388 AFFINITIES CONNECTING Chap. XIII. 



g^roups have since undergone miicli modification in divergent 

 directions. On citlicr view we must suppose that the bizcacha 

 has retained, by inheritance, more of the character of its an- 

 cient progenitor than have other Kodents ; and therefore it 

 will not be specially rchited to any one existing Marsupial, but 

 indirectly to all or nearly all Marsupials, from having partially 

 retained the character of their conimon progenitor, or of some 

 earl}"^ member of the group. On the other hand, of all Mar- 

 suj>ials, as Mr. Waterhousc 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 resemblance 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 

 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 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 common progenitor of a whole family, now broken up by 

 extinction into distinct groups and sub-groups, will have trans- 

 mitted 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 genealogical 

 irco, 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 tliey perceive between the many hving 

 and extinct members of the same great natural class. 



Extinction, as we have seen in the fourth chapter, has 

 played an imjiortant part in defining and M-idening the inter- 

 vals between the several groups in each class. AVe 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 forms of life have been utterly lost, 

 through which the earlv progenitors of l)irds were formerly 



