but if we confine our attention to any one formation, it becomes much more difficult to understand why we do not therein find closely graduated varieties between the allied species which lived at its commencement and at its close.
baina geure arreta formazio bakar batera mugatzen badugu, askoz zailago gertatzen da ulertzea zergatik ez dugun han aurkitzen formazioaren hasieran eta bukaeran bizi izan ziren espezie hurbilen bitarteko aldakien mailaz mailako serierik.