enerations to new and varying conditions, would vary. It is certain
that organisms, living in an _isolated_ country which is undergoing
geological changes, must in the course of time be so subjected to new
conditions; moreover an organism, when by chance transported into a new
station, for instance into an island, will often be exposed to new
conditions, and be surrounded by a new series of organic beings. If
there were no power at work selecting every slight variation, which
opened new sources of subsistence to a being thus situated, the effects
of crossing, the chance of death and the constant tendency to reversion
to the old parent-form, would prevent the production of new races. If
there were any selective agency at work, it seems impossible to assign
any limit{508} to the complexity and beauty of the adaptive structures,
which _might_ thus be produced: for certainly the limit of possible
variation of organic beings, either in a wild or domestic state, is not
known.

{508} In the _Origin_, Ed. i. p. 469, vi. p. 644, Darwin makes a
strong statement to this effect.

It was then shown, from the geometrically increasing tendency of each
species to multiply (as evidenced from what we know of mankind and of
other animals when favoured by circumstances), and from the means of
subsistence of each species on an _average_ remaining constant, that
during some part of the life of each, or during every few generations,
there must be a severe struggle for existence; and that less than a
grain{509} in the balance will determine which individuals shall live
and which perish. In a country, therefore, undergoing changes, and cut
off from the free immigration of species better adapted to the new
station and conditions, it cannot be doubted that there is a most
powerful means of selection, _tending_ to preserve even the slightest
variation, which aided the subsistence or defence of those organic
beings, during any part of their whole existence, whose organization had
been rendered plastic. Moreover, in animals in which the sexes are
distinct, there is a sexual struggle, by which the most vigorous, and
consequently the best adapted, will oftener procreate their kind.

{509} "A grain in the balance will determine which individual shall
live and which shall die," _Origin_, Ed. i. p. 467, vi. p. 642. A
similar statement occurs in the 1842 Essay, p. 8, note 3.{Note 59}

A new race thus formed by natural selection