of development and a group
of hypothetical germinal determiners that tend to be associated within
the germ.

The presence or absence of a determiner in a germ is thus the primary
cause of the corresponding presence or absence of a certain
characteristic in the adult organism.

But whatever the essential nature of the characteristic in this
respect, whether simple or complex, we know further that every
organismal characteristic is subject to variation. In any group of
human individuals, for example, we can find persons of different
stature, different weight, with fingers of different length and form,
with heads of different size and shape, hair and eyes of different
shades, different blood pressures, pulse rates, digestive
possibilities, different degrees of determination, cheerfulness,
alertness, and so forth. This fact of variation is not limited to the
comparison of the individuals of a given group or generation among
themselves, but successive generations considered as the units of
comparison show the same sort of thing. And further successive broods
from the same parents exhibit this same phenomenon of variation when
compared with one another. Variation is a universal fact--not only
among organic things but in the inorganic world as well. The variation
which any company of persons shows in stature is paralleled by the
variation in the diameter of the grains in a handful of sand, or of
the drops in a rainstorm.

When we examine the phenomena of variation carefully we find that
they are of two quite distinct categories. The first kind of
variation, that which we most frequently think of as "variation,"
should properly be termed _variability_. Differences of this type are
small _fluctuations_ in any and every character, centering about an
average or mean, which is itself fairly definite and fixed--less
subject to variation in different groups or through successive
generations. For example, if we measure by inches the stature of a
thousand or more persons chosen at random we find that they may vary
from fifty-four to seventy-six inches; the most frequent heights might
be about sixty-nine and sixty-four inches among the men and women
respectively. The results of such a measurement may be expressed
graphically as in Figure 3, which is an expression of the measurement
of 1,052 mothers. The measurement of almost any characteristic in a
large group of any organisms usually gives a result of the kind
figured. The most s