rue that the past record of experimental
breeding had been mainly one of disappointment. It was true also that there
was no tangible clue by which experiments might be directed in the present.
Nevertheless in this kind of work alone there seemed any promise of
ultimate success.

A few years later appeared the first volume of de Vries' remarkable book on
_The Mutation Theory_. From a prolonged study of the evening primrose
(_Oenothera_) de Vries concluded that new varieties suddenly arose from
older ones by sudden sharp steps or mutations, and not by any process
involving the gradual accumulation of minute {16} differences. The number
of striking cases from among widely different plants which he was able to
bring forward went far to convincing biologists that discontinuity in
variation was a more widespread phenomenon than had hitherto been
suspected, and not a few began to question whether the account of the mode
of evolution so generally accepted for forty years was after all the true
account. Such in brief was the outlook in the central problem of biology at
the time of the rediscovery of Mendel's work.

* * * * *

{17}

CHAPTER III

MENDEL'S WORK

The task that Mendel set before himself was to gain some clear conception
of the manner in which the definite and fixed varieties found within a
species are related to one another, and he realised at the outset that the
best chance of success lay in working with material of such a nature as to
reduce the problem to its simplest terms. He decided that the plant with
which he was to work must be normally self-fertilising and unlikely to be
crossed through the interference of insects, while at the same time it must
possess definite fixed varieties which bred true to type. In the common pea
(_Pisum sativum_) he found the plant he sought. A hardy annual, prolific,
easily worked, _Pisum_ has a further advantage in that the insects which
normally visit flowers are unable to gather pollen from it and so to bring
about cross fertilisation. At the same time it exists in a number of
strains presenting well-marked and fixed differences. The flowers may be
purple, or red, or white; the plants may be tall or dwarf; the ripe seeds
may be yellow or green, round or wrinkled--such are a few of the characters
in which the various races of peas differ from one another. {18}

In planning his crossing experiments Mendel adopted an attitude which
marked hi