the conditions of existence-第4章

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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　seed；　and　the　animals　that　manure　it　with　their
dung；I　say；　when　these　things　are　considered；　it　seems　impossible
that　any　variation　which　may　arise　in　a　species　in　nature　should　not
tend　in　some　way　or　other　either　to　be　a　little　better　or　worse　than
the　previous　stock；　if　it　is　a　little　better　it　will　have　an　advantage
over　and　tend　to　extirpate　the　latter　in　this　crush　and　struggle；　and　if
it　is　a　little　worse　it　will　itself　be　extirpated。

I　know　nothing　that　more　appropriately　expresses　this；　than　the　phrase；
〃the　struggle　for　existence〃；　because　it　brings　before　your　minds；　in　a
vivid　sort　of　way；　some　of　the　simplest　possible　circumstances
connected　with　it。　　When　a　struggle　is　intense　there　must　be　some　who
are　sure　to　be　trodden　down；　crushed；　and　overpowered　by　others；　and
there　will　be　some　who　just　manage　to　get　through　only　by　the　help　of
the　slightest　accident。　　I　recollect　reading　an　account　of　the　famous
retreat　of　the　French　troops；　under　Napoleon；　from　Moscow。　　Worn　out；
tired；　and　dejected；　they　at　length　came　to　a　great　river　over　which
there　was　but　one　bridge　for　the　passage　of　the　vast　army。　Disorganised
and　demoralised　as　that　army　was；　the　struggle　must　certainly　have　been
a　terrible　oneevery　one　heeding　only　himself；　and　crushing　through
the　ranks　and　treading　down　his　fellows。　　The　writer　of　the　narrative；
who　was　himself　one　of　those　who　were　fortunate　enough　to　succeed　in
getting　over；　and　not　among　the　thousands　who　were　left　behind　or
forced　into　the　river；　ascribed　his　escape　to　the　fact　that　he　saw
striding　onward　through　the　mass　a　great　strong　fellow；one　of　the
French　Cuirassiers；　who　had　on　a　large　blue　cloakand　he　had　enough
presence　of　mind　to　catch　and　retain　a　hold　of　this　strong　man's
cloak。　　He　says；　〃I　caught　hold　of　his　cloak；　and　although　he　swore　at
me　and　cut　at　and　struck　me　by　turns；　and　at　last；　when　he　found　he
could　not　shake　me　off；　fell　to　entreating　me　to　leave　go　or　I　should
prevent　him　from　escaping；　besides　not　assisting　myself；　I　still　kept
tight　hold　of　him；　and　would　not　quit　my　grasp　until　he　had　at　last
dragged　me　through。〃　　Here　you　see　was　a　case　of　selective　savingif
we　may　so　term　itdepending　for　its　success　on　the　strength　of　the
cloth　of　the　Cuirassier's　cloak。　　It　is　the　same　in　nature；　every
species　has　its　bridge　of　Beresina；　it　has　to　fight　its　way　through　and
struggle　with　other　species；　and　when　well　nigh　overpowered；　it　may　be
that　the　smallest　chance；　something　in　its　colour；　perhapsthe
minutest　circumstancewill　turn　the　scale　one　way　or　the　other。

Suppose　that　by　a　variation　of　the　black　race　it　had　produced　the　white
man　at　any　timeyou　know　that　the　Negroes　are　said　to　believe　this　to
have　been　the　case；　and　to　imagine　that　Cain　was　the　first　white　man；
and　that　we　are　his　descendantssuppose　that　this　had　ever　happened；
and　that　the　first　residence　of　this　human　being　was　on　the　West　Coast
of　Africa。　　There　is　no　great　structural　difference　between　the　white
man　and　the　Negro；　and　yet　there　is　something　so　singularly　different
in　the　constitution　of　the　two；　that　the　malarias　of　that　country；　which
do　not　hurt　the　black　at　all；　cut　off　and　destroy　the　white。　　Then　you
see　there　would　have　been　a　selective　operation　performed；　if　the　white
man　had　risen　in　that　way；　he　would　have　been　selected　out　and　removed
by　means　of　the　malaria。　　Now　there　really　is　a　very　curious　case　of
selection　of　this　sort　among　pigs；　and　it　is　a　case　of　selection　of
colour　too。　　In　the　woods　of　Florida　there　are　a　great　many　pigs；　and
it　is　a　very　curious　thing　that　they　are　all　black；　every　one　of　them。
Professor　Wyman　was　there　some　years　ago；　and　on　noticing　no　pigs　but
these　black　ones；　he　asked　some　of　the　people　how　it　was　that　they　had
no　white　pigs；　and　the　reply　was　that　in　the　woods　of　Florida　there　was
a　root　which　they　called　the　Paint　Root；　and　that　if　the　white　pigs
were　to　eat　any　of　it；　it　had　the　effect　of　making　their　hoofs　crack；
and　they　died；　but　if　the　black　pigs　eat　any　of　it；　it　did　not　hurt
them　at　all。　　Here　was　a　very　simple　case　of　natural　selection。　　A
skilful　breeder　could　not　more　carefully　develope　the　black　breed　of
pigs；　and　weed　out　all　the　white　pigs；　than　the　Paint　Root　does。

To　show　you　how　remarkably　indirect　may　be　such　natural　selective
agencies　as　I　have　referred　to；　I　will　conclude　by　noticing　a　case
mentioned　by　Mr。　Darwin；　and　which　is　certainly　one　of　the　most　curious
of　its　kind。　　It　is　that　of　the　Humble　Bee。　　It　has　been　noticed　that
there　are　a　great　many　more　humble　bees　in　the　neighbourhood　of　towns；
than　out　in　the　open　country；　and　the　explanation　of　the　matter　is
this：　the　humble　bees　build　nests；　in　which　they　store　their　honey　and
deposit　the　larvae　and　eggs。　　The　field　mice　are　amazingly　fond　of　the
honey　and　larvae；　therefore；　wherever　there　are　plenty　of　field　mice；　as
in　the　country；　the　humble　bees　are　kept　down；　but　in　the　neighbourhood
of　towns；　the　number　of　cats　which　prowl　about　the　fields　eat　up　the
field　mice；　and　of　course　the　more　mice　they　eat　up　the　less　there　are
to　prey　upon　the　larvae　of　the　beesthe　cats　are　therefore　the　INDIRECT
HELPERS　of　the　bees！*　　Coming　back　a　step　farther　we　may　say　that　the
old　maids　are　also　indirect　friends　of　the　humble　bees；　and　indirect
enemies　of　the　field　mice；　as　they　keep　the　cats　which　eat　up　the
latter！　　This　is　an　illustration　somewhat　beneath　the　dignity　of　the
subject；　perhaps；　but　it　occurs　to　me　in　passing；　and　with　it　I　will
conclude　this　lecture。

＇footnote＇　*The　humble　bees；　on　the　other　hand；　are　direct　helpers　of
some　plants；　such　as　the　heartsease　and　red　clover；　which　are
fertilized　by　the　visits　of　the　bees；　and　they　are　indirect　helpers　of
the　numerous　insects　which　are　more　or　less　completely　supported　by　the
heartsease　and　red　clover。







End