78 Variation 
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 ex- 
ternal 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 de- 
scribing 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 ob- 
servation 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 without historical data. 
