andthereareindicationsthattheplanetsdifferfromoneanotherintheir
　　properheats，aswellasintheheatswhichtheyreceivefromthesun。Bearing
　　inmindthattheyalsodifferintheinclinationsoftheirorbits，theinclinations
　　oftheiraxes，intheirspecificgravities，andintheirphysicalconstitutions，
　　weseehowdecidedisthecomplexitywroughtintheSolarSystembythosesecondaryredistributionswhichhaveaccompaniedtheprimaryredistribution。§118。Passingfromillustrations，which，asassumingthenebular
　　hypothesis，mustbeclassedasmoreorlesshypothetical，letusdescendtoevidencelessopentoobjection。ItisnowagreedamonggeologiststhattheEarthwasonceamoltenmass。
　　Originally，then，itwascomparativelyhomogeneousinconsistence;and，because
　　ofthecirculationwhichtakesplaceinheatedliquids，musthavebeencomparatively
　　homogeneousintemperature。Itmust，too，havebeensurroundedbyanatmosphere
　　consistingpartlyoftheelementsofairandwater，andpartlyofthosevarious
　　otherelementswhichassumegaseousformsathightemperatures。Coolingby
　　radiationmust，afteranimmensetime，haveresultedindifferentiatingthe
　　portionmostabletopartwithitsheat;namely，thesurface。Afurthercooling，
　　leadingtodepositionofallsolidifiableelementscontainedintheatmosphere，
　　andthentoprecipitationofthewater，leavingbehindtheair，mustthus
　　havecausedasecondmarkeddifferentiation;andasthecondensationcommenced
　　onthecoolestpartsofthesurface—namely，aboutthepolestheremustsohaveresultedthefirstgeographicaldistinctions。Totheseillustrationsofgrowingheterogeneity，inferredfromknownlaws，
　　Geologyaddsanextensiveseriesthathavebeeninductivelyestablished。
　　TheEarth'sstructurehasbeenageafteragefurthercomplicatedbyadditions
　　tothestratawhichformitscrust;andithasbeenageafteragemademore
　　variousbytheincreasingcompositionofthesestrata;themorerecentof
　　which，formedfromthedetritusofthemoreancient，aremanyofthemrendered
　　highlycomplexbythemixturesofmaterialstheycontain。Thisheterogeneity
　　hasbeenvastlyaugmentedbytheactionsoftheEarth'snucleusonitsenvelope;
　　whencehaveresultednotonlymanykindsofigneousrocks，butthetilting
　　upofsedimentarystrataatallangles，theformationoffaultsandmetallic
　　veins，theproductionofendlessdislocationsandirregularities。Again，
　　geologiststeachusthattheEarth'ssurfacehasbeengrowingmorevaried
　　inelevation——thatthemostancientmountain—systemsarethesmallest，
　　andtheAndesandHimalayasthemostmodern;while，inallprobability，there
　　havebeencorrespondingchangesinthebedoftheocean。Asaconsequence
　　ofthisceaselessmultiplicationofdifferences，wenowfindthatnoconsiderable
　　portionoftheEarth'sexposedsurface，islikeanyotherportion，eitherincontour，ingeologicstructure，orinchemicalcomposition。Therehasbeensimultaneouslygoingonagradualdifferentiationofclimates。
　　AsfastastheEarthcooledanditscrustsolidified，inequalitiesoftemperature
　　arosebetweenthosepartsofitssurfacemostexposedtotheSunandthose
　　lessexposed;andthusintimetherecametobethemarkedcontrastsbetween
　　regionsofperpetualiceandsnowregionswherewinterandsummeralternately
　　reignforperiodsvaryingaccordingtothelatitude，andregionswheresummer
　　followssummerwithscarcelyanappreciablevariation。Meanwhile，elevations
　　andsubsidences，recurringhereandthereovertheEarth'scrust，andproducing
　　irregulardistributionsoflandandsea，haveentailedvariousmodifications
　　ofclimatebeyondthosedependentonlatitude;whileayetfurtherseries
　　ofsuchmodificationshasbeencausedbyincreaseddifferencesofheight
　　inthesurface，whichinsundryplaceshavebroughtarctic，temperate，and
　　tropicalclimatestowithinafewmilesofoneanother。Thegeneralresults
　　are，thateveryextensiveregionhasitsownmeteorologicalconditions，and
　　thateverylocalityineachregiondiffersmoreorlessfromothersinthoseconditions:asalsoinitsstructure，itscontour，itssoil。ThusbetweenourexistingEarth，thephenomenaofwhosevariedcrustneither
　　geographers，geologists，mineralogists，normeteorologistshaveyetenumerated，
　　andthemoltenglobeoutofwhichitwasevolved，thecontrastinheterogeneityisstriking。§119。Theclearest，mostnumerous，andmostvariedillustrations
　　oftheadvanceinmultiformitythataccompaniestheadvanceinintegration，
　　arefurnishedbylivingbodies。Distinguishedasthesearebythegreatquantity
　　oftheircontainedmolecularmotion，theyexhibitinanextremedegreethe
　　secondaryre—distributionswhichcontainedmotionfacilitates。Thehistory
　　ofeveryplantandeveryanimal，whileitisahistoryofincreasingbulk，
　　isalsoahistoryofsimultaneously—increasingdifferencesamongtheparts。Thistransformationhasseveralaspects。Thechemicalcompositionwhichisalmostuniformthroughoutthesubstance
　　ofagerm，vegetaloranimal，graduallyceasestobeuniform。Theseveral
　　compounds，nitrogenousandnon—nitrogenous，whichwerehomogeneouslymixed，
　　segregatebydegrees，becomediverselyproportionedindiverseplaces，and
　　producenewcompoundsbytransformationormodification。Inplantsthealbuminous
　　andamylaceousmatterswhichformthesubstanceoftheembryo，giveorigin
　　heretoapreponderanceofchlorophyllandtheretoapreponderanceofcellulose。
　　Overthepartsthatarebecomingleaf—surfaces，certainofthematerials
　　aremetamorphosedintowax。Inthisplacestarchpassesintooneofitsisomeric
　　equivalents，sugar;andinthatplaceintoanotherofitsisomericequivalents，
　　gum。Bysecondarychangesomeofthecelluloseismodifiedintowood;while
　　someofitismodifiedintothealliedsubstancewhich，inlargemasses，
　　wecallcork。Andthemorenumerouscompoundsthusarising，initiatefurther
　　unlikenessesbyminglinginunlikeratios。Theyelk，oressentialpartof
　　ananimal—ovum，havingcomponentswhichareatfirstevenlydiffusedamong
　　oneanother，chemicallytransformsitselfinlikemanner。Itsprotein，its
　　fats，itssalts，becomedissimilarlyproportionedindifferentlocalities;
　　andmultiplicationofisomericformsleadstofurthermixturesandcombinations
　　thatconstituteminordistinctionsofparts。Hereamassdarkeningbyaccumulation
　　ofhematine，presentlydissolvesintoblood。Therefattyandalbuminousmatters
　　uniting，composenerve—tissue。Atthisspotthenitrogenoussubstancetakes
　　onthecharacterofcartilage;andatthat，calcareoussalts，gatheringtogether
　　inthecartilage，laythefoundationofbone。Allthesechemicaldifferentiationsslowlybecomemoremarkedandmorenumerous。Simultaneouslyarisecontrastsofminutestructure。Distincttissuestake
　　theplaceofmatterthathadpreviouslynorecognizableunlikenessesofparts;
　　andeachofthetissuesfirstproducedundergoessecondarymodifications，
　　causingsub—speciesoftissues。Thegranularprotoplasmofthevegetalgerm，
　　equallywiththatwhichformstheunfoldingpointofeveryshoot，givesorigin
　　tocellsthatareatfirstalike。Someofthese，astheygrow，flattenand
　　unitebytheiredgestoformtheouterlayer。Otherslengthen，andatthe
　　sametimejointogetherinbundlestolaythefoundationofwoody—fibre。
　　Beforemuchelongating，certainofthesecellsshowabreaking—upofthe
　　liningdeposit，which，duringelongation，becomesaspiralthread，orareticulated
　　framework，oraseriesofrings;andbythelongitudinalunionofcellsso
　　lined，vesselsareformed。Meanwhileeachofthesedifferentiatedtissues
　　isre—differentiated:instancethatconstitutingtheessentialpartofa
　　leaf，theupperstratumofwhichiscomposedofchlorophyll—cellsremaining
　　closelypacked，whilethelowerstratumbecomesspongy。Ofthesamegeneral
　　characterarethetransformationsundergonebythefertilizedovum，which，
　　atfirstaclusterofsimilarcells，quicklyreachesastagemarkedbydissimilarity
　　ofthecells。Morefrequentlyrecurringfissionofthesuperficialcells，
　　aresultingsmallersizeofthem，andsubsequentunionofthemintoanouter
　　layer，constitutethefirstdifferentiation;andthemiddleareaofthis
　　layerisrenderedunliketherestbystillmoreactiveprocessesoflike
　　kind。Bysuchmodificationsuponmodifications，manyandvarious，arisethe
　　classesandsub—classesoftissueswhich，intricatelycombinedonewithanother，composeorgans。Equallyconformingtothelawarethechangesingeneralshapeandin
　　theshapesoforgans。Allgermsareatfirstspheresandalllimbsareat
　　firstbudsormereroundedlumps。Fromthisprimordialuniformityandsimplicity，
　　theretakeplacedivergences，bothofthewholesandoftheleadingparts，
　　towardsmultiformityofcontourandtowardscomplexityofcontour。Remove
　　thecompactly—foldedyoungleavesthatterminateeveryshoot，andthenucleus
　　isfoundtobeacentralknobbearinglateralknobs，oneofwhichmaygrow
　　intoeitheraleaf，asepal，apetal，astamen，oracarpel:alltheseeventually
　　——unlikepartsbeingatfirstalike。Theshootsthemselvesalsodepartfrom
　　theirprimitiveunityofform;andwhileeachbranchbecomesmoreorless
　　differentfromtherest，thewholeexposedpartoftheplantbecomesdifferent
　　fromtheimbeddedpart。So，too，isitwiththeorgansofanimals。Oneof
　　theArthropoda，forinstance，haslimbsthatwereoriginallyindistinguishable
　　fromoneanother——composedahomogeneousseries;butbycontinuousdivergences
　　therehavearisenamongthemunlikenessesofsizeandform，suchaswesee
　　inthecrabandthelobster。Vertebratecreaturesequallyexemplifythis
　　truth。Thewingsandlegsofabirdareofsimilarshapeswhentheybud—outfromthesidesoftheembryo。Thusineveryplantandanimal，conspicuoussecondaryre—distributions
　　accompanytheprimaryre—distribution。Afirstdifferencebetweentwoparts;
　　ineachofthesepartsotherdifferenceswhichpresentlybecomeasmarked
　　asthefirst;andalikemultiplicationofdifferencesingeometricalprogression，
　　untilthereisreachedthatcomplexcombinationconstitutingtheadult。This
　　isthehistoryofalllivingthingswhatsoever。PursuinganideawhichHarvey
　　setafloat，ithasbeenshownbyWolffandVonBaer，thatduringitsdevelopment
　　eachorganismpassesfromastateofhomogeneitytoastateofheterogeneity。Foragenerationthistruthhasbeenacceptedbybiologists。*