is　a　special　skeleton　for　every　one　of　these　polypes　in　addition　to
  that　for　the　whole　body。　　There　is　a　skeleton　formed　in　the　body　of
  each　of　them；　like　a　cup　divided　by　a　number　of　radiating　partitions
  towards　the　outside；　and　that　cup　is　formed　of　carbonate　of　lime；　only
  not　stained　red；　as　in　the　case　of　the　red　coral。　　And　all　these　cups
  are　joined　together　into　a　common　branch；　the　result　of　which　is　the
  formation　of　a　beautiful　coral　tree。　　This　is　a　great　mass　of
  madrepore；　and　in　the　living　state　every　one　of　the　ends　of　these
  branches　was　terminated　by　a　beautiful　little　polype；　like　a　sea
  anemone；　and　all　the　skeleton　was　covered　by　a　soft　body　which　united
  the　polypes　together。　　You　must　understand　that　all　this　skeleton　has
  been　formed　in　the　interior　of　the　body；　to　suit　the　branched　body　of
  the　polype　mass；　and　that　it　is　as　much　its　skeleton　as　our　own　bones
  are　our　skeleton。　　In　this　next　coral　the　creature　which　has　formed　the
  skeleton　has　divided　itself　as　it　grew；　and　consequently　has　formed　a
  great　expansion；　but　scattered　all　over　this　surface　there　were　polype
  bodies　like　those　I　previously　described。　　Again；　when　this　great　cup
  was　alive；　the　whole　surface　was　covered　with　a　beautiful　body　upon
  which　were　set　innumerable　small　polype　flowers；　if　we　may　so　call
  them；　often　brilliantly　coloured；　and　the　whole　cup　was　built　up　in　the
  same　fashion　by　the　deposit　of　carbonate　of　lime　in　the　interior　of　the
  combined　polype　body；　formed　by　budding　and　by　fission　in　the　way　I
  described。　　You　will　perceive　that　there　is　no　necessary　limit　to　this
  process。　　There　is　no　reason　why　we　should　not　have　coral　three　or　four
  times　as　big；　and　there　are　certain　creatures　of　this　kind　that　do
  fabricate　very　large　masses；　or　half　spheres　several　feet　in　diameter。
  Thus　the　activity　of　these　animals　in　separating　carbonate　of　lime　from
  the　sea　and　building　it　up　into　definite　shapes　is　very　considerable
  indeed。
  Now　I　think　I　have　said　sufficientas　much　as　I　can　without　taking　you
  into　technical　details；　of　the　general　nature　of　these　creatures　which
  form　coral。　　The　animals　which　form　coral　are　scattered　over　the　seas
  of　all　countries　in　the　world。　　The　red　coral　is　comparatively　limited；
  but　the　polypes　which　form　the　white　coral　are　widely　scattered。　　There
  are　some　of　them　which　remain　single；　or　which　give　rise　to　only　small
  accumulations；　and　the　skeletons　of　these；　as　they　die；　accumulate　upon
  the　bottom　of　the　sea；　but　they　do　not　come　to　much；　they　are　washed
  about　and　do　not　adhere　together；　but　become　mixed　up　with　the　mud　of
  the　sea。　But　there　are　certain　parts　of　the　world　in　which　the　coral
  polypes　which　live　and　grow　are　of　a　kind　which　remain；　adhere
  together；　and　form　great　masses。　　They　differ　from　the　ordinary　polypes
  just　in　the　same　way　as　those　plants　which　form　a　peat…bog　or
  meadow…turf　differ　from　ordinary　plants。　　They　have　a　habit　of　growing
  together　in　masses　in　the　same　place；　　they　are　what　we　call
  〃gregarious〃　things；　and　the　consequence　of　this　is；　that　as　they　die
  and　leave　their　skeletons；　those　skeletons　form　a　considerable　solid
  aggregation　at　the　bottom　of　the　sea；　and　other　polypes　perch　upon
  them；　and　begin　building　upon　them；　and　so　by　degrees　a　great　mass　is
  formed。　　And　just　as　we　know　there　are　some　ancient　cities　in　which　you
  have　a　British　city；　and　over　that　the　foundations　of　a　Roman　city；　and
  over　that　a　Saxon　city；　and　over　that　again　a　modern　city；　so　in　these
  localities　of　which　I　am　speaking；　you　have　the　accumulations　of　the
  foundations　of　the　houses；　if　I　may　use　the　term；　of　nation　after　nation
  of　these　coral　polypes；　and　these　accumulations　may　cover　a　very
  considerable　space；　and　may　rise　in　the　course　of　time　from　the　bottom
  to　the　surface　of　the　sea。
  Mariners　have　a　name　which　they　apply　to　all　sorts　of　obstacles
  consisting　of　hard　and　rocky　matter　which　comes　in　their　way　in　the
  course　of　their　navigation；　they　call　such　obstacles　〃reefs；〃　and　they
  have　long　been　in　the　habit　of　calling　the　particular　kind　of　reef；
  which　is　formed　by　the　accumulation　of　the　skeletons　of　dead　corals；　by
  the　name　of　〃coral　reefs；〃　therefore；　those　parts　of　the　world　in　which
  these　accumulations　occur　have　been　termed　by　them　〃coral　reef　areas；〃
  or　regions　in　which　coral　reefs　are　found。　　There　is　a　very　notable
  example　of　a　simple　coral　reef　about　the　island　of　Mauritius；　which　I
  dare　say　you　all　know；　lies　in　the　middle　of　the　Indian　Ocean。　　It　is　a
  very　considerable　and　beautiful　island；　and　is　surrounded　on　all　sides
  by　a　mass　of　coral；　which　has　been　formed　in　the　way　I　have　described；
  so　that　if　you　could　get　upon　the　top　of　one　of　the　peaks　of　the　island；
  and　look　down　upon　the　Indian　Ocean；　you　would　see　that　the　beach　round
  the　Island　was　continued　outward　by　a　kind　of　shallow　terrace；　which　is
  covered　by　the　sea；　and　where　the　sea　is　quite　shallow；　and　at　a
  distance　varying　from　three…quarters　of　a　mile　to　a　mile　and　a　half　from
  the　proper　beach；　you　would　see　a　line　of　foam　or　surf　which　looks　most
  beautiful　in　contrast　with　the　bright　green　water　in　the　inside；　and
  the　deep　blue　of　the　sea　beyond。　　That　line　of　surf　indicates　the　point
  at　which　the　waters　of　the　ocean　are　breaking　upon　the　coral　reef　which
  surrounds　the　island。　　You　see　it　sweep　round　the　island　upon　all
  sides；　except　where　a　river　may　chance　to　come　down；　and　that　always
  makes　a　gap　in　the　shore。
  There　are　two　or　three　points　which　I　wish　to　bring　clearly　before　your
  notice　about　such　a　reef　as　this。　　In　the　first　place；　you　perceive　it
  forms　a　kind　of　fringe　round　the　island；　and　is　therefore　called　a
  〃fringing　reef。〃　　In　the　next　place；　if　you　go　out　in　a　boat；　and　take
  soundings　at　the　edge　of　the　reef；　you　find　that　the　depth　of　the　water
  is　not　more　than　from　20　to　25　fathomsthat　is　about　120　to　150　feet。
  Outside　that　point　you　come　to　the　natural　sea　bottom；　but　all　inside
  that　depth　is　coral；　built　up　from　the　bottom　by　the　accumulation　of　the
  skeletons　of　innumerable　generations　of　coral　polypes。　　So　that　you　see
  the　coral　forms　a　very　considerable　rampart　round　the　island。　　What　the
  exact　circumference　may　be　I　do　not　remember；　but　it　cannot　be　less
  than　100　miles；　and　the　outward　height　of　this　wall　of　coral　rock
  nowhere　amounts　to　less　than　about　100　or　150　feet。
  When　the　outward　face　of　the　reef　is　examined；　you　find　that　the　upper
  edge；　which　is　exposed　to　the　wash　of　the　sea；　and　all　the　seaward
  face；　is　covered　with　those　living　plant…like　flowers　which　I　have
  described　to　you。　　They　are　the　coral　polypes　which　grow；　flourish；　and
  add　to　the　mass　of　calcareous　matter　which　already　forms　the　reef。　　But
  towards　the　lower　part　of　the　reef；　at　a　depth　of　about　120　feet；　these
  creatures　are　less　active；　and　fewer　of　them　at　work；　and　at　greater
  depths　than　that　you　find　no　living　coral　polype　at　all；　and　it　may　be
  laid　down　as　a　rule；　derived　from　very　extensive　observation；　that
  these　reef…building　corals　cannot　live　in　a　greater　depth　of　water　than
  about　120　to　150　feet。　　I　beg　you　to　recollect　that　fact；　because　it　is
  one　I　shall　have　to　come　back　to　by　and　by；　and　to　show　to　what　very
  curious　consequences　that　rule　leads。　　Well　then；　coming　back　to　the
  margin　of　the　reef；　you　find　that　part　of　it　which　lies　just　within　the
  surf　to　be　coated　by　a　very　curious　plant；　a　sort　of　seaweed；　which
  contains　in　its　substance　a　very　great　deal　of　carbonate　of　lime；　and
  looks　almost　like　rock；　this　is　what　is　called　the　nulli　pore。　　More
  towards　the　land；　we　come　to　the　shallow　water　upon　the　inside　of　the
  reef；　which　has　a　particular　name；　derived　from　the　Spanish　or　the
  Portugueseit　is　called　a　〃lagoon；〃　or　lake。　　In　this　lagoon　there　is
  comparatively　little　living　coral；　the　bottom　of　it　is　formed　of　coral
  mud。　　If　we　pounded　this　coral　in　water；　it　would　be　converted　into
  calcareous　mud；　and　the　waves　during　storms　do　for　the　coral　skeletons
  exactly　what　we　might　do　for　this　coral　in　a　mortar；　the　waves　tear　off
  great　fragments　and　crush　them　with　prodigious　force；　until　they　are
  ground　into　the　merest　powder；　and　that　powder　is　washed　into　the
  interior　of　the　lagoon；　and　forms　a　muddy　coating　at　the　bottom。　　Beside
  that　there　are　a　great　many　animals　that　prey　upon　the　coralfishes；
  worms；　and　creatures　of　that　kind；　and　all　these；　by　their　digestive
  processes；　reduce　the　coral　to　the　same　state；　and　contribute　a　very
  important　element　to　this　fine　mud。　　The　living　coral　found　in　the
  lagoon；　is　not　the　reef　building　coral；　it　does　not　give　rise　to　the
  same　massive　skeletons。　　As　you　go　in　a　boat　over　these　shallow　pools；
  you　see　these　beautiful　things；　coloured　red；　blue；　green；　and　all
  colours；　building　their　houses；　but　these　are　mere　tenements；　and　not　to
  be　compared　in　magnitude　and　importance　to　the　masses　which　are　built
  by　the　reef…builders　themselves。　　Now　such　a　structure　as　this　is　what
  is　termed　a　〃fringing　reef。〃　　You　meet　with　fringing　reefs　of　this　kind
  not　only　in　the　Mauritius；　but　in　a　number　of　other　parts　of　the　world。
  If　these　were　the　only　reefs　to　be　seen　anywhere；　the　problem　of　the
  formation　of　coral　reefs　would　never　have　been　a　difficult　one。　Nothin