the conditions of existence-第4节

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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　miles；　constituting　the　surface　of　the
dry　land；　and　as　the　number　of　square　feet　are　placed　under　and
subtracted　from　the　number　of　seeds　that　would　be　produced　in　the　ninth
year；　you　can　see　at　once　that　there　would　be　an　immense　number　more　of
plants　than　there　would　be　square　feet　of　ground　for　their
accommodation。　　This　is　certainly　quite　enough　to　prove　my　point；　that
between　the　eighth　and　ninth　year　after　being　planted　the　single　plant
would　have　stocked　the　whole　available　surface　of　the　earth。

This　is　a　thing　which　is　hardly　conceivableit　seems　hardly
imaginableyet　it　is　so。　　It　is　indeed　simply　the　law　of　Malthus
exemplified。　　Mr。　Malthus　was　a　clergyman；　who　worked　out　this　subject
most　minutely　and　truthfully　some　years　ago；　he　showed　quite
clearly；and　although　he　was　much　abused　for　his　conclusions　at　the
time；　they　have　never　yet　been　disproved　and　never　will　behe　showed
that　in　consequence　of　the　increase　in　the　number　of　organic　beings　in
a　geometrical　ratio；　while　the　means　of　existence　cannot　be　made　to
increase　in　the　same　ratio；　that　there　must　come　a　time　when　the　number
of　organic　beings　will　be　in　excess　of　the　power　of　production　of
nutriment；　and　that　thus　some　check　must　arise　to　the　further　increase
of　those　organic　beings。　　At　the　end　of　the　ninth　year　we　have　seen　that
each　plant　would　not　be　able　to　get　its　full　square　foot　of　ground；　and
at　the　end　of　another　year　it　would　have　to　share　that　space　with　fifty
others　the　produce　of　the　seeds　which　it　would　give　off。

What；　then；　takes　place？　　Every　plant　grows　up；　flourishes；　occupies　its
square　foot　of　ground；　and　gives　off　its　fifty　seeds；　but　notice　this；
that　out　of　this　number　only　one　can　come　to　anything；　there　is　thus；
as　it　were；　forty…nine　chances　to　one　against　its　growing　up；　it
depends　upon　the　most　fortuitous　circumstances　whether　any　one　of　these
fifty　seeds　shall　grow　up　and　flourish；　or　whether　it　shall　die　and
perish。　　This　is　what　Mr。　Darwin　has　drawn　attention　to；　and　called　the
〃STRUGGLE　FOR　EXISTENCE〃；　and　I　have　taken　this　simple　case　of　a　plant
because　some　people　imagine　that　the　phrase　seems　to　imply　a　sort　of
fight。

I　have　taken　this　plant　and　shown　you　that　this　is　the　result　of　the
ratio　of　the　increase；　the　necessary　result　of　the　arrival　of　a　time
coming　for　every　species　when　exactly　as　many　members　must　be　destroyed
as　are　born；　that　is　the　inevitable　ultimate　result　of　the　rate　of
production。　　Now；　what　is　the　result　of　all　this？　　I　have　said　that
there　are　forty…nine　struggling　against　every　one；　and　it　amounts　to
this；　that　the　smallest　possible　start　given　to　any　one　seed　may　give
it　an　advantage　which　will　enable　it　to　get　ahead　of　all　the　others；
anything　that　will　enable　any　one　of　these　seeds　to　germinate　six　hours
before　any　of　the　others　will；　other　things　being　alike；　enable　it　to
choke　them　out　altogether。　　I　have　shown　you　that　there　is　no
particular　in　which　plants　will　not　vary　from　each　other；　it　is　quite
possible　that　one　of　our　imaginary　plants　may　vary　in　such　a　character
as　the　thickness　of　the　integument　of　its　seeds；　it　might　happen　that
one　of　the　plants　might　produce　seeds　having　a　thinner　integument；　and
that　would　enable　the　seeds　of　that　plant　to　germinate　a　little　quicker
than　those　of　any　of　the　others；　and　those　seeds　would　most　inevitably
extinguish　the　forty…nine　times　as　many　that　were　struggling　with　them。

I　have　put　it　in　this　way；　but　you　see　the　practical　result　of　the
process　is　the　same　as　if　some　person　had　nurtured　the　one　and
destroyed　the　other　seeds。　　It　does　not　matter　how　the　variation　is
produced；　so　long　as　it　is　once　allowed　to　occur。　　The　variation　in　the
plant　once　fairly　started　tends　to　become　hereditary　and　reproduce
itself；　the　seeds　would　spread　themselves　in　the　same　way　and　take　part
in　the　struggle　with　the　forty…nine　hundred；　or　forty…nine　thousand；
with　which　they　might　be　exposed。　　Thus；　by　degrees；　this　variety；　with
some　slight　organic　change　or　modification；　must　spread　itself　over　the
whole　surface　of　the　habitable　globe；　and　extirpate　or　replace　the
other　kinds。　　That　is　what　is　meant　by　NATURAL　SELECTION；　that　is　the
kind　of　argument　by　which　it　is　perfectly　demonstrable　that　the
conditions　of　existence　may　play　exactly　the　same　part　for　natural
varieties　as　man　does　for　domesticated　varieties。　　No　one　doubts　at　all
that　particular　circumstances　may　be　more　favourable　for　one　plant　and
less　so　for　another；　and　the　moment　you　admit　that；　you　admit　the
selective　power　of　nature。　　Now；　although　I　have　been　putting　a
hypothetical　case；　you　must　not　suppose　that　I　have　been　reasoning
hypothetically。　　There　are　plenty　of　direct　experiments　which　bear　out
what　we　may　call　the　theory　of　natural　selection；　there　is　extremely
good　authority　for　the　statement　that　if　you　take　the　seed　of　mixed
varieties　of　wheat　and　sow　it；　collecting　the　seed　next　year　and　sowing
it　again；　at　length　you　will　find　that　out　of　all　your　varieties　only
two　or　three　have　lived；　or　perhaps　even　only　one。　　There　were　one　or
two　varieties　which　were　best　fitted　to　get　on；　and　they　have　killed
out　the　other　kinds　in　just　the　same　way　and　with　just　the　same
certainty　as　if　you　had　taken　the　trouble　to　remove　them。　　As　I　have
already　said；　the　operation　of　nature　is　exactly　the　same　as　the
artificial　operation　of　man。

But　if　this　be　true　of　that　simple　case；　which　I　put　before　you；　where
there　is　nothing　but　the　rivalry　of　one　member　of　a　species　with
others；　what　must　be　the　operation　of　selective　conditions；　when　you
recollect　as　a　matter　of　fact；　that　for　every　species　of　animal　or
plant　there　are　fifty　or　a　hundred　species　which　might　all；　more　or
less；　be　comprehended　in　the　same　climate；　food；　and　station；that
every　plant　has　multitudinous　animals　which　prey　upon　it；　and　which　are
its　direct　opponents；　and　that　these　have　other　animals　preying　upon
them；that　every　plant　has　its　indirect　helpers　in　the　birds　that
scatter　abroad　its　s