillbemanytimesdeeper；inshort，yourfringingreefwillbeconvertedintoanencirclingreef。Andif，insteadofthisbeinganisland，itwereagreatcontinentlikeAustralia，thenyouwillhavethephenomenonofabarrierreefwhichI
havedescribed。ThebarrierreefofAustraliawasoriginallyafringingreef；thelandhasgoneslowlydown；theconsequenceisthelagoonhasdeepeneduntilitsdepthisnow25fathomsandthecoralshavegrownupattheouteredgeuntilyouhavethatprodigiousaccumulationwhichformsthebarrierreefatpresent。Nowletthisprocessgoonfurtherstill；letustakethelandafurtherstepdown，soastosubmergeeventhepeak。Thecoral，stillgrowingup，willcoverthesurfaceoftheland，andyouwillhaveanatollreef；thatistosay，amoreorlesscircularorovalringofcoralrockwithalagooninthemiddle。Thusyouseethateverypeculiarityandphenomenonofthesedifferentformsofcoralreefwasexplainedatoncebythesimplestofallpossiblesuppositions，namely，bysupposingthatthelandhasgonedownataratenotgreaterthanthatatwhichthecoralpolypeshavegrownup。YouexplainaFringingReefasareefwhichisformedroundlandcomparativelystationary；anEncirclingReefasonewhichisformedroundlandgoingdown；andanAtollasareefformeduponlandgonedown；andthethingissosimplethatachildmayunderstanditwhenitisonceexplained。
Butthiswouldbynomeanssatisfytheconditionsofascientifichypothesis。Nomanwhoiscautiouswoulddreamoftrustingtoanexplanationofthiskindsimplybecauseitexplainedoneparticularsetoffacts。BeforeyoucanpossiblybesafeindealingwithNature——whoisveryproperlymadeofthefemininegender，onaccountoftheastonishingtrickswhichsheplaysuponheradmirers！——IsaybeforeyoucanbesafeindealingwithNature，youmustgettwoorthreekindsofcrossproofs，soastomakesurenotonlythatyourhypothesisfitsthatparticularsetoffacts，butthatitisnotcontradictedbysomeothersetoffactswhichisjustasclearandcertain。Anditsohappens，thatinthiscaseMr。Darwinsuppliedthecrossproofsaswellastheimmediateevidence。Youhaveallheardofvolcanoes，thosewonderfulventsinthesurfaceoftheearthoutofwhichpourmassesoflava，cindersandashes，andthelike。Now，itisamatterofobservationandexperiencethatallvolcanoesareplacedinareasinwhichthesurfaceoftheearthisundergoingelevation，oratanyrateisstationary；theyarenotplacedinpartsoftheworldinwhichthelevelofthelandisbeinglowered。Theyareallindicationsofagreatsubterraneanactivity，ofasomethingbeingpushedup，andthereforenaturallythelandeithergiveswayandletsitcomethrough，orelseisraisedupbyitsviolence。AndsoMr。Darwin，beingdesirousnottomerelyputoutaflashyhypothesis，buttogetatthetruthofthematter，saidtohimself，\"Ifmynotionofthismatterisright，thenatollsandencirclingreefs，inasmuchastheyaredependentuponsubsidence，oughtnottobefoundincompanywithvolcanoes；and，’viceversa’，volcanoesoughtnottobefoundincompanywithatolls，buttheyoughttobefoundincompanywithfringingreefs。\"AndifyouturntoMr。Darwin’sgreatworkuponthecoralreefs，youwillseeaverybeautifulchartoftheworld，whichhepreparedwithgreatpainsandlabour，showingthedistributionontheonehandofthereefs，andontheotherofthevolcanoes；youwillfindthatinnocasedoestheatollaccompanythevolcano，orthevolcanoburstupamongtheatolls。
ItismostinstructivetolookatthegreatareaofthePacificonthemap，andseethegreatmassesofatollsforminginoneregionofitamostenormousbelt，runningfromnorth—westtosouth—east；whilethevolcanoes，whichareverynumerousinthatregion，goroundthemargin，sothatwecanpicturethePacifictoourselvesasectionofakindofveryshallowbasin——shallowinproportiontoitswidth，withtheatollsrisingfromthebottomofit，andatthemarginsthevolcanoes。Itisexactlyasifyouhadtakenaflatmassandlifteduptheedgesofit；
thesubterraneanforcewhichlifteduptheedgesshowsitselfinvolcanoes，andastheedgeshavebeenraised，themiddlepartofthemasshasgonedown。Inotherwords，thefactsofphysicalgeographypreciselyandexactlycorrespondwiththehypothesiswhichaccountsfortheinfinitevarietiesofcoralreefs。
Oneotherpoint，beforeIconclude，aboutthismatter。Thesereefs，asyouhavejustperceived，areinamostsingularandunexpectedmannerindicationsofphysicalchangesofelevationsanddepressionsgoingonuponthesurfaceoftheglobe。Idaresayitmayhavesurprisedyoutohearmetalkinthisfamiliarsortofwayoflandgoingupanddown；butitisoneoftheuniversallessonsofgeologythatthelandisgoingdownandgoingup，andhasbeengoingupanddown，inallsortsofplacesandtoallsortsofdistances，throughallrecordedtime。
Geologistswouldbequiterightinmaintainingtheseemingparadoxthatthestablethingintheworldisthefluidseaandtheshiftingthingisthesolidland。Thatmaysoundaveryhardsayingatfirst，butthemoreyoulookintogeology，themoreyouwillseegroundforbelievingthatitisnotamereparadox。
Inanunexpectedmanner，again，thesereefsaffordusnotonlyanindicationofchangeofplace，buttheyaffordanindicationoflapseoftime。Thereefisatimekeeperofaverycuriouscharacter；andyoucaneasilyunderstandwhy。Thecoralpolype，likeeverythingelse，takesacertaintimetogrowtoitsfullsize；itdoesnotdoitinaminute；justasachildtakesacertaintimetogrowintoamansodoestheembryopolypetaketimetogrowintoaperfectpolypeandformitsskeleton。Consequentlyeveryparticleofcorallimestoneisanexpressionoftime。Itmusthavetakenacertaintimetoseparatethelimefromtheseawater。Itisnotpossibletoarriveatanaccuratecomputationofthetimeitmusthavetakentoformthesecoralislands，becausewelackthenecessarydata；butwecanformaroughcalculation，whichleadstoverycuriousandstrikingresults。Thecomputationsoftherateatwhichcoralsgrowaresoexceedinglyvariable，thatwemustallowthewidestpossiblemarginforerror；anditisbetterinthiscasetomaketheallowanceuponthesideofexcess。IthinkthatanybodywhoknowsanythingaboutthematterwilltellyouthatIammakingacomputationfarinexcessofwhatisprobable，ifIsaythataninchofcorallimestonemaybeaddedtooneofthesereefsinthecourseofayear。Ithinkmostnaturalistswouldbeinclinedtolaughatmeformakingsuchanassumption，andwouldputthegrowthatcertainlynotmorethanhalfthatamount。Butsupposingitisso，whataverycuriousnotionoftheantiquityofsomeofthesegreatlivingpyramidscomesoutbyaverysimplecalculation。Thereisnodoubtwhateverthattheseafacesofsomeofthemarefullyathousandfeethigh，andifyoutakethereckoningofaninchayear，thatwillgiveyou12，000yearsfortheageofthatparticularpyramidorconeofcorallimestone；12，000longyearshavethesecreaturesbeenlabouringinconditionswhichmusthavebeensubstantiallythesameastheyarenow，otherwisethepolypescouldnothavecontinuedtheirwork。ButI
believeIverymuchunderstateboththeheightofsomeofthesemasses，andoverstatetheamountwhichtheseanimalscanforminthecourseofayear；sothatyoumightverysafelydoubletheperiodasthetimeduringwhichthePacificOcean，thegeneralstateoftheclimate，andthesea，andthetemperaturehasbeensubstantiallywhatitisnow；andyetthatstateofthingswhichnowobtainsinthePacificOceanistheyesterdayofthehistoryofthelifeoftheglobe。Thosepyramidsofcoralrockarebuiltuponafoundationwhichisitselfformedbythedepositswhichthegeologisthastodealwith。Ifwegobackintimeandsearchthroughtheseriesoftherocks，wefindateveryageoftheworld’shistorywhichhasyetbeenexamined，accumulationsoflimestone，manyofwhichhavecertainlybeenbuiltupinjustthesamewayasthosecoralreefswhicharenowformingthebottomofthePacificOcean。Andevenifweturntotheoldestperiodsofgeologichistory，althoughthenatureofthematerialsischanged，althoughwecannotapplytothemthesamereasoningsthatwecantotheexistingcorals，yetstilltherearevastmassesoflimestoneformedofnothingelsethantheaccumulationsoftheskeletonsofsimilaranimals，andtestifyingthateveninthoseremoteperiodsoftheworld’shistory，asnow，theorderofthingsimpliesthattheearthhadalreadyenduredforaperiodofwhichourordinarystandardsofchronologygiveusnottheslightestconception。Inotherwords，thehistoryofthesecoralreefs，tracedouthonestlyandcarefully，andwiththesamesortofreasoningthatyouwoulduseintheordinaryaffairsoflife，testifies，likeeveryfactthatIknowof，totheprodigiousantiquityoftheearthsinceitexistedinaconditioninthemainsimilartothatinwhichitnowis。