CLASSIFICATION 387 



(groups have since undergone much modification in 

 divergent directions. On either view we may suppose 

 that the bizcacha has retained, by inheritance, more 

 of the character of its ancient progenitor than have 

 other Rodents ; and therefore it will not be specially 

 related to any one existing Marsupial, but indirectly 

 to all or nearly all Marsupials, from having partially 

 retained the character of their common progenitor, or 

 of an early member of the group. On the other hand, 

 of all Marsupials, as Mr. Waterhouse has remarked, 

 the phascolomys resembles most nearly, not any one 

 species, but the general order of Rodents. In this 

 case, however, it may be strongly suspected that the re- 

 semblance is only analogical, owing to the phascolomys 

 having become adapted to habits like those of a Rodent. 

 The elder De Candolle has made nearly similar observa- 

 tions on the general nature of the affinities of distinct 

 orders of plants. 



On the principle of the multiplication and gradual 

 divergence in character of the species descended from 

 a common parent, 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 

 common parent of a whole family of species, 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 ; and the 

 several species 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 this without this aid, we can under- 

 stand the extraordinary difficulty which naturalists 

 have experienced in describing, without the aid of a 

 diagram, the various affinities which they perceive 



