386 ON THE ORIGIN OF SPECIES 



forms of life often present characters in some slight 

 degree intermediate between existing groups. A few 

 old and intermediate parent-forms having occasionally 

 transmitted to the present day descendants but little 

 modified, will give to us our so-called osculant or 

 aberrant groups. The more aberrant any form is, the 

 greater must be the number of connecting forms which 

 on my theory have been exterminated and utterly lost. 

 And we have some evidence of aberrant forms having 

 suffered severely from extinction, for they are gener- 

 ally 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 by a single 

 one ; but such richness in species, as I find after some 

 investigation, does not commonly fall to the lot of 

 aberrant genera. We can, I think, account for this 

 fact only by looking at aberrant forms as failing groups 

 conquered by more successful competitors, with a few 

 members preserved by some unusual coincidence of 

 favourable circumstances. 



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. 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 bizcacha, branched off from some very 

 ancient Marsupial, which will have had a character in 

 some degree intermediate with respect to all existing 

 Marsupials ; or that both Rodents and Marsupials 

 branched off from a common progenitor, and that both 



