ral shape
of body and structure of limbs from a common ancestor. So it is with
fishes.

As members of distinct classes have often been adapted by successive
slight modifications to live under nearly similar circumstances,--to
inhabit for instance the three elements of land, air, and water,--we can
perhaps understand how it is that a numerical parallelism has sometimes
been observed between the sub-groups in distinct classes. A naturalist,
struck by a parallelism of this nature in any one class, by arbitrarily
raising or sinking the value of the groups in other classes (and all our
experience shows that this valuation has hitherto been arbitrary), could
easily extend the parallelism over a wide range; and thus the septenary,
quinary, quaternary, and ternary classifications have probably arisen.

As the modified descendants of dominant species, belonging to the 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 fro