Chap. XIV.] ORGANIC 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 with- 

 in 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 

 afi&nities. 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 natu- 

 ral 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 Si- 

 lurian 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 dis- 

 tinguished from their more immediate parents and de- 

 scendants. Yet the arrangement in the diagram would 

 still hold good and would be natural; for, on the prin- 

 ciple of inheritance, all the forms descended, for in- 

 stance, 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, whether large or small. 



