It is not surprising that the difficulty in crossing any two species, and the sterility of their hybrid offspring, should in most cases correspond, even if due to distinct causes:
Baina, espezie ezberdinen kasuan, ez dakigu zergatik geratu behar izan duten haien sexu-elementuak hain eskuarki gutxi-asko eraldaturik, haiek elkarrekiko antzutasunera eramanez;