not always those which are selected to continue the stock.
In nature, too, the species always extends over a larger area and
consists of much greater numbers, and thus a difference of constitution
soon arises in different parts of the area, which is wanting in the
limited numbers of pure bred domestic animals. From a consideration of
these varied facts we conclude that an occasional disturbance of the
organic equilibrium is what is essential to keep up the vigour and
fertility of any organism, and that this disturbance may be equally well
produced either by a cross between individuals of somewhat different
constitutions, or by occasional slight changes in the conditions of
life. Now plants which have great powers of dispersal enjoy a constant
change of conditions, and can, therefore, exist permanently, or at all
events, for very long periods, without intercrossing; while those which
have limited powers of dispersal, and are restricted to a comparatively
small and uniform area, need an occasional cross to keep up their
fertility and general vigour. We should, therefore, expect that those
groups of plants which are adapted both for cross-and
self-fertilisation, which have showy flowers and possess great powers of
seed-dispersal, would be the most abundant and most widely distributed;
and this we find to be the case, the Compositae possessing all these
characteristics in the highest degree, and being the most generally
abundant group of plants with conspicuous flowers in all parts of the
world.

_How the Struggle for Existence Acts among Flowers._

Let us now consider what will be the action of the struggle for
existence under the conditions we have seen to exist.

Everywhere and at all times some species of plants will be dominant and
aggressive; while others will be diminishing in numbers, reduced to
occupy a smaller area, and generally having a hard struggle to maintain
themselves. Whenever a self-fertilising plant is thus reduced in numbers
it will be in danger of extinction, because, being limited to a small
area, it will suffer from the effects of too uniform conditions which
will produce weakness and infertility. But while this change is in
progress, any crosses between individuals of slightly different
constitution will be beneficial, and all variations favouring either
insect agency on the one hand, or wind-dispersal of pollen on the other,
will lead to the production of a somewhat stronger and more fertile