Secondly, it must be possible to demand separate and independent returns for the different increments of goods, so that cutting the rate charged for one part of the traffic does not entail cutting the rate charged for the other. In the case of a manufacturer this is secured, either by carrying some goods to a remote and entirely independent market, or by producing some new kind of goods the low price of which will have no effect on the sales or the prices of the other kinds. In the case of the carrier it is accomplished in a variety of similar ways. He can take return cargoes at a low rate. If he stops at different ports along his route he can charge less for goods landed at certain ports than for those landed at others. He can classify his freight and carry some of it at a rate at which he could not afford to carry the whole. With the growth of traffic, however, this condition tends to disappear. Its existence requires that the carrier should have facilities only partially used. As the ship acquires fuller and fuller cargoes, it ceases to be advantageous to fill the hold with goods which pay lower rates than others; just as a mill, which may have run for a time partly on goods that yield a large return and partly on those which yield a small one, gradually discards the making of the cheaper goods as the demand for the dearer kind increases. The vessel which can get full cargoes of profitable merchandise will cease to devote any space to what is less profitable. In the end the ship in our illustration will be transporting in both directions all the first-class freight it can take, and will accept neither the stone nor the merchandise consigned to ports to which it can be carried only at the cheap rates.