wo directions of excess and defect
in relation to the mean amount, whichever kind of variation is wanted is
always present in some degree, and thus the difficulty as to
"beneficial" variations occurring, as if they were a special and rare
class, falls to the ground. No doubt some organs may vary in three or
perhaps more directions, as in the length, breadth, thickness, or
curvature of the bill. But these may be taken as separate variations,
each of which again occurs as "more" or "less"; and thus the "right" or
"beneficial" or "useful" variation must always be present so long as any
variation at all occurs; and it has not yet been proved that in any
large or dominant species, or in any part, organ, or faculty of such
species, there is no variation. And even were such a case found it would
prove nothing, so long as in numerous other species variation was shown
to exist; because we know that great numbers of species and groups
throughout all geological time have died out, leaving no descendants;
and the obvious and sufficient explanation of this fact is, that they
did _not_ vary enough at the time when variation was required to bring
them into harmony with changed conditions. The objection as to the
"right" or "beneficial" variation occurring when required, seems
therefore to have no weight in view of the actual facts of variation.

_Isolation to prevent Intercrossing._

Most writers on the subject consider the isolation of a portion of a
species a very important factor in the formation of new species, while
others maintain it to be absolutely essential. This latter view has
arisen from an exaggerated opinion as to the power of intercrossing to
keep down any variety or incipient species, and merge it in the parent
stock. But it is evident that this can only occur with varieties which
are not useful, or which, if useful, occur in very small numbers; and
from this kind of variations it is clear that new species do not arise.
Complete isolation, as in an oceanic island, will no doubt enable
natural selection to act more rapidly, for several reasons. In the first
place, the absence of competition will for some time allow the new
immigrants to increase rapidly till they reach the limits of
subsistence. They will then struggle among themselves, and by survival
of the fittest will quickly become adapted to the new conditions of
their environment. Organs which they formerly needed, to defend
themselves against, or to escape fr