uestion which confronts us. How are
the differences between the level of earnings of the relatively
separate groups of wage earners determined?

The factors which determine the relative levels of earnings of each of
the different groups may be put into two sets. First, those factors in
regard to which each group stands alone and separate. Second, those
which arise out of the dealings between the several groups.

"The relative plenty or scarcity of the different kinds of labor" falls
in the first set. It will be remembered that this was among the three
forces which, earlier in the book, were stated to be among the most
constant and important in the determination of wages. The processes by
and through which the facts of relative plenty or scarcity work out
their effect in the distributive result have already been examined. If
the numbers in any group of wage earners are high relative to the uses
in which the employment of the members of that group results in a
considerable addition to the product of market values, the wages of that
group will be low, and vice versa. The need of the productive system for
any kind of labor, relative to the supply available to fill that need is
an important factor in determining the reward paid for that labor.

Furthermore, the statements in regard to the interactions to which the
action of the factor of relative plenty or scarcity was subject, apply
with equal force to the problem under discussion. Every human quality
plays its part in the actual processes and negotiations by which the
wages of the various groups of wage earners are settled. The outcome
depends on many forces, some stable, some shifting and difficult to
trace. Among those forces labor unionism, as the assertion of group
economic power, holds a significant place.

In one respect, indeed, the previous analysis does not apply accurately
to the question of different, though closely related wage levels. It is
probable that the opportunities for the substitution of one type or
group of labor for another type or group are more extensive and numerous
than the opportunities for the substitution of one agent of production
for another. And this fact limits the differences of wage levels that
may arise between different kinds or groups of labor. For substitution
of one type or group of labor for another is one of the ways in which
changes in the relative plenty or scarcity of the different types or
groups are brought about.

So mu