formation　of　coral　reefs　would　never　have　been　a　difficult　one。　Nothing
  can　be　easier　than　to　understand　how　there　must　have　been　a　time　when
  the　coral　polypes　came　and　settled　on　the　shores　of　this　island；
  everywhere　within　the　20　to　25　fathom　line；　and　how；　having　perched
  there；　they　gradually　grew　until　they　built　up　the　reef。
  But　these　are　by　no　means　the　only　sort　of　coral　reefs　in　the　world；　on
  the　contrary；　there　are　very　large　areas；　not　only　of　the　Indian　ocean；
  but　of　the　Pacific；　in　which　many　many　thousands　of　square　miles　are
  covered　either　with　a　peculiar　kind　of　reef；　which　is　called　the
  〃encircling　reef；〃　or　by　a　still　more　curious　reef　which　goes　by　the
  name　of　the　〃atoll。〃　　There　is　a　very　good　picture；　which　Professor
  Roscoe　has　been　kind　enough　to　prepare　for　me；　of　one　of　these　atolls；
  which　will　enable　you　to　form　a　notion　of　it　as　a　landscape。　　You　have
  in　the　foreground　the　waters　of　the　Pacific。　　You　must　fancy　yourself
  in　the　middle　of　the　great　ocean；　and　you　will　perceive　that　there　is
  an　almost　circular　island；　with　a　low　beach；　which　is　formed　entirely
  of　coral　sand；　growing　upon　that　beach　you　have　vegetation；　which　takes；
  of　course；　the　shape　of　the　circular　land；　and　then；　in　the　interior　of
  the　circle；　there　is　a　pool　of　water；　which　is　not　very　deepprobably
  in　this　case　not　more　than　eight　or　nine　fathomsand　which　forms　a
  strange　and　beautiful　contrast　to　the　deep　blue　water　outside。　　This
  circular　island；　or　atoll；　with　a　lagoon　in　the　middle；　is　not　a
  complete　circle；　upon　one　side　of　it　there　is　a　break；　exactly　like　the
  entrance　into　a　dock；　and；　as　a　matter　of　course；　these　circular
  islets；　or　atolls；　form　most　efficient　break…waters；　for　if　you　can　only
  get　inside　your　ship　is　in　perfect　safety；　with　admirable　anchorage　in
  the　interior。　　If　the　ship　were　lying　within　a　mile　of　that　beach；　the
  water　would　be　one　or　two　thousand　feet　deep；　therefore；　a　section　of
  that　atoll；　with　the　soundings　as　deep　as　this　all　round；　would　give
  you　the　notion　of　a　great　cone；　cut　off　at　the　top；　and　with　a　shallow
  cup　in　the　middle　of　it。　　Now；　what　a　very　singular　fact　this　is；　that
  we　should　have　rising　from　the　bottom　of　the　deep　ocean　a　great　pyramid；
  beside　which　all　human　pyramids　sink　into　the　most　utter
  insignificance！　　These　singular　coral　limestone　structures　are　very
  beautiful；　especially　when　crowned　with　cocoa…nut　trees。　　There　you　see
  the　long　line　of　land；　covered　with　vegetationcocoa…nut　treesand　you
  have　the　sea　upon　the　inner　and　outer　sides；　with　a　vessel　very
  comfortably　riding　at　anchor。　　That　is　one　of　the　remarkable　forms　of
  reef　in　the　Pacific。　　Another　is　a　sort　of　half…way　house；　between　the
  atoll　and　the　fringing　reef；　it　is　what　is　called　an　〃encircling　reef。〃
  In　this　case　you　see　an　Island　rising　out　of　the　sea；　and　at　two　or
  three　miles　distance；　or　more；　and　separated　by　a　deep　channel；　which
  may　be　eight　to　twelve　fathoms　deep；　there　is　a　reef；　which　encircles
  it　like　a　great　girdle；　and　outside　that　again　the　water　is　one　or　two
  thousand　feet　deep。　　I　spent　three　or　four　years　of　my　life　in　cruising
  about　a　modification　of　one　of　these　encircling　reefs；　called　a
  〃barrier　reef；〃　upon　the　east　coast　of　Australiaone　of　the　most
  wonderful　accumulations　of　coral　rock　in　the　world。　　It　is　about　1；100
  miles　long；　and　varies　in　width　from　one　or　two　to　many　miles。　　It　is
  separated　from　the　coast　of　Australia　by　a　channel　of　about　25　fathoms
  deep；　while　outside；　looking　toward　America；　the　water　is　two　or　three
  thousand　feet　deep　at　a　mile　from　the　edge　of　the　reef。　　This　is　an
  accumulation　of　limestone　rock；　built　up　by　corals；　to　which　we　have　no
  parallel　anywhere　else。　　Imagine　to　yourself　a　heap　of　this　material
  more　than　one　thousand　miles　long；　and　several　miles　wide。　　That　is　a
  barrier　reef；　but　a　barrier　reef　is　merely　as　it　were　a　fragment　of　an
  encircling　reef　running　parallel　to　the　coast　of　a　great　continent。
  I　told　you　that　the　polypes　which　built　these　reefs　were　not　able　to
  live　at　a　greater　depth　than　20　to　25　fathoms　of　water；　and　that　is　the
  reason　why　the　fringing　reef　goes　no　farther　from　the　land　than　it
  does。　　And　for　the　same　reason；　if　the　Pacific　could　be　laid　bare　we
  should　have　a　most　singular　spectacle。　　There　would　be　a　number　of
  mountains　with　truncated　tops　scattered　over　it；　and　those　mountains
  would　have　an　appearance　just　the　very　reverse　of　that　presented　by　the
  mountains　we　see　on　shore。　　You　know　that　the　mountains　on　shore　are
  covered　with　vegetation　at　their　bases；　while　their　tops　are　barren　or
  covered　with　snow；　but　these　mountains　would　be　perfectly　bare　at　their
  bases；　and　all　round　their　tops　they　would　be　covered　with　a　beautiful
  vegetation　of　coral　polypes。　　And　not　only　would　this　be　the　case；　but
  we　should　find　that　for　a　considerable　distance　down；　all　the　material
  of　these　atoll　and　encircling　reefs　was　built　up　of　precisely　the　same
  coral　rock　as　the　fringing　reef。　　That　is　to　say；　you　have　an　enormous
  mass　of　coral　rock　at　a　depth　below　the　surface　of　the　water　where　we
  know　perfectly　well　that　the　coral　animals　could　not　have　lived　to　form
  it。　　When　those　two　facts　were　first　put　together；　naturalists　were
  quite　as　much　puzzled　as　I　daresay　you　are；　at　present；　to　understand
  how　these　two　seeming　contradictions　could　be　reconciled；　and　all　sorts
  of　odd　hypotheses　were　resorted　to。　　It　was　supposed　that　the　coral　did
  not　extend　so　far　down；　but　that　there　was　a　great　chain　of　submarine
  mountains　stretching　through　the　Pacific；　and　that　the　coral　had　grown
  upon　them。　　But　only　fancy　what　supposition　that　was；　for　you　would
  have　to　imagine　that　there　was　a　chain　of　mountains　a　thousand　miles　or
  more　long；　and　that　the　top　of　every　mountain　came　within　20　fathoms　of
  the　surface　of　the　sea；　and　neither　rose　above　nor　sunk　beneath　that
  level。　　That　is　highly　improbable：　　such　a　chain　of　mountains　was　never
  known。　　Then　how　can　you　possibly　account　for　the　curious　circular　form
  of　the　atolls　by　any　supposition　of　this　kind？　　I　believe　there　was
  some　one　who　imagined　that　all　these　mountains　were　volcanoes；　and　that
  the　reefs　had　grown　round　the　tops　of　the　craters；　so　we　all　stuck
  fast。　　I　may　say　〃we；〃　though　it　was　rather　before　my　time。　　And　when　we
  all　stick　fast；　it　is　just　the　use　of　a　man　of　genius　that　he　comes　and
  shows　us　the　meaning　of　the　thing。　He　generally　gives　an　explanation
  which　is　so　ridiculously　simple　that　everybody　is　ashamed　that　he　did
  not　find　it　out　before；　and　the　way　such　a　discoverer　is　often　rewarded
  is　by　finding　out　that　some　one　had　made　the　discovery　before　him！　　I
  do　not　mean　to　say　that　it　was　so　in　this　particular　instance；　because
  the　great　man　who　played　the　part　of　Columbus　and　the　egg　on　this
  occasion　had；　I　believe；　always　had　the　full　credit　which　he　so　well
  deserves。　　The　discoverer　of　the　key　to　these　problems　was　a　man　whose
  name　you　know　very　well　in　connection　with　other　matters；　and　I　should
  not　wonder　if　some　of　you　have　heard　it　said　that　he　was　a　superficial
  kind　of　person　who　did　not　know　much　about　the　subject　on　which　he
  writes。　　He　was　Mr。　Darwin；　and　this　brilliant　discovery　of　his　was　made
  public　thirty　years　ago；　long　before　he　became　the　celebrated　man　he
  now　is；　and　it　was　one　of　the　most　singular　instances　of　that
  astonishing　sagacity　which　he　possesses　of　drawing　consequences　by　way
  of　deduction　from　simple　principles　of　natural　sciencea　power　which
  has　served　him　in　good　stead　on　other　occasions。　　Well；　Mr。　Darwin；
  looking　at　these　curious　difficulties　and　having　that　sort　of　knowledge
  of　natural　phenomena　in　general；　without　which　he　could　not　have　made　a
  step　towards　the　solution　of　the　problem；　said　to　himself〃It　is
  perfectly　clear　that　the　coral　which　forms　the　base　of　the　atolls　and
  fringing　reefs　could　not　possibly　have　been　formed　there　if　the　level
  of　the　sea　has　always　been　exactly　where　it　is　now；　for　we　know　for
  certain　that　these　polypes　cannot　build　at　a　greater　depth　than　20　to
  25　fathoms；　and　here　we　find　them　at　50　to　100　fathoms。〃
  That　was　the　first　point　to　make　clear。　　The　second　point　to　deal　with
  wasif　the　polypes　cannot　have　built　there　while　the　level　of　the　sea
  has　remained　stationary；　then　one　of　two　things　must　have
  happenedeither　the　sea　has　gone　up；　or　the　land　has　gone　down。
  There　is　no　escape　from　one　of　these　two　alternatives。　　Now　the
  objections　to　the　notion　of　the　sea　having　gone　up　are　very
  considerable　indeed；　for　you　will　readily　perceive　that　the　sea　could
  not　possibly　have　risen　a　thousand　feet　in　the　Pacific　without　rising
  pretty　much　the　same　distance　everywhere　else；　and　if　it　had　risen　that
  height　everywhere　else　since　the　reefs　began　to　be　formed；　the
  geography　of　the　world　in　general　must　have　been　very　different　indeed；
  at　that　time；　from　what　it　is　now。　　And　we　have　very　good　means　of
  knowing　that　any　such　rise　as　this　certainly　has　not　taken　place　in　the
  level　of　the　sea　since　the　time　that　the　corals　have　been　building
  their　houses。　　And　so　the　only　other　alternative　was　to　suppose　that
  the　land　had　gone　down；　and　at　so　slow　a　rate　that　the　corals　were　able
  to　grow　upward　as　fast　as　it　went　downwa