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 the term polymorphy. It does not imply that the single
constituents of the varieties are at present really changing their
characters. On the other hand, it does not exclude the possibility of
such changes. It simply states that observation shows the existence of
different forms; how these have originated is a question which it
does not deal with. In his well-known discussion of the variability of
cereals, Darwin is mainly concerned with the question, whether under
cultivation they have undergone great changes or only small ones.
The decision ultimately depends on the question, how many forms have
originally been taken into cultivation. Assuming five or six initial
species, the variability must be assumed to have been very large, but
on the assumption that there were between ten and fifteen types, the
necessary range of variability is obviously much smaller. But in regard
to this point, we are of course entirely withou