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 

 fi-om 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 fatiia), 

 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 Avhich 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, hoAV 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. 



