common, the extremes are
rare. And yet an unusual individual may really be an outlying member
of a normal group.

In describing the facts of hereditary resemblance between successive
generations two formulas are available. One deals ostensibly with the
individual--the Mendelian formula: the other deals with the group--the
statistical formula. It seems entirely probable that these are not
formulas for describing two essentially different processes or forms
of heredity, but that in reality these are two ways of describing the
same facts seen from two different points of view. The Mendelian
formula regards each individual separately and describes its heredity
thus. The statistical formula regards the whole group as the unit and
considers the individual not as such, but as one of the crowd,
concerning which statements can be made only in terms of averages and
probabilities; black sheep and white. Of these two formulas the
Mendelian is obviously of much the greater importance on account of
its more exact, more particular character; its greater definiteness
gives it a value in the treatment of eugenic problems that statistical
statements must inherently lack. While much has been written of late
regarding the Mendelian formula of heredity, we shall find it
profitable to repeat here its general outlines and to recall a few of
the essential features of this important law that we shall make much
use of later.

Let us have a concrete illustration. One of the simplest cases is that
of the heredity of color in the Andalusian fowl which has been so
clearly described by Bateson. There are two established color
varieties of this fowl, one with a great deal of black and one that is
white with some black markings or "splashes"; for convenience we may
refer to these as the black and white varieties respectively. Each of
these breeds true by itself. Black mated with black produce none but
black offspring, white mated with white produce none but white
offspring. Crossing black and white, however, results in the
production of fowls with a sort of grayish color, called "blue" by the
fancier, though in reality it is a fine mixture of black and white. At
first sight we seem to have a gray hybrid race through the mixture of
the black and the white races. Not so: for if we continue to breed
successive generations from these blue hybrid fowls we get three
differently colored forms. Some will be blue like the parents, some
black like one grandp