Itisnotsurprisingthatthedegreeofdifficultyinunitingtwospecies，andthedegreeofsterilityoftheirhybridoffspring，shouldgenerallycorrespond，thoughduetodistinctcauses；forbothdependontheamountofdifferenceofsomekindbetweenthespecieswhicharecrossed。Norisitsurprisingthatthefacilityofeffectingafirstcross，thefertilityofhybridsproducedfromit，andthecapacityofbeinggraftedtogether——thoughthislattercapacityevidentlydependsonwidelydifferentcircumstances——shouldallruntoacertainextentparallelwiththesystematicaffinityoftheformswhicharesubjectedtoexperiment；forsystematicaffinityattemptstoexpressallkindsofresemblancebetweenallspecies。
\"Firstcrossesbetweenformsknowntobevarieties，orsufficientlyaliketobeconsideredasvarieties，andtheirmongreloffspring，areverygenerally，butnotquiteuniversally，fertile。Noristhisnearlygeneralandperfectfertilitysurprising，whenwerememberhowliablewearetoargueinacirclewithrespecttovarietiesinastateofNature；andwhenwerememberthatthegreaternumberofvarietieshavebeenproducedunderdomesticationbytheselectionofmereexternaldifferences，andnotofdifferencesinthereproductivesystem。Inallotherrespects，excludingfertility，thereisaclosegeneralresemblancebetweenhybridsandmongrels。\"——Pp。276-8。
Wefullyagreewiththegeneraltenorofthisweightypassage；butforcibleasarethesearguments，andlittleasthevalueoffertilityorinfertilityasatestofspeciesmaybe，itmustnotbeforgottenthatthereallyimportantfact，sofarastheinquiryintotheoriginofspeciesgoes，is，thattherearesuchthingsinNatureasgroupsofanimalsandofplants，whosemembersareincapableoffertileunionwiththoseofothergroups；andthattherearesuchthingsashybrids，whichareabsolutelysterilewhencrossedwithotherhybrids。For，ifsuchphenomenaasthesewereexhibitedbyonlytwoofthoseassemblagesoflivingobjects，towhichthenameofspecies（whetheritbeusedinitsphysiologicalorinitsmorphologicalsense）isgiven，itwouldhavetobeaccountedforbyanytheoryoftheoriginofspecies，andeverytheorywhichcouldnotaccountforitwouldbe，sofar，imperfect。
Uptothispoint，wehavebeendealingwithmattersoffact，andthestatementswhichwehavelaidbeforethereaderwould，tothebestofourknowledge，beadmittedtocontainafairexpositionofwhatisatpresentknownrespectingtheessentialpropertiesofspecies，byallwhohavestudiedthequestion。Andwhatevermaybehistheoreticalviews，nonaturalistwillprobablybedisposedtodemurtothefollowingsummaryofthatexposition：——
Livingbeings，whetheranimalsorplants，aredivisibleintomultitudesofdistinctlydefinablekinds，whicharemorphologicalspecies。Theyarealsodivisibleintogroupsofindividuals，whichbreedfreelytogether，tendingtoreproducetheirlike，andarephysiologicalspecies。Normallyresemblingtheirparents，theoffspringofmembersofthesespeciesarestillliabletovary；andthevariationmaybeperpetuatedbyselection，asarace，whichrace，inmanycases，presentsallthecharacteristicsofamorphologicalspecies。Butitisnotasyetprovedthataraceeverexhibits，whencrossedwithanotherraceofthesamespecies，thosephenomenaofhybridizationwhichareexhibitedbymanyspecieswhencrossedwithotherspecies。Ontheotherhand，notonlyisitnotprovedthatallspeciesgiverisetohybridsinfertile’interse’，butthereismuchreasontobelievethat，incrossing，speciesexhibiteverygradationfromperfectsterilitytoperfectfertility。