the conditions of existence-第3部分

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head　on　the　blackboard；　in　the　case　of　the　last　the　breadth　would　be
about　seven…tenths；　and　in　the　other　it　would　be　nine…tenths　of　the
total　length。　　So　that　you　see　there　is　abundant　evidence　of　variation
among　men　in　their　natural　condition。　　And　if　you　turn　to　other　animals
there　is　just　the　same　thing。　　The　fox；　for　example；　which　has　a　very
large　geographical　distribution　all　over　Europe；　and　parts　of　Asia；　and
on　the　American　Continent；　varies　greatly。　　There　are　mostly　large
foxes　in　the　North；　and　smaller　ones　in　the　South。　　In　Germany　alone；
the　foresters　reckon　some　eight　different　sorts。

Of　the　tiger；　no　one　supposes　that　there　is　more　than　one　species；　they
extend　from　the　hottest　parts　of　Bengal；　into　the　dry；　cold；　bitter
steppes　of　Siberia；　into　a　latitude　of　50　degrees；so　that　they　may
even　prey　upon　the　reindeer。　　These　tigers　have　exceedingly　different
characteristics；　but　still　they　all　keep　their　general　features；　so　that
there　is　no　doubt　as　to　their　being　tigers。　　The　Siberian　tiger　has　a
thick　fur；　a　small　mane；　and　a　longitudinal　stripe　down　the　back；　while
the　tigers　of　Java　and　Sumatra　differ　in　many　important　respects　from
the　tigers　of　Northern　Asia。　　So　lions　vary；　so　birds　vary；　and　so；　if
you　go　further　back　and　lower　down　in　creation；　you　find　that　fishes
vary。　　In　different　streams；　in　the　same　country　even；　you　will　find
the　trout　to　be　quite　different　to　each　other　and　easily　recognisable　by
those　who　fish　in　the　particular　streams。　　There　is　the　same
differences　in　leeches；　leech　collectors　can　easily　point　out　to　you
the　differences　and　the　peculiarities　which　you　yourself　would　probably
pass　by；　so　with　fresh…water　mussels；　so；　in　fact；　with　every　animal
you　can　mention。

In　plants　there　is　the　same　kind　of　variation。　　Take　such　a　case　even　as
the　common　bramble。　The　botanists　are　all　at　war　about　it；　some　of　them
wanting　to　make　out　that　there　are　many　species　of　it；　and　others
maintaining　that　they　are　but　many　varieties　of　one　species；　and　they
cannot　settle　to　this　day　which　is　a　species　and　which　is　a　variety！

So　that　there　can　be　no　doubt　whatsoever　that　any　plant　and　any　animal
may　vary　in　nature；　that　varieties　may　arise　in　the　way　I　have
described；as　spontaneous　varieties；and　that　those　varieties　may　be
perpetuated　in　the　same　way　that　I　have　shown　you　spontaneous　varieties
are　perpetuated；　I　say；　therefore；　that　there　can　be　no　doubt　as　to　the
origin　and　perpetuation　of　varieties　in　nature。

But　the　question　now　is：Does　selection　take　place　in　nature？　　is　there
anything　like　the　operation　of　man　in　exercising　selective　breeding；
taking　place　in　nature？　　You　will　observe　that；　at　present；　I　say
nothing　about　species；　I　wish　to　confine　myself　to　the　consideration　of
the　production　of　those　natural　races　which　everybody　admits　to　exist。
The　question　is；　whether　in　nature　there　are　causes　competent　to
produce　races；　just　in　the　same　way　as　man　is　able　to　produce　by
selection；　such　races　of　animals　as　we　have　already　noticed。

When　a　variety　has　arisen；　the　CONDITIONS　OF　EXISTENCE　are　such　as　to
exercise　an　influence　which　is　exactly　comparable　to　that　of　artificial
selection。　　By　Conditions　of　Existence　I　mean　two　things；there　are
conditions　which　are　furnished　by　the　physical；　the　inorganic　world；
and　there　are　conditions　of　existence　which　are　furnished　by　the
organic　world。　　There　is；　in　the　first　place；　CLIMATE；　under　that　head
I　include　only　temperature　and　the　varied　amount　of　moisture　of
particular　places。　　In　the　next　place　there　is　what　is　technically
called　STATION；　which　meansgiven　the　climate；　the　particular　kind　of
place　in　which　an　animal　or　a　plant　lives　or　grows；　for　example；　the
station　of　a　fish　is　in　the　water；　of　a　fresh…water　fish　in　fresh
water；　the　station　of　a　marine　fish　is　in　the　sea；　and　a　marine　animal
may　have　a　station　higher　or　deeper。　　So　again　with　land　animals：　the
differences　in　their　stations　are　those　of　different　soils　and
neighbourhoods；　some　being　best　adapted　to　a　calcareous；　and　others　to
an　arenaceous　soil。　　The　third　condition　of　existence　is　FOOD；　by　which
I　mean　food　in　the　broadest　sense；　the　supply　of　the　materials　necessary
to　the　existence　of　an　organic　being；　in　the　case　of　a　plant　the
inorganic　matters；　such　as　carbonic　acid；　water；　ammonia；　and　the
earthy　salts　or　salines；　in　the　case　of　the　animal　the　inorganic　and
organic　matters；　which　we　have　seen　they　require；　then　these　are　all；
at　least　the　two　first；　what　we　may　call　the　inorganic　or　physical
conditions　of　existence。　　Food　takes　a　mid…place；　and　then　come　the
organic　conditions；　by　which　I　mean　the　conditions　which　depend　upon　the
state　of　the　rest　of　the　organic　creation；　upon　the　number　and　kind　of
living　beings；　with　which　an　animal　is　surrounded。　　You　may　class　these
under　two　heads：　there　are　organic　beings；　which　operate　as
'opponents'；　and　there　are　organic　beings　which　operate　as　'helpers'　to
any　given　organic　creature。　　The　opponents　may　be　of　two　kinds：　there
are　the　'indirect　opponents'；　which　are　what　we　may　call　'rivals'；　and
there　are　the　'direct　opponents'；　those　which　strive　to　destroy　the
creature；　and　these　we　call　'enemies'。　By　rivals　I　mean；　of　course；　in
the　case　of　plants；　those　which　require　for　their　support　the　same　kind
of　soil　and　station；　and；　among　animals；　those　which　require　the　same
kind　of　station；　or　food；　or　climate；　those　are　the　indirect　opponents；
the　direct　opponents　are；　of　course；　those　which　prey　upon　an　animal　or
vegetable。　　The　'helpers'　may　also　be　regarded　as　direct　and　indirect：
in　the　case　of　a　carnivorous　animal；　for　example；　a　particular
herbaceous　plant　may　in　multiplying　be　an　indirect　helper；　by　enabling
the　herbivora　on　which　the　carnivore　preys　to　get　more　food；　and　thus
to　nourish　the　carnivore　more　abundantly；　the　direct　helper　may　be　best
illustrated　by　reference　to　some　parasitic　creature；　such　as　the
tape…worm。　　The　tape…worm　exists　in　the　human　intestines；　so　that　the
fewer　there　are　of　men　the　fewer　there　will　be　of　tape…worms；　other
things　being　alike。　　It　is　a　humiliating　reflection；　perhaps；　that　we
may　be　classed　as　direct　helpers　to　the　tape…worm；　but　the　fact　is　so：
we　can　all　see　that　if　there　were　no　men　there　would　be　no　tape…worms。

It　is　extremely　difficult　to　estimate；　in　a　proper　way；　the　importance
and　the　working　of　the　Conditions　of　Existence。　　I　do　not　think　there
were　any　of　us　who　had　the　remotest　notion　of　properly　estimating　them
until　the　publication　of　Mr。　Darwin's　work；　which　has　placed　them
before　us　with　remarkable　clearness；　and　I　must　endeavour；　as　far　as　I
can　in　my　own　fashion；　to　give　you　some　notion　of　how　they　work。　　We
shall　find　it　easiest　to　take　a　simple　case；　and　one　as　free　as
possible　from　every　kind　of　complication。

I　will　suppose；　therefore；　that　all　the　habitable　part　of　this
globethe　dry　land；　amounting　to　about　51；000；000　square　miles；I
will　suppose　that　the　whole　of　that　dry　land　has　the　same　climate；　and
that　it　is　composed　of　the　same　kind　of　rock　or　soil；　so　that　there　will
be　the　same　station　everywhere；　we　thus　get　rid　of　the　peculiar
influence　of　different　climates　and　stations。　I　will　then　imagine　that
there　shall　be　but　one　organic　being　in　the　world；　and　that　shall　be　a
plant。　　In　this　we　start　fair。　　Its　food　is　to　be　carbonic　acid；　water
and　ammonia；　and　the　saline　matters　in　the　soil；　which　are；　by　the
supposition；　everywhere　alike。　　We　take　one　single　plant；　with　no
opponents；　no　helpers；　and　no　rivals；　it　is　to　be　a　〃fair　field；　and　no
favour〃。　　Now；　I　will　ask　you　to　imagine　further　that　it　shall　be　a
plant　which　shall　produce　every　year　fifty　seeds；　which　is　a　very
moderate　number　for　a　plant　to　produce；　and　that；　by　the　action　of　the
winds　and　currents；　these　seeds　shall　be　equally　and　gradually
distributed　over　the　whole　surface　of　the　land。　　I　want　you　now　to
trace　out　what　will　occur；　and　you　will　observe　that　I　am　not　talking
fallaciously　any　more　than　a　mathematician　does　when　he　expounds　his
problem。　　If　you　show　that　the　conditions　of　your　problem　are　such　as
may　actually　occur　in　nature　and　do　not　transgress　any　of　the　known
laws　of　nature　in　working　out　your　proposition；　then　you　are　as　safe　in
the　conclusion　you　arrive　at　as　is　the　mathematician　in　arriving　at　the
solution　of　his　problem。　　In　science；　the　only　way　of　getting　rid　of　the
complications　with　which　a　subject　of　this　kind　is　environed；　is　to
work　in　this　deductive　method。　　What　will　be　the　result；　then？　　I　will
suppose　that　every　plant　requires　one　square　foot　of　ground　to　live
upon；　and　the　result　will　be　that；　in　the　course　of　nine　years；　the
plant　will　have　occupied　every　single　available　spot　in　the　whole
globe！　　I　have　chalked　upon　the　blackboard　the　figures　by　which　I
arrive　at　the　result：…

Plants。　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Plants
　　　　　　　　　　　　　　　　　　1　x　50　in　1st　year　=　　　　　　　　　　　　　　　　　　　　　50
　　　　　　　　　　　　　　　　　50　x　50　〃　　2nd　　　〃　　=　　　　　　　　　　　　　　　　　　2；500
　　　　　　　　　　　　　　2；500　x　50　〃　　3rd　　　〃　　=　　　　　　　　　　　　　　　　125；000
　　　　　　　　　　　　125；000　x　50　〃　　4th　　　〃　　=　　　　　　　　　　　　　　6；250；000
　　　　　　　　　　6；250；000　x　50　〃　　5th　　　〃　　=　　　　　　　　　　　　312；500；000
　　　　　　　　312；500；000　x　50　〃　　6th　　　〃　　=　　　　　　　　　15；625；000；000
　　　　　15；625；000；000　x　50　〃　　7th　　　〃　　=　　　　　　　　781；250；000；000
　　　　781；250；000；000　x　50　〃　　8th　　　〃　　=　　　　　39；062；500；000；000
39；062；500；000；000　x　50&　〃　　9th　　　〃　　=　　1；953；125；000；000；000

51；000；000　sq。　milesthe　dry　surface　of　the　earth　x　27；878；400the
number　of　sq。　ft。　in　1　sq。　　mile　=　sq。　ft。　1；421；798；400；000；000　being
531；326；600；000；000　square　feet　less　than　would　be　required　at　the　end
of　the　ninth　year。

You　will　see　from　this　that；　at　the　end　of　the　first　year　the　single
plant　will　have　produced　fifty　more　of　its　kind；　by　the　end　of　the
second　year　these　will　have　increased　to　2；500；　and　so　on；　in
succeeding　years；　you　get　beyond　even　trillions；　and　I　am　not　at　all
sure　that　I　could　tell　you　what　the　proper　arithmetical　denomination　of
the　total　number　really　is；　but；　at　any　rate；　you　will　understand　the
meaning　of　all　those　noughts。　　Then　you　see　that；　at　the　bottom；　I　have
taken　the　51；000；000　of　square　mi