No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, around which, according to the theory, there has been much extinction. Or again, if we take an organ common to all the members of a class, for in this latter case the organ must have been originally formed at a remote period, since which all the many members of the class have been developed; and in order to discover the early transitional grades through which the organ has passed, we should have to look to very ancient ancestral forms, long since become extinct.
Organo asko dira, dudarik gabe, guk ezagutzen ez ditugun iragaitza mailak dituztenak, batez ere espezie oso isolatuak kontuan hartzen baditugu, haien inguruan, teoriaren arabera, iraungipen handia izan delarik. Eta gauza bera gertatzen da, klase bateko kide guztiek duten organo bat hartzen badugu ere, organo horrek aldi urrunen batean formatua izan behar duelako, klase horretako kide ugari guztiak horren ondoren garatu izan direlarik; eta organoak pasa dituen hasierako iragaitza-mailak aurkitzeko, oso antzinako arbasoen formak, aspaldi iraungiak, bilatu beharko genituzke.