The principal cause of forced or under-prices (_Schleuderpreise_) is the facility with which the product deteriorates, and must, therefore, find a quick sale, especially when its storage or transportation is attended by further difficulties.(667) But, very durable commodities are also subject to under-prices, and especially those which last longest, because the supply of them can be diminished only very slowly. Thus, for instance, houses, in a declining city. Distress-prices are found most usually in the case of such commodities as are produced without any intention to produce them, as for instance, rags and excrementitious substances. The more the mere forces of nature preponderate in production, the less can the supply be increased or decreased at pleasure, the more frequently, as a consequence, do we find monopoly-prices and under-prices. (Compare § 131 ff.) Thus the production of wheat is invariably connected with the order of the seasons. Between seed-time and harvest, there are a number of months which neither capital nor skill can shorten to any extent. The cultivation of land, to be very much greater and more lasting, supposes so many conditions precedent, increase of live stock, buildings etc., that it can be attained only after a series of years. Hence it happens that wheat, much more than manufactured products, is subject to oppressively high prices and oppressively low ones, during a long period of time. No matter what the influence of the forces operating in the opposite direction may be, the price of wheat depends most largely on the result of the last crop.(668) Section CXIII. Exceptions. (Continued.) Other impediments in the way of freedom of competition have their origin in social conditions. The rule governing prices applies only where the vendor and purchaser are equally ready to exchange. But in every case in which the producer carries on his business, not for the sake of free gain, but simply to obtain a means of livelihood, it may be subject to many important exceptions.(669) The richer a seller is, the longer can he wait for a favorable opportunity to sell. Thus, for instance, wheat is somewhat lower in price at times when payments are universally made than at other seasons of the year, because a great many country people are then compelled to sell. Where the country population are universally needy, it sinks after a harvest to an unusually low figure, and in spring rises again very high.