places in the economy of nature. The
larger and more dominant groups thus tend to go on increasing in size; and
they consequently supplant many smaller and feebler groups. Thus we can
account for the fact that all organisms, recent and extinct, are included
under a few great orders, under still fewer classes, and all in one great
natural system. As showing how few the higher groups are in number, and how
widely spread they are throughout the world, the fact is striking, that the
discovery of Australia has not added a single insect belonging to a new
class; and that in the vegetable kingdom, as I learn from Dr. Hooker, it
has added only two or three orders of small size.

In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some slight degree intermediate
between existing groups. A few {429} old and intermediate parent-forms
having occasionally transmitted to the present day descendants but little
modified, will give to us our so-called osculant or aberrant groups. The
more aberrant any form is, the greater must be the number of connecting
forms which on my theory have been exterminated and utterly lost. And we
have some evidence of aberrant forms having suffered severely from
extinction, for they are generally represented by extremely few species;
and such species as do occur are generally very distinct from each other,
which again implies extinction. The genera Ornithorhynchus and Lepidosiren,
for example, would not have been less aberrant had each been represented by
a dozen species instead of by a single one; but such richness in species,
as I find after some investigation, does not commonly fall to the lot of
aberrant genera. We can, I think, account for this fact only by looking at
aberrant forms as failing groups conquered by more successful competitors,
with a few members preserved by some unusual coincidence of favourable
circumstances.

Mr. Waterhouse has remarked that, when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in
the points in which it approaches this order, its relations are general,
and not