In the first place, it may be observed that the amount of external difference between two species is no sure guide to their degree of mutual sterility, so that similar differences in the case of varieties would be no sure guide.
Lehenik, egiazta daiteke espezie bik kanpo-itxuran duten aldea ez dela haien elkarrekiko antzutasun mailaren seinale seguru; era horretan, aldakien kasuan antzeko aldea ez litzateke seinale seguru.