As species have generally diverged in character during their long course of descent and modification, we can understand why it is that the more ancient forms, or early progenitors of each group, so often occupy a position in some degree intermediate between existing groups.
Espezieak, eskuarki, beren ondorengotza eta eraldaketetako bide luzean, ezaugarrietan urrundu egin direnez gero, uler dezakegu nola gertatzen den formarik zaharrenek, edo talde bakoitzaren gurasolehenek, hain sarri izatea neurri batean egun diren taldeen bitartekoa den lekua.