ledge, and of ethical, social
and domestic habits."[26] Secondly, the tide of immigration and
emigration. The conditions which are summarized under the first heading
govern the supply of labor in many different ways. They govern the
length of the working day; they settle the regularity of work. They
determine the number of the members of the family that seek work. They
regulate the ages of entrance into industry and retirement from
industry. They tend to govern the rate of growth of the population--both
through the birth and the death rate. It should be clearly understood,
however, that many of these habits or conditions are themselves, in a
measure, a function of the level of production and of earnings. For
example, the state of knowledge within a community is to-day very
considerably affected by the financial support of education--by the
amount the community can (as well as does) spend upon it.

The importance of immigration and emigration is firstly, the addition or
subtraction thereby made to or from the supply of labor, and, secondly,
the influence of the immigrants upon those habits of the community,
which in turn affect the supply of labor.

10.--The third of the forces quoted earlier in the chapter, as among
those which play a constant and important part in the determination of
wages, is the relative plenty or scarcity of different kinds of labor.
The statement of this force acknowledges the existence of facts which up
to this point have been barely recognized. It calls attention to the
existence of considerable differences in the levels of earnings of
different groups or kinds of labor. It suggests also that the relative
plenty or scarcity of the different kinds of labor is the chief
explanation of these wage differences. We shall investigate at some
length the causes of these differences in the next chapter. Before going
on to that subject, however, it is well to trace out the connection
between the idea of "a general rate of wages" as it has been held, and
the existence of different wage levels.

The idea of a general rate of wages, as it appears in economic theory,
rests upon certain broad assumptions. One of the most important of
these is that there are no "differences of inborn gifts," which would
lead to a limitation of the flow of labor into the upper grades, and
thus lead to a separation of grades. A second important assumption is
that of complete mobility of labor--no obstacles of habit, expense or