sub-families, families, &c., &c., has been
quite arbitrary{438}; without the clearest definition, how can it be
possible to decide whether two groups of species are of equal value, and
of what value? whether they should both be called genera or families; or
whether one should be a genus, and the other a family{439}?

{437} _Origin_, Ed. i. pp. 419, 427, vi. pp. 575, 582.

{438} This is discussed from the point of view of divergence in the
_Origin_, Ed. i. pp. 420, 421, vi. pp. 576, 577.

{439} <Footnote by the author.> I discuss this because if Quinarism
true, I false. <The Quinary System is set forth in W. S. Macleay's
_Horae Entomologicae_, 1821.>

_On the kind of relation between distinct groups._

I have only one other remark on the affinities of organic beings; that
is, when two quite distinct groups approach each other, the approach is
_generally_ generic{440} and not special; I can explain this most easily
by an example: of all Rodents the Bizcacha, by certain peculiarities in
its reproductive system, approaches nearest to the Marsupials; of all
Marsupials the Phascolomys, on the other hand, appears to approach in
the form of its teeth and intestines nearest to the Rodents; but there
is no special relation between these two genera{441}; the Bizcacha is no
nearer related to the Phascolomys than to any other Marsupial in the
points in which it approaches this division; nor again is the
Phascolomys, in the points of structure in which it approaches the
Rodents, any nearer related to the Bizcacha than to any other Rodent.
Other examples might have been chosen, but I have given (from
Waterhouse) this example as it illustrates another point, namely, the
difficulty of determining what are analogical or adaptive and what real
affinities; it seems that the teeth of the Phascolomys though _appearing
closely_ to resemble those of a Rodent are found to be built on the
Marsupial type; and it is thought that these teeth and consequently the
intestines may have been adapted to the peculiar life of this animal and
therefore may not show any real relation. The structure in the Bizcacha
that connects it with the Marsupials does not seem a peculiarity related
to its manner of life, and I imagine that no one would doubt that this
shows a real affinity, though not more with any one Marsupial species
than with another. The difficulty of determining what relations are real
and what analogical is far