e
between certain weights put into cylinders, the cylinders, when arranged
according to the size of the beans, will appear as shown in figure 72. An
imaginary line running over the tops of the piles will give a curve (fig.
73) that corresponds to the curve of probability (fig. 74).

[Illustration: FIG. 72. Beans put into cylindrical jars according to the
sizes of the beans. The jars arranged according to size of contained beans.
(After de Vries.)]

[Illustration: FIG. 73. A curve resulting from arrangement of beans
according to size. (After de Vries.)]

If we stand men in lines according to their height (fig. 75) we get a
similar arrangement.

[Illustration: FIG. 74. Curve of probability.]

[Illustration: FIG. 75. Students arranged according to size. (After
Blakeslee.)]

The differences in size shown by the individual beans or by the individual
men are due in part to heredity, in part to the environment in which they
have developed. This is a familiar fact of almost every-day observation. It
is well shown in the following example. In figure 76 the two boys and the
two varieties of corn, which they are holding, differ in height. The
pedigrees of the boys (fig. 77) make it probable that their height is
largely inherited and the two races of corn are known to belong to a tall
and a short race respectively. Here, then, the chief effect or difference
is due to heredity. On the other hand, if individuals of the same race
develop in a favorable environment the result is different from the
development in an unfavorable environment, as shown in figure 78. Here to
the right the corn is crowded and in consequence dwarfed, while to the left
the same kind of corn has had more room to develop and is taller.

[Illustration: FIG. 76. A short and a tall boy each holding a stalk of
corn--one stalk of a race of short corn, the other of tall corn. (After
Blakeslee.)]

[Illustration: FIG. 77. Pedigree of boys shown in Fig. 76. (After
Blakeslee.)]

Darwin knew that if selection of particular kinds of individuals of a
population takes place the next generation is affected. If the taller men
of a community are selected _the average_ of their offspring will be taller
than the average of the former population. If selection for tallness again
takes place, still taller men will _on the average_ arise. If, amongst
these, selection again makes a choice the process would, he thought,
continue (fig. 79).

[Illustration: FIG. 78. A race of