From the point of view of economic theory, the warrant for a differentiation of charges between various classes of commodities offered for transportation, may be considered primarily from two distinct points of view. The first is that of operation, which determines cost. The second is from the standpoint of traffic whereby the value of service, so-called, is measured. The reasonableness of making a distinction in freight rates according to the character of goods is easily apparent, as judged on the basis of cost of service. A multitude of factors enter into consideration at this point. The railway ought in self-protection to charge more for hauling a thing, if it actually costs it more in the long run to perform that service. Some of the factors which enter into this cost were well put by the Interstate Commerce Commission in 1897.[334] "Whether commodities were crude, rough, or finished; liquid or dry; knocked down or set up; loose or in bulk; nested or in boxes, or otherwise packed; if vegetables, whether green or dry, desiccated or evaporated; the market value and shippers' representations as to their character; the cost of service, length and direction of haul; the season and manner of shipment; the space occupied and weight; whether in carload or less-than-carload lots; the volume of annual shipments to be calculated on; the sort of car required, whether flat, gondola, box, tank, or special; whether ice or heat must be furnished; the speed of trains necessary for perishable or otherwise rush goods; the risk of handling, either to the goods themselves or other property; the weights, actual and estimated; the carrier's risk or owner's release from damage or loss." Instances of approval of classification on the basis of such cost of operation are frequently found in the decisions of the Interstate Commerce Commission. For example, special service or equipment, as in the rapid transport of fresh vegetables and fruit from the South, justify the carriers in a specially high classification.[335] Rates on live hogs by comparison with rates on hog products, as well as on live cattle and dressed beef, have likewise been adjusted in terms of cost of carriage. A classification on hogs yielding a rate equal to two-thirds of that on hog products has been held equitably to represent the relative expense.[336] Even the indefinite element of risk has been accepted as justifying a higher classification for live stock as compared with other commodities.[337]