Nevertheless, this difficulty, though appearing to our imagination insuperably great, cannot be considered real if we admit the following propositions, namely, that all parts of the organisation and instincts offer, at least individual differences-that there is a struggle for existence leading to the preservation of profitable deviations of structure or instinct-and, lastly, that gradations in the state of perfection of each organ may have existed, each good of its kind.
Hala ere, zailtasun hau, gaindiezina dirudien arren, ezin da benetakotzat jo, baldin jarraian datozen proposizioak onartzen baditugu, alegia, organismoaren atal eta instintu guztiek dituztela gutxienez ezberdintasun indibidualak, existentziaren aldeko borroka dagoela, egitura edo instintuen eraldaketa onuragarriak kontserbatzera daramana, eta, azkenez, organo ororen hobezintasun egoeran mailaketak izan zitezkeela, guztiak onak, zein bere mailan.