Agricult。Soc。’volume4page43。）Althoughfreecrossingisadangerontheonesidewhicheveryonecansee，toocloseinterbreedingisahiddendangerontheotherside。Unfavourableconditionsoflifeoverrulethepowerofselection。Ourimprovedheavybreedsofcattleandsheepcouldnothavebeenformedonmountainouspastures;norcoulddray—horseshavebeenraisedonabarrenandinhospitableland，suchastheFalklandIslands，whereeventhelighthorsesofLaPlatarapidlydecreaseinsize。ItseemsimpossibletopreserveseveralEnglishbreedsofsheepinFrance;forassoonasthelambsareweanedtheirvigourdecaysastheheatofthesummerincreases（21/42。
Malingie—Nouel’JournalR。Agricult。Soc。’volume141853pages215，217。）:itwouldbeimpossibletogivegreatlengthofwooltosheepwithinthetropics;
yetselectionhaskepttheMerinobreednearlytrueunderdiversifiedandunfavourableconditions。Thepowerofselectionissogreat，thatbreedsofthedog，sheep，andpoultry，ofthelargestandsmallestsize，longandshortbeakedpigeons，andotherbreedswithoppositecharacters，havehadtheircharacteristicqualitiesaugmented，thoughtreatedineverywayalike，beingexposedtothesameclimateandfedonthesamefood。Selection，however，iseithercheckedorfavouredbytheeffectsofuseorhabit。Ourwonderfully—
improvedpigscouldneverhavebeenformediftheyhadbeenforcedtosearchfortheirownfood;theEnglishracehorseandgreyhoundcouldnothavebeenimproveduptotheirpresenthighstandardofexcellencewithoutconstanttraining。
Asconspicuousdeviationsofstructureoccurrarely，theimprovementofeachbreedisgenerallytheresultoftheselectionofslightindividualdifferences。Hencetheclosestattention，thesharpestpowersofobservation，andindomitableperseverance，areindispensable。Itis，also，highlyimportantthatmanyindividualsofthebreedwhichistobeimprovedshouldberaised;