per ton mile can be earned above the expenses incident to hauling that sand or coal, it enables the rates on the dry goods or hardware to be maintained at a lower point than they otherwise would be. It is unnecessary to elaborate this principle further. It is everywhere accepted as valid. And it in a measure substantiates Mavor's statement that "freight rates, like rent, are rather the effect of price movements than the cause of them." When tariffs are high because prices are high, we are afforded a fair illustration of value of service as an element in rate making. Value of service, therefore, as affording a warrant for classification, has also been recognized in a number of Interstate Commerce decisions since 1887. A relation between the grade of the charge and fluctuations in the market price of the commodity--in other words, charging what the traffic will bear--is at times discernible. It is to the interest of the public that carriers should be satisfied with relatively smaller profits from the transportation of commodities of low price which are in general demand.[339] Under these circumstances changes in price of such staple commodities as iron and steel or the lower priced grains, should be reflected in a corresponding modification of rates.[340] Akin to this is recognition of a relation in general between the value of a commodity and its classification. Where, for example, articles representing different stages of manufacture have to be graded, it is but fair that the raw material, or the partly-made product should be graded lower than the finished article.[341] Similarly, articles which may fairly be substituted for one another ought to be classified with reference to their common market value.[342] The relative value of commodities, as controlling classification, clearly governs the treatment of hard and soft coal.[343] The practical difficulty, of course, is to know where to stop in admitting such considerations. Shall "small-vein" soft coal, because it cannot compete on even terms with the "big-vein" product, be accepted for carriage on a more favored basis?[344] Some rather nice questions, both of business and public policy, would be suggested by such a precedent.