maintains that the largest amount of life can be supported on any area, by
great diversification or divergence in the structure and constitution of
its inhabitants. Every meadow and every forest affords a proof of this
thesis. The numerical proportion of the different species of the flora is
always changing according to external influences. Thus, in a given meadow,
some species will flower abundantly in one year and then almost disappear,
until, after a series of years, circumstances allow them again to multiply
rapidly. Other species, which have taken their places, will then become
rare. It follows from this principle, that notwithstanding the constantly
changing conditions, a suitable selection from the constituents of a meadow
will ensure a continued high production. But, although the principle is
quite clear, artificial selection has, as yet, done very little towards
reaching a really high standard.

The same holds good for cereals. In ordinary circumstances a field will
give a greater yield, if the crop grown consists of a number of
sufficiently differing types. Hence it happens that almost all older
varieties of wheat are mixtures of more or less diverging forms. In the
same variety the numerical composition will vary from year to year, and in
oats this may, in bad years, go so far as to destroy more than half of the
harvest, the wind-oats (Avena fatua), which scatter their grain to the
winds as soon as it ripens, increasing so rapidly that they assume the
dominant place. A severe winter, a cold spring and other extreme
conditions of life will destroy one form more completely than another, and
it is evident that great changes in the numerical composition of the
mixture may thus be brought about.

This mixed condition of the common varieties of cereals was well known to
Darwin. For him it constituted one of the many types of variability. It
is of that peculiar nature to which, in describing other groups, he applies