When the same organ is found in several members of the same class, especially if in members having very different habits of life, we may generally attribute its presence to inheritance from a common ancestor; and its absence in some of the members to loss through disuse or natural selection.
Organo bera klase bereko zenbait kidetan aurkitzen denean, bereziki bizi-aztura oso ezberdinak dituztenetan, haren presentzia jeneralean arbaso komun bategandiko herentziari iratxeki diezaiokegu; eta haren ausentzia, erabilerarik eza edo hautespen naturala bide, kideetako batzuengan galdu izanari.