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.
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