MUTUAL AFFINITIES OF ORGANIC BEINGS 387 



some degree intermediate between existing groups. As some few 

 of the old and intermediate forms have transmitted to the present 

 day descendants 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 al- 

 most 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 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 conquered by more successful competitors, with a few mem- 

 bers 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 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 ap- 

 proaches 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 accordance with our view, to inheritance from a com- 

 mon progenitor. Therefore, we must suppose either that all Ro- 

 dents, including the bizcacha, branched off from some ancient 

 Marsupial, which will naturally have been more or less inter- 

 mediate 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 sup- 

 pose 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 hav- 

 ing partially retained the character of their common progenitor, or 

 of some early member of the group. On the other hand, of all Mar- 

 supials, 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 



