of the several sub-regions; and as the organic beings of every great
region are in some degree allied, and as even the physical conditions
are often in some respects alike, we might expect that a modification in
structure, which gave our species some advantage over antagonist species
in one sub-region, would be followed by other modifications in other of
the sub-regions. The races or new species supposed to be formed would be
closely related to each other; and would either form a new genus or
sub-genus, or would rank (probably forming a slightly different section)
in the genus to which the parent species belonged. In the course of
ages, and during the contingent physical changes, it is probable that
some of the six new species would be destroyed; but the same advantage,
whatever it may have been (whether mere tendency to vary, or some
peculiarity of organization, power of mind, or means of distribution),
which in the parent-species and in its six selected and changed
species-offspring, caused them to prevail over other antagonist species,
would generally tend to preserve some or many of them for a long period.
If then, two or three of the six species were preserved, they in their
turn would, during continued changes, give rise to as many small groups
of species: if the parents of these small groups were closely similar,
the new species would form one great genus, barely perhaps divisible
into two or three sections: but if the parents were considerably
unlike, their species-offspring would, from inheriting most of the
peculiarities of their parent-stocks, form either two or more sub-genera
or (if the course of selection tended in different ways) genera. And
lastly species descending from different species of the newly formed
genera would form new genera, and such genera collectively would form a
family.

{449} The discussion here following corresponds more or less to the
_Origin_, Ed. i. pp. 411, 412, vi. pp. 566, 567; although the
doctrine of divergence is not mentioned in this Essay (as it is in
the _Origin_) yet the present section seems to me a distinct
approximation to it.

The extermination of species follows from changes in the external
conditions, and from the increase or immigration of more favoured
species: and as those species which are undergoing modification in any
one great region (or indeed over the world) will very often be allied
ones from (as just explained) partaking of many c