ORGANIC BEINGS. 419 



ants but little modified, these constitute our so-called osculant 

 or aberrant species. The more aberrant any form is, the 

 greater must be the number of connecting forms which have 

 been exterminated and utterly lost. And we have evidence 

 of aberrant groups having suffering severely from extinction, 

 for they are almost always represented by extremely few 

 species, and such species as do occur are generally very dis- 

 tinct from each other, which again implies extinction. The 

 genera Ornithorhynchus and Lepidosiren, for example, would 

 not have been less aberrant had each been represented 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 

 by looking at aberrant groups as forms which have been con- 

 quered by more successful competitors, with a few members 

 still preserved under unusually favorable conditions. 



Mr. Waterhouse has remarked that when a member be- 

 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 

 Rodents, the bizcacha is most nearly related to Marsupials; 

 but in the points in which it approaches this order, its rela- 

 tions 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 

 accordance with 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 re- 

 tained, by inheritance, more of the characters of its ancient 

 progenitor than hare 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 par- 

 tially 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 Phas- 

 colomys resembles- most nearly, not any one species, but the 

 general- order <)f Rodents. In this case, however, it may.be 

 strongly suspected that the resemblance, -is only analogical, 

 owing to the Phascolouiys having becoiae adapted to habito 



