So that we might obtain the parent-species and its several modified descendants from the lower and upper beds of the same formation, and unless we obtained numerous transitional gradations, we should not recognise their blood-relationship, and should consequently rank them as distinct species. It is notorious on what excessively slight differences many palaeontologists have founded their species;
Era honetan, guraso-espeziea eta haren ondorengo eraldatu ezberdinak formazio bereko goi-eta behe-geruzetan aurki genitzake, eta iragaitzazko maila asko aurkitu ezean, ez genuke haien arteko odolezko ahaidetasunik hautemanen, eta, beraz, espezie ezberdintzat sailkatuko genituzke.