the conditions of existence-第3章

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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　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。