2;>6 THE ORiaiX OF SPECIES 



groups having suffered severely from extinction, for tliey 

 are almost always represented by extremely few species; 

 and such species as do occur are generally very distinct 

 from each other, which again implies extinction. The 

 genera Ornithorhynchus and Lepidosiren, for example, 

 would not have been less aberrant had each been rep- 

 resented by a dozen species, instead of as at present 

 by a single one, or by two or three. We can, I think, 

 account for this fact only bj' looking at aberrant groups 

 as forms which have been conquered by more successful 

 competitors, with a few members still preserved under 

 unusually favorable conditions. 



Mr. Waterhouse has remarked that, when a member 

 belonging to one group of animals exhibits an affinity to 

 a quite distinct group, this affinity in most cases is gen- 

 eral and not special; thus, according to Mr. Waterhouse, 

 of all Rodents, the bizcacha is most nearly related to 

 Marsupials; but in the points in which it approaches this 

 order, its relations arc 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 adap- 

 tive, the)'" must be due in accordance with our view to 

 inheritance from a common progenitor. Therefore we 

 must suppose either that all Rodents, including the biz- 

 cacha, branched off from some ancient Marsupial, which 

 will naturally have been more or less intermediate in 

 character with respect to all existing Marsui»ials; or that 

 both Rodents and Marsupials branched off from a com- 

 mon progenitor, and that both groups have since un- 

 dergone much modification in divergent directions. On 

 either view we must suppose that the bizcacha has re- 

 tained, by inheritance, more of the characters of its 



