430 CLASSIFICATION. Chap. XIII. 



belonging 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. Water- 

 house, of all Rodents, the bizcacha is most nearly related 

 to Marsupials ; but in the points in which it approaches 

 this order, its relations are general, and not to any one 

 marsupial species more than to another. As the points 

 of affinity of the bizcacha to Marsupials are believed 

 to be real and not merely adaptive, they are due on 

 my theory to inheritance in common. Therefore we 

 must suppose either that all Rodents, including the biz- 

 cacha, branched off from some very ancient Marsupial, 

 which will have had a character in some degree inter- 

 mediate 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 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 re- 

 sembles 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 orders of plants. 

 On the principle of the multiplication and gradual 

 divergence in character of the species descended from 



