coral and coral reefs-第3部分

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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　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