As to what may be expected in future--Professor Johnston anticipates the gradual diminution of the supply, from the circumstance, already adverted to, of the progressive exhaustion of the newer lands of the Union, and the rapid increase of population in the old. If several of the Western States, he argues, have even already ceased to raise enough wheat for the supply of their present inhabitants, and are compelled to draw largely on the produce of the remote States of Illinois, Ohio, &c.--and if the productive power of these new lands is annually becoming less, the virgin soils more distant, and the transport of subsistence more difficult--if this is the state of matters now, what will it be in 1860, when immigration and natural increase will probably have raised the population of the Union to some thirty-four millions? "It is very safe," he concludes, "to say that in 1860 their wheat-exporting capability will have become so small as to give our British farmers very little cause for apprehension." It may perchance occur to these gentlemen, that the consolation Professor Johnston here offers them is not very cheering after all; and as long as they see the provision stores in every market town piled up with the interloping flour barrels of New York, and their own waggons returning home with their loads unsold, it is not to be wondered at if they are not greatly exhilarated with the prospect of what may possibly happen nine years hence. And slender as is the hope deferred here held out to them, it rests, we fear, on very questionable grounds. Professor Johnston's opinion is founded on two suppositions: 1st, That the exhaustion of the Western States, on which he dwells so much, is proceeding so rapidly as already to affect the markets of the eastern districts; 2d, That these older districts will be unable to increase the quantity of produce raised within their own boundaries, without so adding to its cost as to prevent its being profitably exported.