The second phase of the problem of relativity arises in connection with classification.[156] How shall goods be graded in respect of their freight charges for identical services in carriage? Besides illustrating the interplay of the two fundamental principles, this topic serves also to clear up another possible confusion of terms. Proportioning transportation charges to the value of the service must always be clearly distinguished from basing them upon the mere value of the goods. Nothing is more certain than that no direct causal relation between freight rates and the intrinsic value of commodities is traceable. On wire the freight rate between two given points may be about one-fourth of the commercial value; on sheet iron one-third; on lumber somewhat more, and on hay two-fifths; while on cattle and hogs the freight rate may range as low as one-tenth to one-eighth of their commercial value. On coal, on the other hand, the freight rate often more than equals the price of the coal at the mine, and on very low grade commodities like bricks, the transportation charges may equal two or even three times the worth of the goods.[157] For each locality or even direction, these percentages will change. Positive reasons for these varying relationships are discernible in local trade conditions. While in general cheap goods are rated lower; if for any reason--bulkiness or risk--they cost relatively more to transport, they may very properly be advanced in grade. Normally, raw products move at lower rates than finished products--for instance, wheat and flour or cattle and beef. This is in accord with charging what the traffic will bear in relation to value. But in the making of export rates, it may be to the interest of the carrier to reverse this order, actually according to the finished product the lower rate, thereby encouraging the development of manufactures at home rather than abroad.[158] Classification committees and regulative commissions are thus compelled to waver between the two opposing considerations of cost and value. One cannot avoid the conclusion, however, that, contrary to the usual rule, in this field of classification undue weight is often accorded by railway managers to that small element of total cost of service arising from risks of damage in transit--insurance cost, in other words--to the neglect of the financially more important consideration of what the traffic will bear. This emphasis upon the cost side of the account by classification committees, oddly enough is peculiarly characteristic of ratings in the higher class commodities. Among low grade goods, like grain, lumber or coal, the risk of damage is small, so that insurance cost becomes almost negligible. The insistent consideration among these low grade commodities is much more apt to be that of relative demand; arising from the necessity of close and constant adjustment to the behests of trade. Special or commodity