the　land　had　gone　down；　and　at　so　slow　a　rate　that　the　corals　were　able
  to　grow　upward　as　fast　as　it　went　downward。　　You　will　see　at　once　that
  this　is　the　solution　of　the　mystery；　and　nothing　can　be　simpler　or　more
  obvious　when　you　come　to　think　about　it。　　Suppose　we　start　with　a　coral
  sea　and　put　in　the　middle　of　it　an　island　such　as　the　Mauritius。　　Now
  let　the　coral　polypes　come　and　perch　on　the　shore　and　build　a　fringing
  reef；　which　will　stop　when　they　come　to　20　or　25　fathoms；　and　you　will
  have　a　fringing　reef　like　that　round　the　island　in　the　illustration。　So
  long　as　the　land　remains　stationary；　so　long　as　it　does　not　descend　so
  long　will　that　reef　be　unable　to　get　any　further　out；　because　the
  moment　the　polype　embryos　try　to　get　below　they　die。　　But　now　suppose
  that　the　land　sinks　very　gradually　indeed。　　Let　it　subside　by　slow
  degrees；　until　the　mountain　peak；　which　we　have　in　the　middle　of　it；
  alone　projects　beyond　the　sea　level。　　The　fringing　reef　would　be
  carried　down　also；　but　we　suppose　that　the　sinking　is　so　slow　that　the
  coral　polypes　are　able　to　grow　up　as　fast　as　the　land　is　carried　down；
  consequently　they　will　add　layer　upon　layer　until　they　form　a　deep　cup；
  because　the　inner　part　of　the　reef　grows　much　more　slowly　than　the
  outer　part。　　Thus　you　have　the　reef　forming　a　bed　thicker　upon　the
  flanks　of　the　island；　but　the　edge　of　the　reef　will　be　very　much　further
  out　from　the　land；　and　the　lagoon　will　be　many　times　deeper；　in　short；
  your　fringing　reef　will　be　converted　into　an　encircling　reef。　　And　if；
  instead　of　this　being　an　island；　it　were　a　great　continent　like
  Australia；　then　you　will　have　the　phenomenon　of　a　barrier　reef　which　I
  have　described。　　The　barrier　reef　of　Australia　was　originally　a
  fringing　reef；　the　land　has　gone　slowly　down；　the　consequence　is　the
  lagoon　has　deepened　until　its　depth　is　now　25　fathoms　and　the　corals
  have　grown　up　at　the　outer　edge　until　you　have　that　prodigious
  accumulation　which　forms　the　barrier　reef　at　present。　　Now　let　this
  process　go　on　further　still；　let　us　take　the　land　a　further　step　down；
  so　as　to　submerge　even　the　peak。　　The　coral；　still　growing　up；　will
  cover　the　surface　of　the　land；　and　you　will　have　an　atoll　reef；　that　is
  to　say；　a　more　or　less　circular　or　oval　ring　of　coral　rock　with　a
  lagoon　in　the　middle。　　Thus　you　see　that　every　peculiarity　and
  phenomenon　of　these　different　forms　of　coral　reef　was　explained　at　once
  by　the　simplest　of　all　possible　suppositions；　namely；　by　supposing　that
  the　land　has　gone　down　at　a　rate　not　greater　than　that　at　which　the
  coral　polypes　have　grown　up。　　You　explain　a　Fringing　Reef　as　a　reef
  which　is　formed　round　land　comparatively　stationary；　an　Encircling　Reef
  as　one　which　is　formed　round　land　going　down；　and　an　Atoll　as　a　reef
  formed　upon　land　gone　down；　and　the　thing　is　so　simple　that　a　child　may
  understand　it　when　it　is　once　explained。
  But　this　would　by　no　means　satisfy　the　conditions　of　a　scientific
  hypothesis。　　No　man　who　is　cautious　would　dream　of　trusting　to　an
  explanation　of　this　kind　simply　because　it　explained　one　particular　set
  of　facts。　　Before　you　can　possibly　be　safe　in　dealing　with　Naturewho
  is　very　properly　made　of　the　feminine　gender；　on　account　of　the
  astonishing　tricks　which　she　plays　upon　her　admirers！I　say　before　you
  can　be　safe　in　dealing　with　Nature；　you　must　get　two　or　three　kinds　of
  cross　proofs；　so　as　to　make　sure　not　only　that　your　hypothesis　fits
  that　particular　set　of　facts；　but　that　it　is　not　contradicted　by　some
  other　set　of　facts　which　is　just　as　clear　and　certain。　　And　it　so
  happens；　that　in　this　case　Mr。　Darwin　supplied　the　cross　proofs　as　well
  as　the　immediate　evidence。　　You　have　all　heard　of　volcanoes；　those
  wonderful　vents　in　the　surface　of　the　earth　out　of　which　pour　masses　of
  lava；　cinders　and　ashes；　and　the　like。　　Now；　it　is　a　matter　of
  observation　and　experience　that　all　volcanoes　are　placed　in　areas　in
  which　the　surface　of　the　earth　is　undergoing　elevation；　or　at　any　rate
  is　stationary；　they　are　not　placed　in　parts　of　the　world　in　which　the
  level　of　the　land　is　being　lowered。　　They　are　all　indications　of　a
  great　subterranean　activity；　of　a　something　being　pushed　up；　and
  therefore　naturally　the　land　either　gives　way　and　lets　it　come　through；
  or　else　is　raised　up　by　its　violence。　　And　so　Mr。　　Darwin；　being
  desirous　not　to　merely　put　out　a　flashy　hypothesis；　but　to　get　at　the
  truth　of　the　matter；　said　to　himself；　〃If　my　notion　of　this　matter　is
  right；　then　atolls　and　encircling　reefs；　inasmuch　as　they　are　dependent
  upon　subsidence；　ought　not　to　be　found　in　company　with　volcanoes；　and；
  'vice　versa'；　volcanoes　ought　not　to　be　found　in　company　with　atolls；
  but　they　ought　to　be　found　in　company　with　fringing　reefs。〃　　And　if　you
  turn　to　Mr。　Darwin's　great　work　upon　the　coral　reefs；　you　will　see　a
  very　beautiful　chart　of　the　world；　which　he　prepared　with　great　pains
  and　labour；　showing　the　distribution　on　the　one　hand　of　the　reefs；　and
  on　the　other　of　the　volcanoes；　you　will　find　that　in　no　case　does　the
  atoll　accompany　the　volcano；　or　the　volcano　burst　up　among　the　atolls。
  It　is　most　instructive　to　look　at　the　great　area　of　the　Pacific　on　the
  map；　and　see　the　great　masses　of　atolls　forming　in　one　region　of　it　a
  most　enormous　belt；　running　from　north…west　to　south…east；　while　the
  volcanoes；　which　are　very　numerous　in　that　region；　go　round　the　margin；
  so　that　we　can　picture　the　Pacific　to　ourselves　a　section　of　a　kind　of
  very　shallow　basinshallow　in　proportion　to　its　width；　with　the　atolls
  rising　from　the　bottom　of　it；　and　at　the　margins　the　volcanoes。　　It　is
  exactly　as　if　you　had　taken　a　flat　mass　and　lifted　up　the　edges　of　it；
  the　subterranean　force　which　lifted　up　the　edges　shows　itself　in
  volcanoes；　and　as　the　edges　have　been　raised；　the　middle　part　of　the
  mass　has　gone　down。　　In　other　words；　the　facts　of　physical　geography
  precisely　and　exactly　correspond　with　the　hypothesis　which　accounts　for
  the　infinite　varieties　of　coral　reefs。
  One　other　point；　before　I　conclude；　about　this　matter。　　These　reefs；　as
  you　have　just　perceived；　are　in　a　most　singular　and　unexpected　manner
  indications　of　physical　changes　of　elevations　and　depressions　going　on
  upon　the　surface　of　the　globe。　　I　dare　say　it　may　have　surprised　you　to
  hear　me　talk　in　this　familiar　sort　of　way　of　land　going　up　and　down；　but
  it　is　one　of　the　universal　lessons　of　geology　that　the　land　is　going
  down　and　going　up；　and　has　been　going　up　and　down；　in　all　sorts　of
  places　and　to　all　sorts　of　distances；　through　all　recorded　time。
  Geologists　would　be　quite　right　in　maintaining　the　seeming　paradox　that
  the　stable　thing　in　the　world　is　the　fluid　sea　and　the　shifting　thing
  is　the　solid　land。　　That　may　sound　a　very　hard　saying　at　first；　but　the
  more　you　look　into　geology；　the　more　you　will　see　ground　for　believing
  that　it　is　not　a　mere　paradox。
  In　an　unexpected　manner；　again；　these　reefs　afford　us　not　only　an
  indication　of　change　of　place；　but　they　afford　an　indication　of　lapse
  of　time。　　The　reef　is　a　timekeeper　of　a　very　curious　character；　and　you
  can　easily　understand　why。　　The　coral　polype；　like　everything　else；
  takes　a　certain　time　to　grow　to　its　full　size；　it　does　not　do　it　in　a
  minute；　just　as　a　child　takes　a　certain　time　to　grow　into　a　man　so　does
  the　embryo　polype　take　time　to　grow　into　a　perfect　polype　and　form　its
  skeleton。　　Consequently　every　particle　of　coral　limestone　is　an
  expression　of　time。　It　must　have　taken　a　certain　time　to　separate　the
  lime　from　the　sea　water。　　It　is　not　possible　to　arrive　at　an　accurate
  computation　of　the　time　it　must　have　taken　to　form　these　coral　islands；
  because　we　lack　the　necessary　data；　but　we　can　form　a　rough　calculation；
  which　leads　to　very　curious　and　striking　results。　　The　computations　of
  the　rate　at　which　corals　grow　are　so　exceedingly　variable；　that　we　must
  allow　the　widest　possible　margin　for　error；　and　it　is　better　in　this
  case　to　make　the　allowance　upon　the　side　of　excess。　　I　think　that
  anybody　who　knows　anything　about　the　matter　will　tell　you　that　I　am
  making　a　computation　far　in　excess　of　what　is　probable；　if　I　say　that
  an　inch　of　coral　limestone　may　be　added　to　one　of　these　reefs　in　the
  course　of　a　year。　　I　think　most　naturalists　would　be　inclined　to　laugh
  at　me　for　making　such　an　assumption；　and　would　put　the　growth　at
  certainly　not　more　than　half　that　amount。　　But　supposing　it　is　so；　what
  a　very　curious　notion　of　the　antiquity　of　some　of　these　great　living
  pyramids　comes　out　by　a　very　simple　calculation。　　There　is　no　doubt
  whatever　that　the　sea　faces　of　some　of　them　are　fully　a　thousand　feet
  high；　and　if　you　take　the　reckoning　of　an　inch　a　year；　that　will　give
  you　12；000　years　for　the　age　of　that　particular　pyramid　or　cone　of
  coral　limestone；　12；000　long　years　have　these　creatures　been　labouring
  in　conditions　which　must　have　been　substantially　the　same　as　they　are
  now；　otherwise　the　polypes　could　not　have　continued　their　work。　　But　I
  believe　I　very　much　understate　both　the　height　of　some　of　these　masses；
  and　overstate　the　amount　which　these　animals　can　form　in　the　course　of　a
  year；　so　that　you　might　very　safely　double　the　period　as　the　time
  during　which　the　Pacific　Ocean