Theconditionsfavourabletoselectionbymanare，——theclosestattentiontoeverycharacter，——long—continuedperseverance，——facilityinmatchingorseparatinganimals，——andespeciallyalargenumberbeingkept，sothattheinferiorindividualsmaybefreelyrejectedordestroyed，andthebetteronespreserved。Whenmanyarekepttherewillalsobeagreaterchanceoftheoccurrenceofwell—markeddeviationsofstructure。Lengthoftimeisall—
important;foraseachcharacter，inordertobecomestronglypronounced，hastobeaugmentedbytheselectionofsuccessivevariationsofthesamekind，thiscanbeeffectedonlyduringalongseriesofgenerations。Lengthoftimewill，also，allowanynewfeaturetobecomefixedbythecontinuedrejectionofthoseindividualswhichrevertorvary，andbythepreservationofthosewhichstillinheritthenewcharacter。Hence，althoughsomefewanimalshavevariedrapidlyincertainrespectsundernewconditionsoflife，asdogsinIndiaandsheepintheWestIndies，yetalltheanimalsandplantswhichhaveproducedstronglymarkedracesweredomesticatedatanextremelyremoteepoch，oftenbeforethedawnofhistory。Asaconsequenceofthis，norecordhasbeenpreservedoftheoriginofourchiefdomesticbreeds。Evenatthepresentdaynewstrainsorsub—breedsareformedsoslowlythattheirfirstappearancepassesunnoticed。Amanattendstosomeparticularcharacter，ormerelymatcheshisanimalswithunusualcare，andafteratimeaslightdifferenceisperceivedbyhisneighbours;——thedifferencegoesonbeingaugmentedbyunconsciousandmethodicalselection，untilatlastanewsub—breedisformed，receivesalocalname，andspreads;butbythistimeitshistoryisalmostforgotten。Whenthenewbreedhasspreadwidely，itgivesrisetonewstrainsandsub—breeds，andthebestofthesesucceedandspread，supplantingotherandolderbreeds;andsoalwaysonwardsinthemarchofimprovement。
Whenawell—markedbreedhasoncebeenestablished，ifnotsupplantedbystillfurtherimprovedsub—breeds，andifnotexposedtogreatlychangedconditionsoflifeinducingfurthervariabilityorreversiontolong—lostcharacters，itmayapparentlylastforanenormousperiod。Wemayinferthatthisisthecasefromthehighantiquityofcertainraces;butsomecautionisnecessaryonthishead，forthesamevariationmayappearindependentlyafterlongintervalsoftime，orindistantplaces。Wemaysafelyassumethatthishasoccurredwiththeturnspit—dog，ofwhichoneisfiguredontheancientEgyptianmonuments——withthesolid—hoofedswine（28/11。Godron’Del’Espece’
tome11859page368。）mentionedbyAristotle——withfive—toedfowlsdescribedbyColumella——andcertainlywiththenectarine。ThedogsrepresentedontheEgyptianmonuments，about2000B。C。，showusthatsomeofthechiefbreedsthenexisted，butitisextremelydoubtfulwhetheranyareidenticallythesamewithourpresentbreeds。AgreatmastiffsculpturedonanAssyriantomb，640B。C。，issaidtobethesamewiththedogstillimportedfromThibetintothesameregion。ThetruegreyhoundexistedduringtheRomanclassicalperiod。
Comingdowntoalaterperiod，wehaveseenthat，thoughmostofthechiefbreedsofthepigeonexistedbetweentwoandthreecenturiesago，theyhavenotallretainedexactlythesamecharactertothepresentday;butthishasoccurredincertaincasesinwhichnoimprovementwasdesired，forinstance，inthecaseoftheSpotandIndianground—tumbler。