y.

Once discovered, this fact was elevated to the rank of a leading
principle and tested on as large a scale as possible. The fields were
again carefully investigated and every single ear, which showed a
distinct divergence from the main type in one character or another,
was selected. A thousand samples were chosen, but this time each sample
consisted of one ear only. Next year, the result corresponded to the
expectation. Uniformity prevailed almost everywhere; only a few
lots showed a discrepancy, which might be ascribed to the accidental
selection of hybrid ears. It was now clear that the progeny of single
ears was, as a rule, pure, whereas that of mixed ears was impure.
The single-ear selection or single-ear sowing, which had fallen into
discredit in Germany and elsewhere in Europe, was rediscovered. It
proved to be the only trustworthy principle of selection. Once isolated,
such single-parent races are constant from seed and remain true to their
type. No further selection is needed; they have simply to be multiplied
and their real value tested.

Patrick Shirreff, in his early experiments, Le Couteur, Hays and others
had observed the rare occurrence of exceptionally good yielders and the
value of their isolation to the agriculturist. The possibility of error
in the choice of such striking specimens and the necessity of judging
their value by their progeny were also known to these investigators, but
they had not the slightest idea of all the possibilities suggested by
their principle. Nilsson, who is a botanist as well as an agriculturist,
discovered that, besides these exceptionably good yielders, every
variety of a cereal consists of hundreds of different types, which find
the best conditions for success when grown together, but which, after
isolation, prove to be constant. Their preference for mixed growth is so
definite, that once isolated, their claims on manure and treatment
are found to be much higher than those of the original mixed variety.
Moreover, the greatest care is necessary to enable them to retain
their purity, and as soon as they are left to themselves they begin to
deteriorate through accidental crosses and admixtures and rapidly return
to the mixed condition.

Reverting now to Darwin's discussion of the variability of cereals, we
may conclude that subsequent investigation has proved it to be exactly
of the kind which he describes. The only difference is that in reality
it reaches a degree, quit