But the subject is surrounded by difficulties, for, looking to varieties produced under nature, if two forms hitherto reputed to be varieties be found in any degree sterile together, they are at once ranked by most naturalists as species.
Baina auzia zailtasunez inguraturik dago; izan ere, naturan jaioriko aldakiei dagokienez, baldin orain arte aldakitzat joriko bi forma neurriren batean elkarrekiko antzuak direla aurkitzen bada, naturalistarik gehienek espezietzat sailkatuko dituzte berehala.