The conditions which govern the need of the productive system for labor may be summarized as follows: Firstly, the consumption habits of the community, by which is decided the direction in which the productive powers are employed; secondly, the state of the productive arts, which governs the manner in which the various agents of production are combined for purposes of production; thirdly, the available supply of the agents of production, other than labor. Each of these are in return governed by a complex set of forces. The conditions determining the supply of labor may be summed up under two headings: Firstly, "the state of knowledge, 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.