us nature. Such forms cannot, of course, ever be bred true, and
where several factors are concerned they may when bred together produce but
a small proportion of offspring like themselves. As soon, however, as their
constitution has been analysed and expressed in terms of Mendelian factors,
pure strains can be built up which when crossed will give nothing but
offspring of the desired heterozygous form.

The points with which the breeder is concerned are often fine ones, not
very evident except to the practised eye. Between an ordinary Dutch rabbit
and a winner, or between the comb of a Hamburgh that is fit to show and one
that is not, the differences are not very apparent to the uninitiated.
Whether Mendelism will assist the breeder in the production of these finer
points is at present doubtful. It may be that these small differences are
heritable, such as those that form the basis of Johannsen's pure lines. In
this case the breeder's outlook is {169} hopeful. But it may be that the
variations which he seeks to perpetuate are of the nature of fluctuations,
dependent upon the earlier life conditions of the individual, and not upon
the constitution of the gametes by which it was formed. If such is the
case, he will get no help from the science of heredity, for we know of no
evidence which might lead us to suppose that variations of this sort can
ever become fixed and heritable.

* * * * *

{170}

CHAPTER XV

MAN

[Illustration: FIG. 32.

Normal and brachydactylous hands placed together for comparison. (From
Drinkwater.)]

[Illustration: FIG. 33.

Radiograph of a brachydactylous hand.]

Though the interest attaching to heredity in man is more widespread than in
other animals, it is far more difficult to obtain evidence that is both
complete and accurate. The species is one in which the differentiating
characters separating individual from individual are very numerous, while
the number of the offspring is comparatively few, and the generations are
far between. For these reasons, even if it were possible, direct
experimental work with man would be likely to prove both tedious and
expensive. There is, however, another method besides the direct one from
which something can be learned. This consists in collecting all the
evidence possible, arranging it in the form of pedigrees, and comparing it
with standard cases already worked out in animals and plants. In this way
it has been pos