ead 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 intermediate 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 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 some 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 affinities of distinct
families of plants.

On the principle of the multiplication and gradual divergence in
character of the species descended from a common progenitor, together
with their retention by inheritance of some characters in common, we can
understand the excessively complex and radiating affinities by which all
the members of the same family or higher group are connected toge