228 Ai^PlNlTIBS CONNECTING [Chap. XlV. 



to the Phascolomys having become adapted to habits 

 like those of a Eodent. The elder 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 un- 

 derstand the excessively complex and radiating affini- 

 ties 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 ex- 

 tinction 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 con- 

 sequently be related to each other by circuitous lines 

 of affinity of variouB lengths (as may be seen in the dia- 

 gram so often referred to), mounting up through many 

 predecessors. As it is difficult to show the blood-re- 

 lationship between the numerous kindred of any an- 

 cient 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 per- 

 ceive 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 instance, of birds from all other 

 vertebrate animals — by the belief that many ancient 



