Extinction, as we have seen in the fourth chapter, has played an important part in defining and widening the intervals between the several groups in each class.

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.

There has been much less extinction of the forms of life which once connected fishes with Batrachians.