We may thus account for the distinctness of whole classes from each other-for instance, of birds from all other vertebrate animals-by the belief that many ancient forms of life have been utterly lost, through which the early progenitors of birds were formerly connected with the early progenitors of the other and at that time less differentiated vertebrate classes.
Klase osoak bata bestetik hain bereiziak izatea, hegaztiak eta gainerako animalia ornodunak adibidez, honako hau suposatuz uler dezakegu, alegia, zeharo galdu direla antzinako forma organiko asko, hegaztien gurasolehenak antzinako garaietan, orduan hain bereizituak ez ziren beste ornodun klase batzuetako hasierako gurasolehenekin lotzen zituztenak.