380 Classification. Chap. xiy. 



ordinary difficulty which 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 may thus account for the distinct- 

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

 value of the differences between them. This is what we should be 

 driven to, if we were ever to succeed in collecting all the forms in 

 any one class which have lived throughout all time and space. 

 Assuredly we shall never succeed in making so perfect a collec- 

 tion : nevertheless, in certain classes, we are tending towards this 

 end ; and Milne Edwards has lately insisted, in an able paper, on 



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