CHAP. XIV.] ORGANIC BEINGS. 35 



in size ; and they consequently supplant many smaller and feebler 

 groups. Thus we can account for the fact that all organisms, 

 recent and extinct, are included under a few great orders, and 

 under still fewer classes. As showing how few the higher groups 

 are in number, and how widely they are spread throughout the 

 world, the fact is striking that the discovery of Australia has not 

 added an insect belonging to a new class ; and that in the vege- 

 table kingdom, as I learn from Dr. Hooker, it has added only two 

 or three families of small size. 



In the chapter on Geological Succession I attempted to show, 

 on the principle of each group having generally diverged much in 

 character during the long-continued process of modification, how 

 it is that the more ancient forms of life often present characters 

 in 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 some 

 evidence of aberrant groups having suffered severely from extinc- 

 tion, for they 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 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 members still preserved under unusually 

 favourable 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 

 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 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 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 



