autobiography and selected essays-第33章

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centre　to　its　circumference。　　When　the　polypes　formed　by　budding　or

division　remain　associated；　the　polypidom　is　sometimes　made　up　of

nothing　but　an　aggregation　of　these　cups；　while　at　other　times　the

cups　are　at　once　separated　and　held　together；　by　an　intermediate

substance；　which　represents　the　branches　of　the　red　coral。　　The　red

coral　polype　again　is　a　comparatively　rare　animal；　inhabiting　a

limited　area；　the　skeleton　of　which　has　but　a　very　insignificant

mass；　while　the　white　corals　are　very　common；　occur　in　almost　all

seas；　and　form　skeletons　which　are　sometimes　extremely　massive。



With　a　very　few　exceptions；　both　the　red　and　the　white　coral

polypes　are；　in　their　adult　state；　firmly　adherent　to　the　sea…

bottom；　nor　do　their　buds　naturally　become　detached　and　locomotive。

But；　in　addition　to　budding　and　division；　these　creatures　possess

the　more　ordinary　methods　of　multiplication；　and；　at　particular

seasons；　they　give　rise　to　numerous　eggs　of　minute　size。　　Within

these　eggs　the　young　are　formed；　and　they　leave　the　egg　in　a

condition　which　has　no　sort　of　resemblance　to　the　perfect　animal。

It　is；　in　fact；　a　minute　oval　body；　many　hundred　times　smaller　than

the　full　grown　creature；　and　it　swims　about　with　great　activity　by

the　help　of　multitudes　of　little　hair…like　filaments；　called　cilia；

with　which　its　body　is　covered。　　These　cilia　all　lash　the　water　in

one　direction；　and　so　drive　the　little　body　along　as　if　it　were

propelled　by　thousands　of　extremely　minute　paddles。　　After　enjoying

its　freedom　for　a　longer　or　shorter　time；　and　being　carried　either

by　the　force　of　its　own　cilia；　or　by　currents　which　bear　it　along；

the　embryo　coral　settles　down　to　the　bottom；　loses　its　cilia；　and

becomes　fixed　to　the　rock；　gradually　assuming　the　polype　form　and

growing　up　to　the　size　of　its　parent。　　As　the　infant　polypes　of　the

coral　may　retain　this　free　and　active　condition　for　many　hours；　or

even　days；　and　as　a　tidal　or　other　current　in　the　sea　may　easily

flow　at　the　speed　of　two　or　even　more　miles　in　an　hour；　it　is　clear

that　the　embryo　must　often　be　transported　to　very　considerable

distances　from　the　parent。　　And　it　is　easily　understood　how　a

single　polype；　which　may　give　rise　to　hundreds；　or　perhaps

thousands；　of　embryos；　may；　by　this　process　of　partly　active　and

partly　passive　migration；　cover　an　immense　surface　with　its

offspring。



The　masses　of　coral　which　may　be　formed　by　the　assemblages　of

polypes　which　spring　by　budding；　or　by　dividing；　from　a　single

polype；　occasionally　attain　very　considerable　dimensions。　　Such

skeletons　are　sometimes　great　plates；　many　feet　long　and　several

feet　in　thickness；　or　they　may　form　huge　half　globes；　like　the

brainstone　corals；　or　may　reach　the　magnitude　of　stout　shrubs　or

even　small　trees。　　There　is　reason　to　believe　that　such　masses　as

these　take　a　long　time　to　form；　and　hence　that　the　age　a　polype

tree；　or　polype　turf；　may　attain；　may　be　considerable。　　But；　sooner

or　later；　the　coral　polypes；　like　all　other　things；　die；　the　soft

flesh　decays；　while　the　skeleton　is　left　as　a　stony　mass　at　the

bottom　of　the　sea；　where　it　retains　its　integrity　for　a　longer　or　a

shorter　time；　according　as　its　position　affords　more　or　less

protection　from　the　wear　and　tear　of　the　waves。



The　polypes　which　give　rise　to　the　white　coral　are　found；　as　has

been　said；　in　the　seas　of　all　parts　of　the　world；　but　in　the

temperate　and　cold　oceans　they　are　scattered　and　comparatively

small　in　size；　so　that　the　skeletons　of　those　which　die　do　not

accumulate　in　any　considerable　quantity。　　But　it　is　otherwise　in

the　greater　part　of　the　ocean　which　lies　in　the　warmer　parts　of　the

world；　comprised　within　a　distance　of　about　eighteen　hundred　miles

on　each　side　of　the　equator。　　Within　the　zone　thus　bounded；　by　far

the　greater　part　of　the　ocean　is　inhabited　by　coral　polypes；　which

not　only　form　very　strong　and　large　skeletons；　but　associate

together　into　great　masses；　like　the　thickets　and　the　meadow　turf；

or；　better　still；　the　accumulations　of　peat；　to　which　plants　give

rise　on　dry　land。　　These　masses　of　stony　matter；　heaped　up　beneath

the　waters　of　the　ocean；　become　as　dangerous　to　mariners　as　so　much

ordinary　rock；　and　to　these；　as　to　the　common　rock　ridges；　the

seaman　gives　the　name　of　〃reefs。〃



Such　coral　reefs　cover　many　thousand　square　miles　in　the　Pacific

and　in　the　Indian　Oceans。　　There　is　one　reef；　or　rather　great

series　of　reefs；　called　the　Barrier　Reef；　which　stretches；　almost

continuously；　for　more　than　eleven　hundred　miles　off　the　east　coast

of　Australia。　　Multitudes　of　the　islands　in　the　Pacific　are　either

reefs　themselves；　or　are　surrounded　by　reefs。　　The　Red　Sea　is　in

many　parts　almost　a　maze　of　such　reefs；　and　they　abound　no　less　in

the　West　Indies；　along　the　coast　of　Florida；　and　even　as　far　north

as　the　Bahama　Islands。　　But　it　is　a　very　remarkable　circumstance

that；　within　the　area　of　what　we　may　call　the　〃coral　zone；〃　there

are　no　coral　reefs　upon　the　west　coast　of　America；　nor　upon　the

west　coast　of　Africa；　and　it　is　a　general　fact　that　the　reefs　are

interrupted；　or　absent；　opposite　the　mouths　of　great　rivers。　　The

causes　of　this　apparent　caprice　in　the　distribution　of　coral　reefs

are　not　far　to　seek。　　The　polypes　which　fabricate　them　require　for

their　vigorous　growth　a　temperature　which　must　not　fall　below　68

degrees　Fahrenheit　all　the　year　round；　and　this　temperature　is　only

to　be　found　within　the　distance　on　each　side　of　the　equator　which

has　been　mentioned；　or　thereabouts。　　But　even　within　the　coral　zone

this　degree　of　warmth　is　not　everywhere　to　be　had。　　On　the　west

coast　of　America；　and　on　the　corresponding　coast　of　Africa；　the

currents　of　cold　water　from　the　icy　regions　which　surround　the

South　Pole　set　northward；　and　it　appears　to　be　due　to　their　cooling

influence　that　the　sea　in　these　regions　is　free　from　the　reef

builders。　　Again；　the　coral　polypes　cannot　live　in　water　which　is

rendered　brackish　by　floods　from　the　land；　or　which　is　perturbed　by

mud　from　the　same　source；　and　hence　it　is　that　they　cease　to　exist

opposite　the　mouths　of　rivers；　which　damage　them　in　both　these

ways。



Such　is　the　general　distribution　of　the　reef…building　corals；　but

there　are　some　very　interesting　and　singular　circumstances　to　be

observed　in　the　conformation　of　the　reefs；　when　we　consider　them

individually。　　The　reefs；　in　fact；　are　of　three　different　kinds；

some　of　them　stretch　out　from　the　shore；　almost　like　a　prolongation

of　the　beach；　covered　only　by　shallow　water；　and　in　the　case　of　an

island；　surrounding　it　like　a　fringe　of　no　considerable　breadth。

These　are　termed　〃fringing　reefs。〃　　Others　are　separated　by　a

channel　which　may　attain　a　width　of　many　miles；　and　a　depth　of

twenty　or　thirty　fathoms　or　more；　from　the　nearest　land；　and　when

this　land　is　an　island；　the　reef　surrounds　it　like　a　low　wall；　and

the　sea　between　the　reef　and　the　land　is；　as　it　were；　a　moat　inside

this　wall。　　Such　reefs　as　these　are　called　〃encircling〃　when　they

surround　an　island；　and　〃barrier〃　reefs；　when　they　stretch　parallel

with　the　coast　of　a　continent。　　In　both　these　cases　there　is

ordinary　dry　land　inside　the　reef；　and　separated　from　it　only　by　a

narrower　or　a　wider；　a　shallower　or　a　deeper；　space　of　sea；　which

is　called　a　〃lagoon；〃　or　〃inner　passage。〃　But　there　is　a　third　kind

of　reef；　of　very　common　occurrence　in　the　Pacific　and　Indian

Oceans；　which　goes　by　the　name　of　〃atoll。〃　　This　is；　to　all　intents

and　purposes；　an　encircling　reef；　without　anything　to　encircle；　or；

in　other　words；　without　an　island　in　the　middle　of　its　lagoon。　　The

atoll　has　exactly　the　appearance　of　a　vast；　irregularly　oval；　or

circular；　breakwater；　enclosing　smooth　water　in　its　midst。　　The

depth　of　the　water　in　the　lagoon　rarely　exceeds　twenty　or　thirty

fathoms；　but；　outside　the　reef；　it　deepens　with　great　rapidity　to

two　hundred　or　three　hundred　fathoms。　　The　depth　immediately

outside　the　barrier；　or