448 AFFINITIES CONNECTING 



naturalists have experienced in describing, without the aid 

 of a diagram, the various affinities which they perceive 

 between the many living and extinct members of the same 

 great natural class. 



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 

 mavthus 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 pro- 

 genitors of birds were formerly connected with the early 

 progenitors of the other and at that time less differentiatecl 

 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 won- 

 derfully diverse forms are still linked together by a long 

 and only partially broken chain of affinities. Extinction 

 has only defined the groups: it had 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 im- 

 possible 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 repre- 

 sent 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 distin- 

 guished from their more immediate parents and descend- 

 ants. 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 dis- 

 tinguish this or that branch, though at the actual fork the 

 two unite and blond 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, 

 whether large or small, and thus give a general idea of the 



