t 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", Jersey, 1837.) on his farm on the isle of Jersey, who cultivated
upwards of 150 varieties of wheat, which he claimed were as pure as
those of any other agriculturalist. But Professor La Gasca of Madrid,
who visited him, drew attention to aberrant ears, and pointed out, that
some of them might be better yielders than the majority of plants in the
crop, whilst others might be poor types. Thence he concluded that the
isolation of the better ones might be a means of increasing his crops.
Le Couteur seems to have considered the constancy of such smaller types
after isolation as absolutely probable, since he did not even discuss
the possibility of their being variable or of their yielding a
changeable or mixed progeny. This curious fact proves that he considered
the types, discovered in his fields by La Gasca to be of the same kind
as his other varieties, which until that time he had relied upon as
being pure and uniform. Thus we see, that for him, the variability of
cereals was what we now call polymorphy. He looked through his fields
for useful aberrations, and collected twenty-three new types of wheat.
He was, moreover, clear about one point, which, on being rediscovered
after half a century, has become the starting-point for the new Swedish
principle of selecting agricultural plants. It was the principle of
single-ear sowing, instead of mixing the grains of all the selected ears
together. By sowing each ear on a separate plot he intended not only
to multiply them, but a