For most purposes, however, it is of more interest to compare land with other agents of production, especially with capital and labor, rather than with ordinary commodities. Now, as we have already noted, there is some doubt as to the manner in which the supply of capital or labor is likely to be affected by alterations in demand price. But the supply of capital and the supply of labor, even if we suppose them to be as entirely unresponsive to price changes as is the supply of land, are at any rate not fixed. Not only _may_ they vary for many reasons, but they are in fact likely to vary in direct proportion to the population. An increase in population implies an increase in the supply of labor; and it is likely to be accompanied by an increase in the supply of capital; in other words, the supply of these agents will expand, as the demand for them expands. But the supply of land will remain what it was. This fact is enormously important in connection with the broad problem of population, which will form the theme of Volume VI. But it is important also in other connections. It has been the dominating factor in many absorbing controversies upon high policy regarding the ownership of land, or the taxation of land values, upon which we can touch but lightly here. It has seemed to many writers a reasonable proposition to lay down, that the ordinary course of the progress of society, the increase of population and industry, must mean, as a broad general rule, a constant increase in the demand for land. And, if that be granted, it seems to follow that the price and rent of land will tend constantly to increase. John Stuart Mill, accordingly, in the middle of the last century, asserted that "the ordinary progress of a society, which increases in wealth, is at all times tending to augment the incomes of landlords; to give them both a greater amount and a greater proportion of the wealth of the community, independently of any trouble or outlay, incurred by themselves,"[1] and upon the strength of this assertion, he justified the policy of imposing a special tax upon what we have come to call the "unearned increment" of land. But how far does actual experience bear his assertion out? In Great Britain we have seen in the last half-century an undoubted increase in urban rents; but over long periods at least, there was a marked fall in both the prices and rents of agricultural land, despite the fact that the country was "increasing in wealth" as rapidly as ever before. This was due, of course, in the main to the increased supplies of wheat and other foodstuffs coming from the New World: and if, accordingly, we choose to lump together not only our own urban and agricultural land, but the land of other countries as well, and to speak vaguely of the demand for land as a whole, it might seem as though we could argue that Mill's generalization still holds good. But even this is by no means certain and in any case such a generalization is of very little