ted to
play parts in the economy of nature, which in other countries were
performed by tribes especially adapted to such parts. We can understand
how it might happen that an otter-like animal might have been formed in
Australia by slow selection from the more carnivorous Marsupial types;
thus we can understand that curious case in the southern hemisphere,
where there are no auks (but many petrels), of a petrel{452} having been
modified into the external general form so as to play the same office
in nature with the auks of the northern hemisphere; although the habits
and form of the petrels and auks are normally so wholly different. It
follows, from our theory, that two orders must have descended from one
common stock at an immensely remote epoch; and we can perceive when a
species in either order, or in both, shows some affinity to the other
order, why the affinity is usually generic and not particular--that is
why the Bizcacha amongst Rodents, in the points in which it is related
to the Marsupial, is related to the whole group{453}, and not
particularly to the Phascolomys, which of all Marsupialia is related
most to the Rodents. For the Bizcacha is related to the present
Marsupialia, only from being related to their common parent-stock; and
not to any one species in particular. And generally, it may be observed
in the writings of most naturalists, that when an organism is described
as intermediate between two _great_ groups, its relations are not to
particular species of either group, but to both groups, as wholes. A
little reflection will show how exceptions (as that of the Lepidosiren,
a fish closely related to _particular_ reptiles) might occur, namely
from a few descendants of those species, which at a very early period
branched out from a common parent-stock and so formed the two orders or
groups, having survived, in nearly their original state, to the present
time.

{451} A similar discussion occurs in the _Origin_, Ed. i. p. 427,
vi. p. 582.

{452} _Puffinuria berardi_, see _Origin_, Ed. i. p. 184, vi. p.
221.

{453} _Origin_, Ed. i. p. 430, vi. p. 591.

Finally, then, we see that all the leading facts in the affinities and
classification of organic beings can be explained on the theory of the
natural system being simply a genealogical one. The similarity of the
principles in classifying domestic varieties and true species, both
those living and extinct, is at once explained; the ru