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 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 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 difficulty which naturalists have experienced in
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