scovery of Mendel's paper a need was felt
for terms of a general nature to express the constitution of
individuals in respect of inherited characters, and Bateson accordingly
proposed the words homozygote and heterozygote. An individual is said
to be homozygous for a given character when it has been formed by two
gametes each bearing the character, and all the gametes of a homozygote
bear the character in respect of which it is homozygous. When, however,
the zygote is formed by two gametes of which one bears the given
character while the other does not, it is said to be heterozygous for
the character in question, and only half the gametes produced by such a
heterozygote bear the character. An individual may be homozygous for
one or more characters, and at the same time may be heterozygous for
others.

* * * * *

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CHAPTER IV

THE PRESENCE AND ABSENCE THEORY

It was fortunate for the development of biological science that the
rediscovery of Mendel's work found a small group of biologists deeply
interested in the problems of heredity, and themselves engaged in
experimental breeding. To these men the extraordinary significance of the
discovery was at once apparent. From their experiments, undertaken in
ignorance of Mendel's paper, de Vries, Correns, and Tschermak were able to
confirm his results in peas and other plants, while Bateson was the first
to demonstrate their application to animals. Thenceforward the record has
been one of steady progress, and the result of ten years' work has been to
establish more and more firmly the fundamental nature of Mendel's
discovery. The scheme of inheritance, which he was the first to enunciate,
has been found to hold good for such diverse things as height, hairiness,
and flower colour and flower form in plants, the shape of pollen grains,
and the structure of fruits; while among animals the coat colour of
mammals, the form of the feathers and of the comb in poultry, the waltzing
habit of Japanese mice, and eye {30} colour in man are but a few examples
of the diversity of characters which all follow the same law of
transmission. And as time went on many cases which at first seemed to fall
without the scheme have been gradually brought into line in the light of
fuller knowledge. Some of these will be dealt with in the succeeding
chapters of this book. Meanwhile we may concern ourselves with the sin