228 AFFINITIES CONNECTING [Chap. XIV. 



to the Phascolomjs having become adapted to habits 

 like those of a Eodent. The elder De CandoUe 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 radiating 

 affinities by which all the members of the same family 

 or higher group are connected together. For the com- 

 mon 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 genea- 

 logical tree, and almost impossible to do so without this 

 aid, we can understand the extraordinary difficulty 

 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 

 intervals between the several groujDs 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 



