Chap. XIV.] OEGANIC BEINGS, 229 



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. There has been still less 

 within some whole classes, for instance the Crustacea, 

 for here the most wonderfully diverse forms are still 

 linked together by a long and only partially broken 

 chain of affinities. Extinction has only defined the 

 groups : it has by no means made them ; for if every 

 form which has ever lived on this earth were suddenly 

 to reappear, though it would be quite impossible to give 

 definitions by which each group could be distinguished, 

 still a natural classification, or at least a natural 

 arrangement, would be possible. We shall see this by 

 turning to the diagram; the letters, A to L,may represent 

 eleven Silurian genera, some of which have produced 

 large groups of modified descendants, with every link in 

 each branch and sub-branch still alive ; and the links 

 not greater than those between existing varieties. In 

 this case it would be quite impossible to give definitions 

 by which the several members of the several groups 

 could be distinguished from their more immediate 

 parents and descendants. Yet the arrangement in the 

 diagram would still hold good and would be natural ; 

 for, on the principle of inheritance, all the forms 

 descended, for instance, from A, would have something 

 in common. In a tree we can distinguish this or that 

 branch, though at the actual fork the two unite and 

 blend together. We could not, as I have said, define 

 the several groups ; but we could pick out types, or 

 forms, representing most of the characters of each group, 



