420 AFFINITIES CONNECTING 



like those of a Rodent. The elder De Candolle has made 

 nearly similar observations on the general nature of the affin- 

 ities of distinct families of plants. 



On the principle of the multiplication and gradual diver- 

 gence 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 exces- 

 sively complex and radiating 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 predeces- 

 sors. As it is difficult to show the blood relationship be- 

 tween the numerous kindred of any ancient and noble family 

 even by the aid of a genealogical tree, and almost impossi- 

 ble to do so without this aid, we can understand the ex- 

 traordinary 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 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 ani- 

 mals — 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 classifica- 

 tion, or at least a natural arrangement, would be possible. 



