al depend, as soon as the material to be made use
of has been properly analysed, the production and fixation of the required
combinations becomes a matter of simple detail.

An excellent example of the practical application of Mendelian principles
is afforded by the experiments which Professor Biffen has recently carried
out in Cambridge. {158} Taken as a whole English wheats compare favourably
with foreign ones in respect of their cropping power. On the other hand,
they have two serious defects. They are liable to suffer from the attacks
of the fungus which causes rust, and they do not bake into a good loaf.
This last property depends upon the amount of gluten present, and it is the
greater proportion of this which gives to the "hard" foreign wheat its
quality of causing the loaf to rise well when baked. For some time it was
held that "hard" wheat with a high glutinous content could not be grown in
the English climate, and undoubtedly most of the hard varieties imported
for trial deteriorated greatly in a very short time. Professor Biffen
managed to obtain a hard wheat which kept its qualities when grown in
England. But in spite of the superior quality of its grain from the baker's
point of view its cropping capacity was too low for it to be grown
profitably in competition with English wheats. Like the latter, it was also
subject to rust. Among the many varieties which Professor Biffen collected
and grew for observation he managed to find one which was completely immune
to the attacks of the rust fungus, though in other respects it had no
desirable quality to recommend it. Now as the result of an elaborate series
of investigations he was able to show that the qualities of heavy cropping
capacity, "hardness" of grain, and immunity to rust can all be expressed in
terms of Mendelian factors. Having once analysed his material {159} the
rest was comparatively simple, and in a few years he has been able to build
up a strain of wheat which combines the cropping capacity of the best
English varieties with the hardness of the foreign kinds, and at the same
time is completely immune to rust. This wheat has already been shown to
keep its qualities unchanged for several years, and there is little doubt
that when it comes to be grown in quantity it will exert an appreciable
influence on wheat-growing in Great Britain.

[Illustration: FIG. 30.

Curves to illustrate the influence of selection.]

It may be objected that it is often