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。*