AFFINITIES CONNECTING ORGANIC BEINGS 469 



longing to one group of animals exhibits an affinity to a 

 quite distinct group, this affinity in most cases is general and 

 not special; thus, according to Mr. Waterhouse, of all Ro- 

 dents, the bizcacha is most nearly related to Marsupials; but 

 in the points in which it approaches this order, its relations 

 are general, that is, not to any one marsupial species more 

 than to another. As these points of affinity are believed to 

 be real and not merely adaptive, they must be due in accord- 

 ance w^ith our view^ to inheritance from a common progenitor. 

 Therefore we must suppose either that all Rodents, including 

 the bizcacha, branched off from some ancient Marsupial, 

 which will naturally have been more or less intermediate in 

 character with respect to all existing Marsupials; or that 

 both Rodents 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 characters 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 some early member of the 

 group. On the other hand, of all Marsupials, as Mr. Water- 

 house 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 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 exces- 

 sively complex and radiating affinities by which all the mem- 

 bers of the same family or higher group are connected to- 

 gether. For the common progenitor of a whole family, now 

 broken up by extinction into distinct groups and sub-groups, 

 will have transmitted some of its characters, motlified in 

 various ways and degrees, to all the species; and they will 



