238 THE ORIGIN OF SPECIES 



the extraordinury difficulty which naturalists have experi- 

 enced 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 distinctness of whole 

 classes from each other --for inptnoce, 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 to- 

 gether by a long and only partially broken chain of affin- 

 ities. 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 classi- 

 fication, or at least a natural arrangement, would be pos- 

 sible. 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 



