OSGANIC BEINQS. 447 



be due in accordance with our view to inheritance from a 

 common progenitor. Therefore, we must suppose either 

 that all Rodents, including thebizcacha, branched off from 

 some ancient Marsupial, which will naturally have been 

 more or less intermediate in character with respect to all 

 existing Mursupials; or that both Eodents and Marsupials 

 branched off from a common progenitor, and that both 

 groups have since undergone much modification in divergent 

 directions. On either view we must suppose that the 

 bizcacha has retained, by inheritance, more of the char- 

 acters 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 Mar- 

 supials, from having partially retained the character of 

 their common progenitor, or of some 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 

 Eodents. In this case, however, it may be strongly sus- 

 pected that the resemblance is only analogical, ovying_ to 

 the Phascolomys having become adapted to habits like 

 those of a Rodent. 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 diver- 

 gence in character of the species descended from a com- 

 mon progenitor, together with their retention by inherit- 

 ance of some characters in common, we can understa,nd 

 the excessively 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 subgroups, 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 



