430 
CLASSIFICATION. 
Chap. XIII. 
that both Kodents 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 inlieritance, more of the character of its 
ancient progenitor than have other Kodents; 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 re¬ 
sembles 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 com¬ 
mon 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 
