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 {430} 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 affinities of
distinct orders of plants.

On the principle of the multiplication and gradual divergence in character
of the species descended from a common parent, 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 together. For the common
parent of a whole family of species, now broken up by extinction into
distinct groups and sub-groups, will have transmitted some of its
characters, modified in various ways and degrees, to all; and the several
species will consequently be related to each other by circuitous lines of
affinity of various lengths (as may be seen in the diagram so often
referred to), mounting up through many predecessors. As it is difficult to
show the blood-relationship between the numerous kindred {431} of any
ancient and noble family, even by the aid of a genealogical tree, and
almost impossible to do this without this aid, we can understand the
extraordinary