In looking to the long succession of past ages, as in looking to distant provinces throughout the world, we find that species in certain classes differ little from each other, whilst those in another class, or only in a different section of the same order, differ greatly from each other.
Iragan aro luzeen joana kontuan hartuz, eta munduan zehar diren herrialde urrunduak ere kontuan hartuz, zenbait klasetako espezieak bata bestetik ezer gutxi ezberdintzen direla aurkitzen dugu, beste zenbait klasetakoak edota ordena bereko sekzio ezberdinetakoak, aldiz, bata bestetik asko bereizten diren artean.