larger genera, tend to inherit the advantages, which made the groups to
which they belong large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more 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
order; 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 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
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