_Slow Change of the Foregoing Adjustments._--The line which represents the comparative density of population is of course slowly changing position as migration goes on from the older centers of population to more newly occupied regions. If the present distribution of population be represented by the line numbered 1, the distribution a hundred years hence may be represented by the dotted line numbered 2, and that which will exist after five hundred years shall have passed may be represented by the dotted line numbered 3. Even within the economic center the comparative density of population in different divisions is therefore not to be treated as strictly permanent, and it is not to be treated as in any sense permanent when we are forecasting effects that will be realized several centuries hence. For a problem involving a score or two of years the general conditions we have described may be treated as, in the main, abiding.[6] [6] The reason for confining attention to the central zone is partly, as we have stated, because here only do we get a quick response to an economic influence. Overproduction of any article quickly lowers the value of it throughout the area, and a mass of unemployed laborers affects wages throughout the area more speedily than it does in the great environing zone. This, however, is only one reason for this limitation of the scope of our immediate study. A serious fact is that, if we include the entire world, we cannot establish, in the way we have proposed, the natural standards toward which values, wages, and interest are tending. It will be recalled that in the static division of this treatise we have attained a "natural" standard of wages by assuming that all dynamic changes were to cease and that labor and capital were to move to and fro in the system of industrial groups till each of these agents produced as much in one subgroup as in another. A computation of this kind might, within a limited area, be made periodically, say once in ten years, and if this were done it would give a series of static standards of wages. Now these standards become higher as time advances. The static rate of pay for labor is, as a rule, higher at any one date than was the standard for a date ten years earlier, and lower than will be that for a date ten years later. The normal rate of pay about which actual wages fluctuate is a rising one.