"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 much for the first set of forces--those in regard to which each group stands alone. The second set--those which arise out of the relationships between the various groups--remains for consideration.