It is in order to object that the principle of progression should not be applied to the taxation of great landed estates, since a considerable part of them is unproductive, and consequently does not directly affect sacrifice. But the same objection can be urged against any taxation of unoccupied land. The obvious reply is that the equal taxation of unproductive with productive land is justified by social reasons, chiefly, the unwisdom of permitting land to be held out of use. The same social reasons apply to the question of levying an exceptionally high tax on large estates, even though they may at present produce no revenue. While the tax is sound in principle, it is probably not much needed in America in connection with agricultural or urban land. Its main sphere of usefulness would seem to be certain great holdings of mineral, timber, and water power lands. "There are many great combinations in other industries whose formation is complete. In the lumber industry, on the other hand, the Bureau now finds in the making a combination caused, fundamentally, by a long standing public policy. The concentration already existing is sufficiently impressive. Still more impressive are the possibilities for the future. In the last forty years concentration has so proceeded that 195 holders, many interrelated, now have practically one-half of the privately owned timber in the investigation area (which contains eighty per cent. of the whole). This formidable process of concentration, in timber and in land, clearly involves grave future possibilities of impregnable monopolistic conditions, whose far reaching consequences to society it is now difficult to anticipate fully or to overestimate."[109] In January, 1916, the Secretary of Agriculture called the attention of Congress to the fact that a small number of corporations closely associated in a policy of community of interest were threatening to secure and exercise a monopoly over the developed water power of the country. Ninety per cent. of the anthracite coal lands of Pennsylvania are owned or controlled by some nine railroads acting as a unit in all important matters. For situations of this kind a supertax on large estates would seem to hold the promise of a large measure of relief.