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 without
historical data.

Few of the varieties of wheat show conspicuous differences, although their
number is great. If we compare the differentiating characters of the
smaller types of cereals with those of ordinary wild species, even within
the same genus or family, they are obviously much less marked. All these
small characters, however, are strictly inherited, and this fact makes it
very probable that the less obvious constituents of the mixtures in
ordinary fields must be constant and pure as long as they do not
intercross. Natural crossing is in most cereals a phenomenon of rare
occurrence, common enough to admit of the production of all possible hybrid
combinations, but requiring the lapse of a long series of years to reach
its full effect.

Darwin laid great stress on this high amount of variability in the plants
of the same variety, and illustrated it by the experience of Colonel Le
Couteur ("On the Varieties, Properties, and Classification of Wheat",