Chap. XIII. 
CLASSIFICATION. 
429 
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 aber¬ 
rant 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 gene¬ 
rally 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 m 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. W^ater- 
hoiise, of all Eodents, 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 Eodents, including the biz¬ 
cacha, branched off from some very ancient Marsupial, 
which will have had a character in some degree inter¬ 
mediate with respect to all existing Marsupials; or 
