Theorganizationofeverybusinessismodified。Easeofcommunicationmakes
itbettertododirectlywhatwasbeforedonebyproxy;agenciesareestablished
wherepreviouslytheywouldnothavepaid;goodsareobtainedfromremote
wholesalehousesinsteadofnearretailones;andcommoditiesareusedwhich
distanceoncerenderedinaccessible。Rapidityandeconomyofcarriagetend
tospecializemorethanevertheindustriesofdifferentdistricts——to
confineeachmanufacturetothepartsinwhich，fromlocaladvantages，it
canbebestcarriedon。Cheapdistributionequalizesprices，andalso，on
theaverage，lowersprices:thusbringingdiversarticleswithinthereach
ofthosebeforeunabletobuythem。Atthesametimethepracticeoftravelling
isimmenselyextended。Peoplewhobeforecouldnotaffordit，takeannual
tripstothesea，visittheirdistantrelations，maketours，andsoarebenefited
inbody，feelings，andintellect。Thepromptertransmissionoflettersand
ofnewsproducesfurtherchanges——makesthepulseofthenationfaster。
Yetmore，therearisesawidedisseminationofcheapliteraturethroughrailway
book—stalls，andofadvertisementsinrailwaycarriages:bothofthemaiding
ulteriorprogress。Sothatbeyondimaginationarethechanges，thusbrieflyindicated，consequentontheinventionofthelocomotiveengine。Itshouldbeaddedthatwehereseemoreclearlythan，ever，howinproportion
astheareaoverwhichanyinfluenceextendsbecomesheterogeneous，theresults
areinayethigherdegreemultipliedinnumberandkind。Whileamongthe
uncivilizedmentowhomitwasfirstknown，caoutchouccausedbutfewchanges，
amongourselvesthechangeshavebeensomanyandvariedthatthehistory
ofthemoccupiesavolume。Uponthesmall，homogeneouscommunityinhabiting
oneoftheHebrides，theelectrictelegraphwouldproduce，wereitused，scarcelyanyresults;butinEnglandtheresultsitproducesaremultitudinous。Spacepermitting，thesynthesismightherebepursuedinrelationtoall
thesubtlerproductsofsociallife。Itmightbeshownhow，inScience，an
advanceofonedivisionpresentlyadvancesotherdivisions——howAstronomy
hasbeenimmenselyforwardedbydiscoveriesinOptics，whileotheroptical
discoverieshaveinitiatedMicroscopicAnatomy，andgreatlyaidedthegrowth
ofPhysiology——howChemistryhasindirectlyincreasedourknowledgeof
Electricity，Magnetism，Biology，Geology——howElectricityhasreactedon
ChemistryandMagnetism，developedourviewsofLightandHeat，anddisclosed
sundrylawsofnervousaction。Butitwouldneedlesslytaxthereader'spatience
todetail，intheirmanyramifications，thesevariouschanges;soinvolvedandsubtleastobefollowedwithdifficulty。§162。Aftertheargumentwhichclosedthelastchapter，aparallel
onehereseemsscarcelyrequired。Forsymmetry'ssake，however，itwillbe
properbrieflytopointouthowthemultiplicationofeffects，liketheinstabilityofthehomogeneous，isacorollaryfromthepersistenceofforce。Thingswhichwecalldifferentarethingswhichreactindifferentways;
andwecanknowthemasdifferentonlybythedifferencesintheirreactions。
Whenwedistinguishbodiesashardorsoft，roughorsmooth，wemeanthat
certainlikemuscularforcesexpendedonthemarefollowedbyunlikereactive
forces，causingunlikesetsofsensations。Objectsclassedasred，blue，
yellowetc。，areobjectswhichdecomposelightincontrastedways;thatis，
weknowcontrastsofcolourascontrastsinthechangesproducedinauniform
incidentforce。Thepropositionthatthedifferentpartsofanywholemust
reactdifferentlyonauniformincidentforce，andmustthusreduceitto
agroupofmultiformforces，isinessenceatruism。Supposewereducethistruismtoitslowestterms。When，fromunlikenessbetweentheeffectstheyproduceonconsciousness，
wepredicateunlikenessbetweentwoobjects，whatisourwarrant？andwhat
dowemeanbytheunlikeness，objectivelyconsidered？Ourwarrantisthe
persistenceofforce。Somekindoramountofchangehasbeenwroughtinus
bytheonewhichhasnotbeenwroughtbytheother。Thischangeweascribe
tosomeforceexercisedbytheonewhichtheotherhasnotexercised。And
wehavenoalternativebuttodothis，ortoassertthatthechangehadno
antecedent，whichistodenythepersistenceofforce。Whenceitisfurther
manifestthatwhatweregardastheobjectiveunlikenessisthepresence
intheoneofsomeforce，orsetofforces，notpresentintheother——something
inthekindsoramountsordirectionsoftheconstituentforcesoftheone，
whichthoseoftheotherdonotparallel。Butnowifthingsorpartsofthings
whichwecalldifferent，arethoseofwhichtheconstituentforcesdiffer
inoneormorerespects，whatmusthappentoanylikeforces，oranyuniform
force，fallingonthem？Suchlikeforces，orpartsofauniformforce，must
bedifferentlymodified。Theforcewhichispresentintheoneandnotin
theother，mustbeanelementintheconflict——mustproduceitsequivalent
reaction;andmustsoaffectthetotalreaction。Tosayotherwiseistosay
thatthisdifferentialforcewillproducenoeffect，whichistosaythatforceisnotpersistent。Ineednotdevelopthiscorollaryfurther。Itmanifestlyfollowsthat
auniformforcefallingonauniformaggregate，mustundergodispersion;
thatfallingonanaggregatemadeupofunlikeparts，itmustundergodispersion
fromeachpart，aswellasqualitativedifferentiations;thatinproportion
asthepartsareunlike，thesequalitativedifferentiationsmustbemarked;
thatinproportiontothenumberoftheparts，theymustbenumerous;that
thesecondaryforcessoproducedmustundergofurthertransformationswhile
workingequivalenttransformationsinthepartsthatchangethem;andsimilarly
withtheforcestheygenerate。Thustheconclusionsthatapart—causeof
Evolutionisthemultiplicationofeffects，andthatthisincreasesingeometrical
progressionastheheterogeneitybecomesgreater，arenotonlytobeestablished
inductively，butarededuciblefromthedeepestofalltruths。
Chapter21Segregation§163。ThegeneralinterpretationofEvolutionisfarfrombeingcompleted
intheprecedingchapters。Wemustcontemplateitschangesunderyetanother
aspect，beforewecanformadefiniteconceptionoftheprocessconstituted
bythem。Thoughthelawsalreadysetforthfurnishakeytothere—arrangement
ofpartswhichEvolutionexhibits，insofarasitisanadvancefromthe
uniformtothemultiform，theyfurnishnokeytothisrearrangementinso
farasitisanadvancefromtheindefinitetothedefinite。Onstudying
theactionsandreactionseverywheregoingon，wehavefoundittofollow
fromacertainprimordialtruth，thatthehomogeneousmustlapseintothe
heterogeneous，andthattheheterogeneousmustbecomemoreheterogeneous;
butwehavenotdiscoveredwhythedifferently—affectedpartsofanysimple
whole，becomeclearlymarkedofffromoneanother，atthesametimethat
theybecomeunlike。Thusfarnoreasonhasbeengivenwhythereshouldnot
ordinarilyariseavaguechaoticheterogeneity，inplaceofthatorderly
heterogeneitydisplayedinEvolution。Itstillremainstofindoutthecause
ofthatlocalintegrationwhichaccompaniedlocaldifferentiation——that
gradually—completedsegregationoflikeunitsintoagroup，distinctlyseparated
fromneighbouringgroupswhichareseverallymadeupofotherkindsofunits。
Therationalewillbeconvenientlyintroducedbyafewinstancesinwhichwemaywatchthissegregativeprocesstakingplace。When，lateinSeptember，thetreesaregainingtheirautumncolours，and
wearehopingsoontoseeafurtherchangeincreasingthebeautyofthelandscape，
wearesometimesdisappointedbytheoccurrenceofanequinoctialgale。Out
ofthemixedmassoffoliageoneachbranch，thestrongcurrentofaircarries
awaythedecayingandbrightly—tintedleaves，butfailstodetachthosewhich
arestillgreen。Andwhiletheselast，frayedandsearedbylong—continued
beatingsagainstoneanother，giveasombrecolourtothewoods，thered
andyellowandorangeleavesarecollectedtogetherinditchesandbehind
wallsandincornerswhereeddiesallowthemtosettle。Thatistosay，
bythatuniformforcewhichthewindexertsonbothkinds，thedyingleaves
arepickedoutfromamongtheirstill—livingcompanionsandgatheredinplaces
bythemselves。Again，theseparationofparticlesofdifferentsizes，as
dustandsandfrompebbles，maybesimilarlyeffected，asweseeonevery
roadinMarch。AndfromthedaysofHomerdownwards，thepowerofcurrents
ofair，naturalandartificial，topartfromoneanotherunitsofunlike
characters，hasbeenhabituallyutilizedinthewinnowingofchafffromwheat。
Ineverybrookweseehowthemixedmaterialscarrieddownareseparately
deposited——howinrapidsthebottomgivesresttonothingbutboulders
andpebbles;howwherethecurrentisnotsostrong，sandisletfall;and
how，instillplaces，thereisasedimentofmud。Thisselectiveaction
ofmovingwateriscommonlyappliedintheartstoobtainmassesofparticles
ofdifferentdegreesoffineness。Emery，forexample，afterbeingground，
iscarriedbyaslowcurrentthroughsuccessivecompartments;inthefirst
ofwhichthelargestgrainssubside;inthesecondofwhichthegrainsthat
settlebeforethewaterhasescaped，aresomewhatsmaller;inthethirdsmaller
still;untilinthelasttherearedepositedthosefinestparticleswhich
havenotpreviouslybeenabletoreachthebottom。Andinawaythatisdifferent
thoughequallysignificant，thissegregativeeffectofwaterinmotion，is
exemplifiedinthecarryingawayofsolublefrominsolublematters——an
applicationofithourlymadeineverylaboratory。Theeffectsoftheuniform
forceswhichaerialandaqueouscurrentsexercise，areparalleledbythose
ofuniformforcesofotherorders。Electricattractionwillseparatesmall
bodiesfromlarge，orlightbodiesfromheavy。Bymagnetism，grainsofiron
maybeselectedfromothergrains;asbytheSheffieldgrinder，whosemagnetized
gauze—maskfiltersoutthesteel—dusthiswheelgivesoff，fromthestone—dust
whichaccompaniesit。Andhowtheaffinityofanyagentactingdifferently
onthemixedcomponentsofabody，enablesustotakeawaysomecomponentandleavetherestbehind，isperpetuallyshowninchemicalexperiments。What，now，isthegeneraltruthherevariouslypresented？Howarethese
facts，andcountlesssimilarones，tobeexpressedintermsthatembrace
themall？Ineachcaseweseeinactionaforcewhichmayberegardedas
simpleoruniform—fluidmotioninacertaindirectionatacertainvelocity;
electricormagneticattractionofagivenamount;chemicalaffinityofa
particularkind;orrather，instrictness，theactingforceiscompounded
ofoneofthesewithsomeotheruniformforce，asgravitation，etc。Ineach
casewehaveanaggregatemadeupofunlikeunits——eitheratomsofdifferent
substancescombinedorintimatelymingled，orfragmentsofthesamesubstance
ofdifferentsizes，orotherconstituentpartsthatareunlikeintheirspecific
gravities，shapes，orotherattributes。Andineachcasetheseunlikeunits，
orgroupsofunits，ofwhichtheaggregateconsists，are，undertheinfluence
ofsomeresultantforceactingindiscriminatelyonthemall，separatedfrom
oneanother——segregatedintominoraggregates，eachconsistingofunits
thatareseverallylikeoneanotherandunlikethoseoftheotherminoraggregates。
Suchbeingthecommonaspectofthesechanges，letuslookforthecommoninterpretationofthem。Inthechapteron"TheInstabilityoftheHomogeneous，"itwas
shownthatauniformforcefallingonanyaggregate，producesunlikemodifications
initsdifferentparts——turnstheuniformintothemultiformandthemultiform
intothemoremultiform。Thetransformationthuswrought，consistsofeither
insensibleorsensiblechangesofrelativepositionamongtheunits，orof
both。Suchportionofthepermanentlyeffectiveforceasreacheseachdifferent
part，ordifferently—conditionedpart，maybeexpendedinmodifyingthemutual
relationsofitsconstituents;oritmaybeexpendedinmovingthepartto
anotherplace;oritmaybeexpendedpartiallyinthefirstandpartially
inthesecond。Andiflittleornoneisabsorbedinre—arrangingthecomponents
ofacompoundunit，muchorthewholemustshowitselfinmotionofsuch
compoundunittosomeotherplaceintheaggregate。andconversely。What
mustfollowfromthis，incaseswherenoneoronlypartoftheforcegenerates
chemicalre—distributions，whatphysicalre—distributionsmustbegenerated？
Partsthataresimilartoeachotherwillbesimilarlyactedonbytheforce，
whilepartsthataredissimilarwillbedissimilarlyactedon。Hencethe
permanentlyeffectiveincidentforce，whenwhollyorpartiallytransformed
intomechanicalmotionoftheunits，willproducelikemotionsinunitsthat
arealike，andunlikemotionsinunitsthatareunlike。Ifthen，inanaggregate
containingtwoormoreordersofmixedunits，thoseofthesameorderwill
bemovedinthesameway，andinawaythatdiffersfromthatinwhichunits
ofotherordersaremoved，therespectiveordersmustsegregate。Agroup
oflikethingsonwhichareimpressedmotionsthatarealikeinamountand
direction，mustbetransferredasagrouptoanotherplace，andiftheyare
mingledwithsomegroupofotherthings，onwhichthemotionsimpressedare
likeoneanother，butunlikethoseofthefirstgroupinamountordirection
orboth，theseotherthingsmustbetransferredasagrouptosomeotherplace——themixedunitsmustundergoasimultaneousselectionandseparation。Furthertoelucidatethisprocess，letmesetdownafewinstancesin
whichwemayseethatthedefinitenessoftheseparationisinproportion
tothedefinitenessofthedifferencesamongtheunits。Takeahandfulof
poundedsubstance，containingfragmentsofallsizes，andletitfallgradually
whileagentlebreezeisblowing。Thelargefragmentswillbecollectedon
thegroundalmostimmediatelyunderthehand;somewhatsmallerfragments
willbecarriedalittletotheleeward;stillsmalleronesfurtheraway;
andthoseminuteparticleswecalldust，willbedriftedfarbeforethey
reachtheearth:thatis，thesegregationisindefinitewherethedifferences
amongthefragmentsareindefinite，thoughthedivergencesaregreatestwhere
thedifferencesaregreatest。If，again，thehandfulbemadeupofdistinct
ordersofunits——aspebbles，coarsesand，anddust——thesewill，under
likeconditions，besegregatedwithgreaterdefiniteness。Thepebbleswill
dropalmostvertically;thesand，fallingobliquely，willdeposititself
withinatolerablycircumscribedspacebeyondthepebbles;whilethedust
willbeblownalmosthorizontallytoagreatdistance。Acaseinwhichanother
kindofforcecomesintoplay，willstillbetterillustratethistruth。Through
amixedaggregateofsolubleandinsolublesubstances，letwaterslowlypercolate。
Therewillinthefirstplacebeadistinctpartingofthesubstancesthat
arethemostwidelyunlike:thesolublewillbecarriedaway;theinsoluble
willremainbehind。Further，someseparation，thoughalessdefiniteone，
willbeeffectedamongthesolublesubstances;sincethefirstpartofthe
currentwillremovethemostsolubleinthelargestamounts，andafterthese
havebeendissolved，itwillcontinuetobringouttheremaininglesssoluble。
Eventheundissolvedmatterswillhavesimultaneouslyundergonesomesegregation;
forthepercolatingfluidwillcarrydowntheminutefragmentsfromamong
thelargeones，andwilloftendepositthoseofsmallspecificgravityin
oneplace，andthoseofgreatspecificgravityinanother。Tocompletethe
elucidationwemustglanceattheobversefact;namelythatmixedunitswhich
differbutslightly，aremovedinbutslightlydifferentwaysbyincident
forces，andcanthereforebeseparatedonlybysuchadjustmentsoftheincident
forcesasallowslightdifferencestobecomeappreciablefactorsintheresult。
Thepartingofalcoholfromwaterbydistillationisagoodexample。Here
wehavemoleculesconsistingofoxygenandhydrogen，mingledwithmolecules
consistingofoxygen，hydrogen，andcarbon。Thetwoordersofmoleculeshave
aconsiderablelikenessofnature:theysimilarlymaintainafluidformat
ordinarytemperatures;theysimilarlybecomegaseousmoreandmorerapidly
asthetemperatureisraised;andtheyboilatpointsnotveryfarapart。
Nowthiscomparativelikenessofthemoleculesisaccompaniedbydifficulty
insegregatingthem。Ifthemixedfluidisundulyheated，muchwaterdistils
overwiththealcohol:itisonlywithinanarrowrangeoftemperaturethat
moleculesoftheonekindaredrivenoffratherthantheothers;andeven
thennotafewoftheothersaccompanythem。Themostinterestingandinstructive
example，however，isfurnishedbycertainphenomenaofcrystallization。When
severalsaltsthathavelittleanalogyofconstitution，aredissolvedin
thesamebodyofwater，theyareseparatedwithoutmuchtrouble，bycrystallization:
subjectastheyaretouniformforces，theysegregate。Thecrystalsofeach
saltdo，indeed，usuallycontaincertainsmallamountsoftheothersalts
presentinthesolution;butfromthesetheyareseverallyfreedbyrepeated
re—solutionsandcrystallizations。Marknow，however，thatthereverseis
thecasewhenthesaltscontainedinthesamebodyofwaterarechemically
homologous。Thenitratesofbarytaandlead，orthesulphatesofzinc，soda，
andmagnesia，uniteinthesamecrystals;norwilltheycrystallizeseparately
ifthesecrystalsbedissolvedafresh，andafreshcrystallized。Onseeking
thecauseofthisanomaly，chemistsfoundthatsuchsaltswereisomorphous—that
theirmolecules，thoughnotchemicallyidentical，areidenticalintheproportions
ofacid，base，andwater，composingthem，andinthecrystallineformsthey
assumewhenuniting。Here，then，weseeclearlythatunitsofunlikekinds
areselectedoutandseparatedwithareadinessproportionatetothedegreeoftheirunlikeness。Thereisaconversecauseofsegregationwhichitisneedlesshereto
treatofwithequalfullness。Ifdifferentunitsactedonbythesameforce，
mustbedifferentlymoved;so，converselyunitsofthesamekindmustbe
differentlymovedbydifferentforces。Supposingsomegroupofunitsforming
partofahomogeneousaggregate，areunitedlyexposedtoaforcewhichis
unlikeinamountordirectiontotheforceactingontherestoftheaggregate，
thenthisgroupofunitswillseparatefromtherest，providedthat，ofthe
forcesoactingonit，thereremainsanyportionnotdissipatedinmolecular
vibrationsorabsorbedinproducingmolecularrearrangements。Afterallthathasbeensaidabove，thispropositionneedsnodefence。Beforeendingourpreliminaryexposition，acomplementarytruthmustbe
specified;namelythatmixedforcesaresegregatedbythereactionofuniform
matters，justasmixedmattersaresegregatedbytheactionofuniformforces。
Ofthistruthacompleteandsufficientillustrationisfurnishedbythe
dispersionofrefractedlight。Abeamoflight，madeupofetherealundulations
ofdifferentorders，isnotuniformlydeflectedbyahomogeneousrefracting
body;butthedifferentordersofundulationsitcontainsaredeflectedat
differentangles:theresultbeingthatthesedifferentordersofundulations
areseparatedandintegrated，andsoproducethecoloursofthespectrum。
Asegregationofanotherkindoccurswhenraysoflighttraverseanobstructing
medium。Thosewhichconsistofcomparativelyshortundulationsareabsorbed
beforethosewhichconsistofcomparativelylongones;andtheredrays，
whichconsistofthelongestundulations，alonepenetratewhentheobstruction
isverygreat。How，conversely，thereisproducedaseparationoflikeforces
bythereactionofunlikematters，isalsomademanifestbythephenomena
ofrefraction;sinceadjacentandparallelbeamsoflight，fallingon，andpassingthrough，unlikesubstances，aremadetodiverge。§164。Invaguewaystheheavenlybodiesexemplifythatcauseofmaterialsegregationlastassigned——theactionofunlikeforcesonlikeunits。IsayinvaguewaysbecauseourSiderealSystemdisplaysmoreofaggregation
thanofsegregation。Thattheirregularswarmsofstarsconstitutingthe
MilkyWaywithitsbranchesandgapsanddenserregions，havebeengathered
togetherfromamorewidelydiffusedstate，maybereasonablyinferred;though
asweknownothingoftheprecedingdistributionsuchachangecannotbeproved:stilllesscantherebeprovedasegregativeprocess。Itistruethatinclustersofstars，beginningwiththosehavingmembers
considerablydispersedandendingwiththosehavingmemberscloselyconcentrated—globular
clusters——weseestrongevidenceofaggregation;anditmaybecontended
thatsincethemutualgravitationsofthestarsformingacluster，differ
intheirdegreesanddirectionsfromthoseofthestarsfromwhichtheyhave
separated，thereisakindofsegregation。Butitmustbeadmittedthattheconformitytotheabove—namedprincipleisbutanindefiniteone。Thereare，however，twoclassesoffactswhichexhibitsegregation，though
theyleaveusignorantofitscauses。Thefirstisthatstar—clustersare
abundantalongthecourseoftheMilkyWay:byfarthelargernumberofthem
lyingintheneighbourhoodofitsplaneandrelativelyfewinregionson
eitherside。Thesecondisthat，contrariwise，thenebulaearesparselyscattered
inandaboutthegalacticcircleandarerelativelynumerousinthespaces
remotefromit。Thoughtherearethuspresentedtwocasesofsegregation
thereisnoevidencethatthesedifferentclassesofbodieshavebeenseparated
fromamixedassemblage，noristhereanyindicationoftheforcesbywhich
thiscontrastindistributionhasbeenproduced。Wecanonlysaythatthe
factsarecongruouswiththebeliefthatsegregation，probablyindirectratherthandirectinitscause，hasbeengoingon。Theformationanddetachmentofanebulousring，illustratesthesame
generalprinciple。Toconclude，asLaplacedid，thattheequatorialportion
ofarotatingnebulousspheroidwill，duringconcentration，acquireacentrifugal
forcesufficienttopreventitfromfollowingtherestofthecontracting
mass，istoconcludethatsuchportionswillremainbehindasareincommon
subjecttoacertaindifferentialforce。Thelineofdivisionbetweenthe
ringandthespheroid，mustbealineinsideofwhichtheaggregativeforce
isgreaterthantheforceresistingaggregation;andoutsideofwhichthe
forceresistingaggregationisgreaterthantheaggregativeforce。Hence
theallegedprocessconformstothelawthatamonglikeunits，exposedtounlikeforces，thesimilarlyconditionedseparatefromthedissimilarlyconditioned。###第37章§165。Thosegeologicchangesusuallyclassedasaqueous，display
undernumerousformsthesegregationofunlikeunitsbyauniformincident
force。Onseashoresthewavesareeversorting—outandseparatingthemixed
materialsagainstwhichtheybreak。Fromeachmassoffallencliff，thetide
carriesawayallthoseparticleswhicharesosmallastoremainlongsuspended
inthewater;and，atsomedistancefromshore，depositsthemintheshape
offinesediment。Largeparticles，sinkingwithcomparativerapidity，are
accumulatedintobedsofsandnearlowwater—mark。Thesmallpebblescollect
togetheratthebottomoftheinclineupwhichthebreakersrush;andon
thetopliethelargerstonesandboulders。Stillmorespecificsegregations
mayoccasionallybeobserved。Flatpebbles，producedbythebreakingdown
oflaminatedrock，aresometimesseparatelycollectedinonepartofashingle
bank。Onthisshorethedepositiswhollyofmud;onthatitiswhollyof
sand。Herewefindashelteredcovefilledwithsmallpebblesalmostofone
size;andthere，inacurvedbayoneendofwhichismoreexposedthanthe
otherweseeaprogressiveincreaseinthemassivenessofthestonesaswe
walkfromthelessexposedtothemoreexposedend。Tracethehistoryof
eachgeologicdeposit，andwearequicklyleddowntothefactthatmixed
fragmentsofmatter，differingintheirsizesorweights，are，whenexposed
tothemomentumandfrictionofwater，joinedwiththeattractionofthe
Earth，selectedfromoneanother，andunitedintogroupsofcomparatively
likefragments。Andweseethat，otherthingsequal，theseparationisdefinite
inproportionasthedifferencesoftheunitsaremarked。Aftertheyhave
beenformed，sedimentarystrataexhibitsegregationsofanotherkind。The
flintsandthenodulesofironpyritesthatarefoundinchalk，aswellas
thesiliciousconcretionswhichsometimesoccurinlimestone，areinterpreted
asaggregationsofmoleculesofsilexorsulphuretofiron，originallydiffused
throughthedeposit，butgraduallycollectedroundcentres，notwithstanding
thesolidorsemi—solidstateofthesurroundingmatter。Bogiron—oresuppliestheconditionsandtheresultinstillmoreobviouscorrelation。Amongigneouschangeswedonotfindsomanyexamplesoftheprocessdescribed。
Nevertheless，geologicalphenomenaofthisorderarenotbarrenofillustrations。
WherethemixedmatterscomposingtheEarth'scrusthavebeenraisedtoa
veryhightemperature，segregationcommonlytakesplaceasthetemperature
falls。Sundryofthesubstancesthatescapeinagaseousformfromvolcanoes，
sublimeintocrystalsoncomingagainstcoolsurfaces;andsolidifying，as
thesesubstancesdo，atdifferenttemperatures，theyaredepositedatdifferent
partsofthecrevicesthroughwhichtheyareemittedtogether。Thebestillustration，
however，isfurnishedbythechangesthatoccurduringtheslowcoolingof
igneousrock。When，throughoneofthefracturesfromtimetotimemadein
theEarth'scrust，aportionofthemoltennucleusisextruded，andwhen
thisiscooledwithcomparativerapidity，thereresultstraporbasalt——
asubstancethatisuniformintexture，thoughmadeupofvariousingredients。
Butwhen，notescapingthroughthesuperficialstrata，suchaportionof
themoltennucleusisslowlycooled，graniteistheresult:themingledparticles
ofquartz，feldspar，andmica，beingkeptforalongtimeinafluidand
semi—fluidstate——astateofcomparativemobility—undergothosechanges
ofpositionwhichtheforcesimpressedonthembytheirfellowunitsnecessitate。
Thedifferentialforcesarisingfrommutualpolarity，segregatethequartz，
feldspar，andmica，intocrystals。Howcompletelythisisdependentonthe
long—continuedagitationofthemixedparticles，andconsequentlong—continued
movablenessbysmalldifferentialforces，isprovedbythefactthatina
granitedykethecrystalsinthecentre，wherethefluidityorsemi—fluidity
continuedforalongertime，aremuchlargerthanthoseatthesides，wherecontactwiththeneighbouringrockcausedmorerapidcoolingandsolidification。§166。Theactionsgoingonthroughoutanorganismaresoinvolved，
thatwecannotexpecttoidentifytheforcesbywhichparticularsegregations
areeffected。Amongthefewinstancesadmitting。ofinterpretation，thebest
arethoseinwhichmechanicalpressuresandtensionsaretheagenciesatwork。Thespineofavertebrateanimalissubjectedtocertaingeneralstrains
——theweightofthebody，togetherwiththereactionsinvolvedbyallconsiderable
muscularefforts;andundertheseconditionsithasbecomesegregatedas
awhole。Atthesametimebeingexposedtodifferentforcesduringthose
lateralbendingswhichthemovementsnecessitate，itspartsretainacertain
separateness。Ifwetraceupthedevelopmentofthevertebralcolumnfrom
itsprimitiveformofacartilaginouscordinthelowestfishes，weseethat，
throughout，itmaintainsanintegrationcorrespondingtotheunityofthe
incidentforces，joinedwithadivisionintosegmentscorrespondingtothe
varietyoftheincidentforces。Eachsegment，consideredapart，exemplifies
thetruthmoresimply。Avertebraisnotasinglebone，butconsistsofa
centralmasswithsundryappendagesorprocesses，andinunfinishedtypes
ofvertebratheseappendagesareseparatefromthecentralmass，and，indeed，
existbeforeitmakesitsappearance。Buttheseseveralindependentbones
constitutingaprimitivespinalsegment，aresubjectedtoacertainaggregate
offorceswhichagreemorethantheydiffer:asthefulcrumtoagroupof
muscleshabituallyactingtogether，theyperpetuallyundergocertainreactions
incommon。Andaccordingly，inthecourseofdevelopment，theygradually
coalesce。Stillcleareristheillustrationfurnishedbyspinalsegments
thatbecomefusedtogetherwheretheyaretogetherexposedtosomepredominant
strain。Thesacrumconsistsofagroupofvertebrafirmlyunited。Inthe
ostrichanditscongenerstherearefromseventeentotwentysacralvertebra;
and，besidesbeingconfluentwithoneanother，theseareconfluentwith
theiliacbones，whichrunoneachsideofthem。If，now，weassumethese
vertebratohavebeenoriginallyseparate，astheystillareintheembryo
bird，andifweconsidertheforcestowhichtheymustinsuchcasehave
beenexposed，weshallseethattheirunionresultsintheallegedway。For
throughthesevertebratheentireweightofthebodyistransferredtothe
legs:thelegssupportthepelvicarch;thepelvicarchsupportsthesacrum;
andtothesacrumisarticulatedtherestofthespine，withalltheorgans
attachedtoitandupheldbyit。Hence，ifseparate，thesacralvertebra
mustbeheldfirmlytogetherbystrongly—contractedmuscles，andmust，by
implication，bepreventedfrompartakinginthoselateralmovementswhich
theothervertebraundergo——theymustbesubjectedtoacommonstrain，
whiletheyarepreservedfromstrainswhichwouldaffectthemdifferently;
andsotheyfulfiltheconditionsunderwhichsegregationoccurs。Butthe
casesinwhichcauseandeffectarebroughtintothemostobviousrelation，
aresuppliedbythelimbs。Themetacarpalbones（thosewhichinmansupport
thepalmofthehand）areseparatefromoneanotherinmostmammals:the
separateactionsofthetoesentailingonthemslightamountsofseparate
movements。Thisisnotsohoweverintheox—tribeandthehorse—tribe。In
theox—tribe，onlythemiddlemetacarpals（thirdandfourth）aredeveloped;
andthese，attainingmassiveproportions，coalescetoformthecannonbone。
Inthehorse—tribe，thesegregationiswhatwemaydistinguishasindirect:
thesecondandfourthmetacarpalsarepresentonlyasrudimentsunitedto
thesidesofthethird，whilethethirdisimmenselydeveloped;thusforming
acannonbonewhichdiffersfromthatoftheoxinbeingasinglecylinder，
insteadoftwocylindersfusedtogether。Themetatarsusinthesequadrupeds
exhibitsparallelchanges。Noweachofthesemetamorphosesoccurswherethe
differentbonesgroupedtogetherhavenolongeranydifferentfunctions，
butretainonlyacommonfunction。Thefeetofoxenandhorsesareusedsolely
forlocomotion——arenotput，likethoseofunguiculatemammals，topurposes
whichinvolvesomerelativemovementsofthemetacarpals。Thustheredirectly
orindirectlyresultsasinglemassofbonewheretheincidentforceissingle。
Andfortheinferencethatthesefactshaveacausalconnexion，wefindconfirmation
throughouttheentireclassofbirds，inthewingsandlegsofwhich，like
segregationsarefoundunderlikeconditions。Whilethissheetispassing
throughthepress（1862），afactillustratingthisgeneraltruthinayet
moreremarkablemanner，hasbeenmentionedtomebyProf。Huxleywhokindly
allowsmetomakeuseofitwhilestillunpublishedbyhim。TheGlyptodon，
anextinctmammalfoundfossilizedinSouthAmerica，haslongbeenknown
asalargeuncouthcreaturealliedtotheArmadillo，buthavingamassive
dermalarmourconsistingofpolygonalplatescloselyfittedtogethersoas
tomakeavastbox，inclosingthebodyinsuchwayaseffectuallytoprevent
itfrombeingbent，laterallyorvertically，intheslightestdegree。This
box，whichmusthaveweighedseveralhundredweight，wassupportedonthe
spinousprocessesofthevertebrae，andontheadjacentbonesofthepelvic
andthoracicarches。Andthesignificantfactisthathere，wherethetrunk
vertebraeweretogetherexposedtothepressureofthisheavydermalarmour，
atthesametimethat，byitsrigidity，theywerepreservedfromallrelativemovements，theywereunitedintoonesolid，continuousbone。Theformationandmaintenanceofaspecies，consideredasanassemblage
ofsimilarorganisms，isinterpretableinananalogousway。Alreadywehave
seenthatinsofarasthemembersofaspeciesaresubjecttodifferent
setsofincidentforces，theyaredifferentiated，ordividedintovarieties。
Hereitremainstoaddthatsuchofthemasaresubjecttolikesetsofincident
forces，aresegregated。Forbytheprocessof"naturalselection，"
thereisacontinualpurificationofeachspeciesfromthoseindividuals
whichdepartfromthecommontypeinwaysthatunfitthemfortheconditions
oftheirexistence。Consequently，thereisacontinualleavingbehindof
thoseindividualswhichareinallrespectsfitfortheconditionsoftheir
existence，andarethereforenearlyalike。Thecircumstancestowhichany
speciesisexposed，beinganinvolvedcombinationofincidentforces;and
themembersofthespecieshavingamongthemsomethatdiffermorethanis
usualfromtheaveragestructurerequiredformeetingtheseforces;itresults
thattheseforcesareconstantlyseparatingsuchdivergentindividualsfrom
therest，andsopreservingtheuniformityoftherest——keepingupits
integrityasaspeciesorvariety。Justasthechangingautumnleavesare
pickedoutbythewindfromamongthegreenonesaroundthem，orjustas，
touseProf。Huxley'ssimile，thesmallerfragmentspassthroughasieve
whilethelargerarekeptback;so，theuniformincidenceofexternalforces
affectsthemembersofagroupoforganismssimilarlyinproportionasthey
aresimilar，anddifferentlyinproportionastheyaredifferent;andthus
iseversegregatingthelikebypartingtheunlikefromthem。Whetherthese
separatedmembersarekilledoff，asmostlyhappens，orwhether，asotherwise
happens，theysurviveandmultiplyintoadistinctvariety，inconsequence
oftheirfitnesstocertainpartially—unlikeconditions，mattersnottothe
argument。Theonecaseconformstothelawthattheunlikeunitsofanaggregate
aresortedintotheirkindsandparted，whenuniformlysubjecttothesame
incidentforces，andtheothertotheconverselawthatthelikeunitsof
anaggregatearepartedandseparatelygroupedwhensubjecttodifferent
incidentforces。AndonconsultingMr。Darwin'sremarksondivergenceof
character，itwillbeseenthatthesegregationsthuscausedtendevertobecomemoredefinite。§167。Mentalevolutionunderoneofitsleadingaspects，wefound
toconsistintheformationinthemindofgroupsoflikeobjectsandlike
relations——adifferentiationofthevariousthingsoriginallyconfounded
togetherinoneassemblage，andanintegrationofeachseparateorderof
thingsintoaseparategroup（§153）。Hereitremainstopointoutthat
whileunlikenessintheincidentforcesisthecauseofsuchdifferentiations，
likenessintheincidentforcesisthecauseofsuchintegrations。Forwhat
istheprocessthroughwhichclassificationsareestablished？Howdoplants
becomegroupedinthemindofthebotanistintoorders，genera，andspecies？
Eachplantheexaminesyieldshimacertaincompleximpression。Nowandthen
hepicksupaplantlikeonebeforeseen;andtherecognitionofitisthe
productioninhimofalikeconnectedgroupofsensations，byalikeconnected
groupofattributes。Thatistosaythereisproducedthroughoutthenerve—centres
concerned，acombinedsetofchanges，similartoacombinedsetofchanges
beforeproduced。Consideredanalytically，eachsuchcombinedsetofchanges
isacombinedsetofmolecularmodificationswroughtintheaffectedpart
oftheorganism。Oneveryrepetitionoftheimpression，alikecombinedset
ofmolecularmodificationsissuperposedonthepreviousones，andmakes
themgreater:thusgeneratinganinternalplexusof。modifications，withits
answeringidea，correspondingtothesesimilarexternalobjects。Meanwhile，
anotherkindofplantproducesinthebrainofthebotanistanothersetof
molecularmodifications——asetwhichdoesnotagreewiththeonewehave
beenconsidering，butdisagreeswithit;andbyrepetitionofsuchthere
isgeneratedadifferentideaansweringtoadifferentspecies。What，now，
isthenatureofthisprocessexpressedingeneralterms？Ontheonehand
therearethelikeandunlikethingsfromwhichseverityemanatethegroups
offorcesbywhichweperceivethem。Ontheotherhand，therearetheorgans
ofsenseandpercipientcentres，throughwhich，inthecourseofobservation，
thesegroupsofforcespass。Inpassingthroughthemthelikegroupsofforces
aresegregated，orseparatedfromtheunlikegroupsofforces;andeachsuch
separateseriesofgroupsofforces，answeringtoanexternalgenusorspecies，
producesanideaofthegenusorspecies。Webeforesawthataswellasa
separationofmixedmattersbythesameforce，thereisaseparationofmixed
forcesbythesamematter;andherewemayfurtherseethattheunlikeforces
soseparated，workunlikestructuralchangesintheaggregatethatseparates
them——structuralchangeseachofwhichthusrepresentstheintegratedseriesofmotionsthathasproducedit。Byaparallelprocess，therelationsofco—existenceandsequenceamong
impressions，becomesortedintokindsandgrouped。Whentwophenomenathat
havebeenexperiencedinagivenorder，arerepeatedinthesameorder，those
nerve—centreswhichbeforewereaffectedbythetransitionareagainaffected;
andsuchmolecularmodificationastheyreceivedfromthefirstmotionpropagated
throughthemisincreasedbythissecondmotion。Eachsuchmotionworksa
structuralalterationwhich，inconformitywiththelawsetforthinChapter
IX，involvesadiminishedresistancetoallsuchmotionsthatafterwards
occur。Thesegregationofthesesuccessivemotions（ormorestrictly，the
permanentlyeffectiveportionsofexpandedtheminovercomingresistance）
thusbecomesthecauseof，andthemeasureof，thementalconnexionsbetween
theimpressionswhichthephenomenaproduced。Meanwhile，phenomenadifferent
fromthese，beingphenomenathataffectdifferentnervouselements，will
havetheirconnexionsseverallyrepresentedbymotionsalongotherroutes;
andalongeachoftheseotherroutes，thenervousdischargeswillseverally
takeplacewithareadinessproportionatetothefrequencywithwhichexperience
repeatstheconnexionsofphenomena。Theclassificationofrelationsmust
hencegoonparipassuwiththeclassificationoftherelatedthings。In
commonwiththemixedsensationsreceivedfromtheexternalworld，themixed
relationsitpresentscannotbeimpressedontheorganism。withoutmoreor
lesssegregationofthemresulting。Andthroughthiscontinuoussortingand
groupingofchangesormotions，whichconstitutesnervousfunction，there
isgraduallywroughtthatsortingandgroupingofmatter，whichconstitutesnervousstructure。§168。Insocialevolution，thecollectingtogetherofthelikeand
theseparationoftheunlikebyincidentforces，isprimarilydisplayedin
thesamemanneraswesawittobeamonggroupsofinferiorcreatures。The
humanracestendtodifferentiateandintegrate，asdoracesofotherlivingforms。Oftheforceswhicheffectandmaintainthesegregationsofmankind，may
firstbenamedthoseexternalonesclassedasphysicalconditions。Theclimate
andfoodwhicharefavourabletoanindigenouspeople，aremoreorlessdetrimental
toanalienpeopleofdifferentbodilyconstitution。Intropicalregions
thenorthernracescannotpermanentlyexist:ifnotkilledoffinthefirst
generation，theyaresointhesecond，and，asinIndia，canmaintaintheir
footingonlybytheartificialprocessofcontinuousimmigrationandemigration。
Thatistosay，theexternalforcesactingequallyontheinhabitantsof
agivenlocality，tendtoexpelallwhoarenotofacertaintype，andthus
tokeepuptheintegrationofthosewhoareofthattype。Evenamongthe
Indianpeoplesthemselvesthelikehappens:someofthehill—tribesbeing
segregatedbysurvivingthemalariousinfluenceswhichkilloffHinduswho
entertheirhabitat。Theotherfoxesconspiringtoproducethesenational
segregations，arethosementalonesshownintheaffinitiesofmenforothers
likethemselves。Unitsofonesocietywhoareobligedtoresideinanother，
generallyformcoloniesinthemidstofthatother——smallsocietiesof
theirown。Raceswhichhavebeenartificiallysevered，showtendenciesto
re—unite。Nowthoughthesesegregationscausedbythemutuallikingsofkindred
men，donotseemduetothegeneralprincipleenunciated，theyreallyare
thusinterpretable。Whentreatingofthedirectionofmotion（§80），
itwasshownthattheactionsperformedbymenforthesatisfactionoftheir
wants，arealwaysmotionsalonglinesofleastresistance。Thefeelingscharacterizing
amemberofagivenrace，arefeelingswhichgetcompletesatisfactiononly
amongothermembersofthatraceasatisfactionpartlyderivedfromsympathy
withthosehavinglikefeelings，butmainlyderivedfromtheadaptedsocial
conditionswhichgrowupwheresuchfeelingsprevail。When，therefore，a
citizenofanynationis，aswesee，attractedtowardsothersofhisnation，
therationaleisthatcertainagencieswhichwecalldesires，movehimin
thedirectionofleastresistance。Humanmotions，likeallothermotions，
beingdeterminedbythedistributionofforces，itfollowsthatsuchsegregations
ofracesasarenotproducedbyincidentexternalforces，areproducedbyforceswhichtheunitsoftheracesexerciseononeanother。Duringthedevelopmentofeachsocietyweseeanalogoussegregationscaused
inanalogousways。Afewofthemresultfromminornaturalaffinities;but
thosemostimportantoneswhichconstitutepoliticalandindustrialorganization，
resultfromtheunionofmeninwhomsimilaritieshavebeenproducedbytraining。
Menbroughtuptobodilylabouraremenwhohavehadwroughtinthemacertain
likeness——alikenesswhich，inrespectoftheirpowersofaction，obscures
andsubordinatestheirnaturaldifferences。Thosetrainedtobrain—workhave
acquiredacertainothercommunityofcharacterwhichmakesthem，associal
units，morelikeoneanotherthanlikethosetrainedtomanualoccupations。
Andthereariseclass—segregationsansweringtothesesuper—inducedlikenesses。
Moredefinitesegregationstakeplaceamongthemoredefinitelyassimilated
membersofanyclasswhoarebroughtuptothesamecalling。Evenwherethe
necessitiesoftheirworkforbidconcentrationinonelocality，asamong
artizanshappenswithmasonsandbricklayers，andamongtradershappenswith
theretaildistributers，andamongprofessionalshappenswiththemedical
men，therearenotwantingOperativeBuilders'Unions，andGrocers'Societies，
andMedicalAssociations，implyingaprocessofsiftingoutandgrouping。
Andwhere，asamongthemanufacturingclasses，thefunctionsdischargeddo
notrequirethedispersionofcitizenswhoareartificiallyassimilated，
thereisanaggregationoftheminspeciallocalities，andaconsequentincrease
inthedefinitenessofindustrialdivisions。If，now，weseekthecauses
ofthesesegregations，consideredasresultsofforceandmotion，weare
broughttothesamegeneralprincipleasbefore。Thislikenessproducedin
themembersofanyclassorsub—classbytraining，isanaptitudeacquired
bythemforsatisfyingtheirwantsinlikeways。Thatis，theoccupation
hasbecometoeachalineofleastresistance。Henceunderthatpressure
whichdeterminesallmentoactivitythesesimilarly——modifiedsocialunits
aresimilarlyaffected，andtendtotakesimilarcourses。If，then，there
beanylocalitywhich，eitherbyitsphysicalpeculiaritiesorbypeculiarities
wroughtonitduringsocialevolution，isrenderedaplacewhereacertain
kindofindustrialactionmeetswithlessresistancethanelsewhere，itfollows
fromthelawofdirectionofmotionthatthosesocialunitswhohavebeen
mouldedtothiskindofindustrialaction，willbesegregatedbymovingtowards
thisplace。If，forinstance，theproximityofcoalandironminestoanavigable
river，givestoGlasgowanadvantageinthebuildingofiron—ships—ifthe
totallabourrequiredtoproduceagivenvessel，andgetitsequivalentin
foodandclothing，islesstherethanelsewhere;thereiscausedaconcentration
ofiron—shipbuildersatGlasgow，eitherbydetentionofthepopulationborn
toiron—shipbuilding，orbyimmigrationofthoseelsewhereengagedinit，
orbyboth。Theprincipleequallyholdswheretheoccupationismercantile
insteadofmanufacturing。Stock—brokersclusterwheretheamountofeffort
tobeseverallygonethroughbythemindischargingtheirfunctions，and
obtainingtheirprofits，islessthanelsewhere。Alocalexchangehaving
oncebeenestablished，becomesaplacewheretheresistancetobeovercome
byeachissmallerthaninanyotherplace;and，beinglikeunitsunderstress
ofcommondesires，pursuitofthecourseofleastresistancebyeachinvolvestheiraggregationaroundthisplace。Ofcourse，withunitssocomplexasthosewhichconstituteasociety，
andwithforcessoinvolvedasthosewhichmovethem，theresultingselections
andseparationsmustbefarmoreentangled，orfarlessdefinite，thanthose
wehavehithertoconsidered。Formen'slikenessesbeingofvariouskinds，
leadtovariousordersofsegregation。Therearelikenessesofdisposition，
likenessesoftaste，likenessesproducedbyeducation，likenessesthatresult
fromclass—habits，likenessesofpoliticalfeeling;anditneedsbuttoglance
roundatthecaste—divisions，theassociationsforphilanthropic，scientific，
andartisticpurposes，thereligiouspartiesandsocialcliques，toseethat
somespeciesoflikenessamongthecomponentmembersofeachbodydetermines
theirunion。Nowthedifferentsegregativeprocesses，bytraversingoneanother
andoftenbytheirindirectantagonism，moreorlessobscureoneanother's
effects，andpreventanyonedifferentiatedclassfromcompletelyintegrating。