m extinction, for they are generally 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.
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, 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 groups
have since undergone much modification in divergent directions.
On either view we may suppose that the bizcacha has retained, by
inheritance, more of the character 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
having partially retained the character of their common progenitor, or
of an early member of the group. On the other hand, of all Marsupials,
as Mr. Waterhouse has remarked, the phascolomys resembles 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 De Candolle has made nearly similar
observations on the general nature of the