Weallknowthatthearmandhandofamonkey,theforelegandfootofadogandofahorse, thewingofabat, andthefinofaporpoise, arefundamentallyidentical; thatthelongneckofthegiraffehasthesameandnomorebonesthantheshortoneoftheelephant;thattheeggsofSurinamfrogshatchintotadpoleswithasgoodtailsforswimmingasanyoftheirkindred,althoughastadpolestheyneverenterthewater; thattheGuinea-pigisfurnishedwithincisorteethwhichitneveruses, asitshedsthembeforebirth; thatembryosofmammalsandbirdshavebranchialslitsandarteriesrunninginloops, inimitationorreminiscenceofthearrangementwhichispermanentinfishes; andthatthousandsofanimalsandplantshaverudimentaryorganswhich, atleastinnumerouscases, arewhollyuselesstotheirpossessors, etc., ; andithasnotbeenexplainedinanyotherway.
Naturalistsareconstantlyspeakingof "relatedspecies," ofthe "affinity" ofagenusorothergroup,andof "familyresemblance"—vaguelyconsciousthatthesetermsofkinshiparesomethingmorethanmeremetaphors, ——withoutknowingit.
Ifitisdifficultandinmanycasespracticallyimpossibletofixthelimitsofspecies, itisstillmoresotofixthoseofgenera; , connectingonegroupwithanotherinamannersadlyperplexingtosystematists, , suchasthatwhichthehypothesisofderivationdemands.
