We ought only to look for a few links, and such assuredly we do find-some more distantly, some more closely, related to each other; and these links, let them be ever so close, if found in different stages of the same formation, would, by many palaeontologists, be ranked as distinct species.
Katemaila batzuk besterik ez ditugu bilatu behar, eta aurkitu ere, aurkituko ditugu, batzuk hurbilxeagotik, beste batzuk urruntxeagotik erlazionatuak; eta katemailok, hurbilenak izanik ere, formazio bereko estratu ezberdinetan aurkituz gero, palentologo askok espezie ezberdintzat sailkatuko dituzte.