432 CLASSIFICATION. Chap. XIII. 



together by a long, but broken, chain of affinities. 

 Extinction has only separated 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 from other groups, as 

 all would blend together by steps as fine as those be- 

 tween the finest existing varieties, nevertheless 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. Every intermediate link between these 

 eleven genera and their primordial parent, and every 

 intermediate link in each branch and sub-branch of 

 their descendants, may be supposed to be still alive ; 

 and the links to be as fine as those between the finest 

 varieties. In this case it would be quite impossible to 

 give any definition by which the several members of the 

 several groups could be distinguished from their more 

 immediate parents ; or these parents from their ancient 

 and unknown progenitor. Yet the natural arrangement 

 in the diagram would still hold good ; and, on the prin- 

 ciple of inheritance, all the forms descended from A, or 

 from I, would have something in common. In a tree we 

 can specify 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 cha- 

 racters 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 class which have lived throughout all time and 

 space. We shall certainly never succeed in making 



