coral and coral reefs-第4部分

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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；　the　general　state　of　the　climate；　and
the　sea；　and　the　temperature　has　been　substantially　what　it　is　now；　and
yet　that　state　of　things　which　now　obtains　in　the　Pacific　Ocean　is　the
yesterday　of　the　history　of　the　life　of　the　globe。　　Those　pyramids　of
coral　rock　are　built　upon　a　foundation　which　is　itself　formed　by　the
deposits　which　the　geologist　has　to　deal　with。　　If　we　go　back　in　time
and　search　through　the　series　of　the　rocks；　we　find　at　every　age　of　the
world's　history　which　has　yet　been　examined；　accumulations　of
limestone；　many　of　which　have　certainly　been　built　up　in　just　the　same
way　as　those　coral　reefs　which　are　now　forming　the　bottom　of　the
Pacific　Ocean。　　And　even　if　we　turn　to　the　oldest　periods　of　geologic
history；　although　the　nature　of　the　materials　is　changed；　although　we
cannot　apply　to　them　the　same　reasonings　that　we　can　to　the　existing
corals；　yet　still　there　are　vast　masses　of　limestone　formed　of　nothing
else　than　the　accumulations　of　the　skeletons　of　similar　animals；　and
testifying　that　even　in　those　remote　periods　of　the　world's　history；　as
now；　the　order　of　things　implies　that　the　earth　had　already　endured　for
a　period　of　which　our　ordinary　standards　of　chronology　give　us　not　the
slightest　conception。　　In　other　words；　the　history　of　these　coral　reefs；
traced　out　honestly　and　carefully；　and　with　the　same　sort　of　reasoning
that　you　would　use　in　the　ordinary　affairs　of　life；　testifies；　like
every　fact　that　I　know　of；　to　the　prodigious　antiquity　of　the　earth
since　it　existed　in　a　condition　in　the　main　similar　to　that　in　which　it
now　is。







End