the conditions of existence-第2部分

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know　at　presentthere　is　no　difficulty　in　breeding　together　the
mongrels。　　Take　the　Carrier　and　the　Fantail；　for　instance；　and　let　them
represent　the　Horse　and　the　Ass　in　the　case　of　distinct　species；　then
you　have；　as　the　result　of　their　breeding；　the　Carrier…Fantail
mongrel；we　will　say　the　male　and　female　mongrel；and；　as　far　as　we
know；　these　two　when　crossed　would　not　be　less　fertile　than　the
original　cross；　or　than　Carrier　with　Carrier。　　Here；　you　see；　is　a
physiological　contrast　between　the　races　produced　by　selective
modification　and　natural　species。　　I　shall　inquire　into　the　value　of
this　fact；　and　of　some　modifying　circumstances　by　and　by；　for　the
present　I　merely　put　it　broadly　before　you。

But　while　considering　this　question　of　the　limitations　of　species；　a
word　must　be　said　about　what　is　called　RECURRENCEthe　tendency　of
races　which　have　been　developed　by　selective　breeding　from　varieties　to
return　to　their　primitive　type。　　This　is　supposed　by　many　to　put　an
absolute　limit　to　the　extent　of　selective　and　all　other　variations。
People　say；　〃It　is　all　very　well　to　talk　about　producing　these
different　races；　but　you　know　very　well　that　if　you　turned　all　these
birds　wild；　these　Pouters；　and　Carriers；　and　so　on；　they　would　all
return　to　their　primitive　stock。〃　　This　is　very　commonly　assumed　to　be
a　fact；　and　it　is　an　argument　that　is　commonly　brought　forward　as
conclusive；　but　if　you　will　take　the　trouble　to　inquire　into　it　rather
closely；　I　think　you　will　find　that　it　is　not　worth　very　much。　　The
first　question　of　course　is；　Do　they　thus　return　to　the　primitive
stock？　　And　commonly　as　the　thing　is　assumed　and　accepted；　it　is
extremely　difficult　to　get　anything　like　good　evidence　of　it。　　It　is
constantly　said；　for　example；　that　if　domesticated　Horses　are　turned
wild；　as　they　have　been　in　some　parts　of　Asia　Minor　and　South　America；
that　they　return　at　once　to　the　primitive　stock　from　which　they　were
bred。　But　the　first　answer　that　you　make　to　this　assumption　is；　to　ask
who　knows　what　the　primitive　stock　was；　and　the　second　answer　is；　that
in　that　case　the　wild　Horses　of　Asia　Minor　ought　to　be　exactly　like　the
wild　Horses　of　South　America。　　If　they　are　both　like　the　same　thing；
they　ought　manifestly　to　be　like　each　other！　　The　best　authorities；
however；　tell　you　that　it　is　quite　different。　　The　wild　Horse　of　Asia
is　said　to　be　of　a　dun　colour；　with　a　largish　head；　and　a　great　many
other　peculiarities；　while　the　best　authorities　on　the　wild　Horses　of
South　America　tell　you　that　there　is　no　similarity　between　their　wild
Horses　and　those　of　Asia　Minor；　the　cut　of　their　heads　is　very
different；　and　they　are　commonly　chestnut　or　bay…coloured。　　It　is　quite
clear；　therefore；　that　as　by　these　facts　there　ought　to　have　been　two
primitive　stocks；　they　go　for　nothing　in　support　of　the　assumption　that
races　recur　to　one　primitive　stock；　and　so　far　as　this　evidence　is
concerned；　it　falls　to　the　ground。

Suppose　for　a　moment　that　it　were　so；　and　that　domesticated　races；　when
turned　wild；　did　return　to　some　common　condition；　I　cannot　see　that
this　would　prove　much　more　than　that　similar　conditions　are　likely　to
produce　similar　results；　and　that　when　you　take　back　domesticated
animals　into　what　we　call　natural　conditions；　you　do　exactly　the　same
thing　as　if　you　carefully　undid　all　the　work　you　had　gone　through；　for
the　purpose　of　bringing　the　animal　from　its　wild　to　its　domesticated
state。　　I　do　not　see　anything　very　wonderful　in　the　fact；　if　it　took
all　that　trouble　to　get　it　from　a　wild　state；　that　it　should　go　back
into　its　original　state　as　soon　as　you　removed　the　conditions　which
produced　the　variation　to　the　domesticated　form。　There　is　an　important
fact；　however；　forcibly　brought　forward　by　Mr。　Darwin；　which　has　been
noticed　in　connection　with　the　breeding　of　domesticated　pigeons；　and　it
is；　that　however　different　these　breeds　of　pigeons　may　be　from　each
other；　and　we　have　already　noticed　the　great　differences　in　these
breeds；　that　if；　among　any　of　those　variations；　you　chance　to　have　a
blue　pigeon　turn　up；　it　will　be　sure　to　have　the　black　bars　across　the
wings；　which　are　characteristic　of　the　original　wild　stock；　the　Rock
Pigeon。

Now；　this　is　certainly　a　very　remarkable　circumstance；　but　I　do　not　see
myself　how　it　tells　very　strongly　either　one　way　or　the　other。　　I
think；　in　fact；　that　this　argument　in　favour　of　recurrence　to　the
primitive　type　might　prove　a　great　deal　too　much　for　those　who　so
constantly　bring　it　forward。　　For　example；　Mr。　Darwin　has　very　forcibly
urged；　that　nothing　is　commoner　than　if　you　examine　a　dun　horseand　I
had　an　opportunity　of　verifying　this　illustration　lately；　while　in　the
islands　of　the　West　Highlands；　where　there　are　a　great　many　dun
horsesto　find　that　horse　exhibit　a　long　black　stripe　down　his　back；
very　often　stripes　on　his　shoulder；　and　very　often　stripes　on　his
legs。　　I；　myself；　saw　a　pony　of　this　description　a　short　time　ago；　in　a
baker's　cart；　near　Rothesay；　in　Bute：　it　had　the　long　stripe　down　the
back；　and　stripes　on　the　shoulders　and　legs；　just　like　those　of　the
Ass；　the　Quagga；　and　the　Zebra。　　Now；　if　we　interpret　the　theory　of
recurrence　as　applied　to　this　case；　might　it　not　be　said　that　here　was
a　case　of　a　variation　exhibiting　the　characters　and　conditions　of　an
animal　occupying　something　like　an　intermediate　position　between　the
Horse；　the　Ass；　the　Quagga；　and　the　Zebra；　and　from　which　these　had
been　developed？　　In　the　same　way　with　regard　even　to　Man。　Every
anatomist　will　tell　you　that　there　is　nothing　commoner；　in　dissecting
the　human　body；　than　to　meet　with　what　are　called　muscular
variationsthat　is；　if　you　dissect　two　bodies　very　carefully；　you　will
probably　find　that　the　modes　of　attachment　and　insertion　of　the　muscles
are　not　exactly　the　same　in　both；　there　being　great　peculiarities　in
the　mode　in　which　the　muscles　are　arranged；　and　it　is　very　singular；
that　in　some　dissections　of　the　human　body　you　will　come　upon
arrangements　of　the　muscles　very　similar　indeed　to　the　same　parts　in　the
Apes。　　Is　the　conclusion　in　that　case　to　be；　that　this　is　like　the
black　bars　in　the　case　of　the　Pigeon；　and　that　it　indicates　a
recurrence　to　the　primitive　type　from　which　the　animals　have　been
probably　developed？　　Truly；　I　think　that　the　opponents　of　modification
and　variation　had　better　leave　the　argument　of　recurrence　alone；　or　it
may　prove　altogether　too　strong　for　them。

To　sum　up；the　evidence　as　far　as　we　have　gone　is　against　the　argument
as　to　any　limit　to　divergences；　so　far　as　structure　is　concerned；　and
in　favour　of　a　physiological　limitation。　　By　selective　breeding　we　can
produce　structural　divergences　as　great　as　those　of　species；　but　we
cannot　produce　equal　physiological　divergences。　　For　the　present　I　leave
the　question　there。

Now；　the　next　problem　that　lies　before　usand　it　is　an　extremely
important　oneis　this：　Does　this　selective　breeding　occur　in　nature？
Because；　if　there　is　no　proof　of　it；　all　that　I　have　been　telling　you
goes　for　nothing　in　accounting　for　the　origin　of　species。　　Are　natural
causes　competent　to　play　the　part　of　selection　in　perpetuating
varieties？　　Here　we　labour　under　very　great　difficulties。　　In　the　last
lecture　I　had　occasion　to　point　out　to　you　the　extreme　difficulty　of
obtaining　evidence　even　of　the　first　origin　of　those　varieties　which　we
know　to　have　occurred　in　domesticated　animals。　　I　told　you；　that　almost
always　the　origin　of　these　varieties　is　overlooked；　so　that　I　could
only　produce　two　of　three　cases；　as　that　of　Gratio　Kelleia　and　of　the
Ancon　sheep。　　People　forget；　or　do　not　take　notice　of　them　until　they
come　to　have　a　prominence；　and　if　that　is　true　of　artificial　cases；
under　our　own　eyes；　and　in　animals　in　our　own　care；　how　much　more
difficult　it　must　be　to　have　at　first　hand　good　evidence　of　the　origin
of　varieties　in　nature！　　Indeed；　I　do　not　know　that　it　is　possible　by
direct　evidence　to　prove　the　origin　of　a　variety　in　nature；　or　to　prove
selective　breeding；　but　I　will　tell　you　what　we　can　proveand　this
comes　to　the　same　thingthat　varieties　exist　in　nature　within　the
limits　of　species；　and；　what　is　more；　that　when　a　variety　has　come　into
existence　in　nature；　there　are　natural　causes　and　conditions；　which　are
amply　competent　to　play　the　part　of　a　selective　breeder；　and　although
that　is　not　quite　the　evidence　that　one　would　like　to　havethough　it
is　not　direct　testimonyyet　it　is　exceeding　good　and　exceedingly
powerful　evidence　in　its　way。

As　to　the　first　point；　of　varieties　existing　among　natural　species；　I
might　appeal　to　the　universal　experience　of　every　naturalist；　and　of
any　person　who　has　ever　turned　any　attention　at　all　to　the
characteristics　of　plants　and　animals　in　a　state　of　nature；　but　I　may　as
well　take　a　few　definite　cases；　and　I　will　begin　with　Man　himself。

I　am　one　of　those　who　believe　that；　at　present；　there　is　no　evidence
whatever　for　saying；　that　mankind　sprang　originally　from　any　more　than
a　single　pair；　I　must　say；　that　I　cannot　see　any　good　ground　whatever；
or　even　any　tenable　sort　of　evidence；　for　believing　that　there　is　more
than　one　species　of　Man。　　Nevertheless；　as　you　know；　just　as　there　are
numbers　of　varieties　in　animals；　so　there　are　remarkable　varieties　of
men。　　I　speak　not　merely　of　those　broad　and　distinct　variations　which
you　see　at　a　glance。　　Everybody；　of　course；　knows　the　difference
between　a　Negro　and　a　white　man；　and　can　tell　a　Chinaman　from　an
Englishman。　　They　each　have　peculiar　characteristics　of　colour　and
physiognomy；　but　you　must　recollect　that　the　characters　of　these　races
go　very　far　deeperthey　extend　to　the　bony　structure；　and　to　the
characters　of　that　most　important　of　all　organs　to　usthe　brain；　so
that；　among　men　belonging　to　different　races；　or　even　within　the　same
race；　one　man　shall　have　a　brain　a　third；　or　half；　or　even　seventy　per
cent。　bigger　than　another；　and　if　you　take　the　whole　range　of　human
brains；　you　will　find　a　variation　in　some　cases　of　a　hundred　per　cent。
Apart　from　these　variations　in　the　size　of　the　brain；　the　characters　of
the　skull　vary。　　Thus　if　I　draw　the　figures　of　a　Mongul　and　of　a　Negro
head　on　the　blackboard；　in　the　case　of　the　last