470 ORIGIN OF SPECIES 



consequently be related to each other by circuitous lines of 

 affinity of various lengths (as may be seen in the diagram so 

 often referred to), mounting up through many predecessors. 

 As it is difficult to show the blood relationship between the 

 numerous kindred of any ancient and noble family even by 

 the aid of a genealogical tree, and almost impossible to do so 

 without this aid, we can understand the extraodinary diffi- 

 culty which naturalists have experienced in describing, with- 

 out 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 inter- 

 vals between the several groups in each class. We may thus 

 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 ut- 

 terly 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 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 natu- 

 ral 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 



