388 THE ORIGIN OF SPECIES 



De Candolle has made nearly similar observations on the general 

 nature of the affinities of distinct families of plants. 



On the principle of the multiplication and gradual divergence 

 in character of the species descended from a common progenitor, 

 together with their retention by inheritance of some characters in 

 common, we can understand the excessively complex and radiat- 

 ing affinities by which all the members of the same family or 

 higher group are connected together. For the common progenitor 

 of a whole family, now broken up by extinction into distinct groups 

 and sub-groups, will have transmitted some of its characters, 

 modified in various ways and degrees, to all the species; and they 

 will 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 extraordinary 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 pro- 

 genitors of birds were formerly connected with the early progeni- 

 tors 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 classi- 

 fication, 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 



