land.  For if it takes the form, say, of the discovery of some new artificial manure, it will very likely facilitate production on the less fertile soils far more than it will on the more fertile soils where artificial manures are not so necessary.  It will thus tend to diminish the differential advantages of working on the more fertile farms, and their rents will accordingly fall, possibly by much more in the aggregate than any increase in the rents of the farms near the margin of cultivation.  The point may, perhaps, be better understood if we pass from agricultural to urban land, and ask what would be the effect on site values of a great improvement in the facilities of internal transport.  Push the case to an extreme, and suppose passenger transport to become so cheap and so quick that there ceases to be any advantage in living in a town so as to be near your place of work.  Urban landlords would no longer be able to obtain the high rents they now receive for the sites of houses in or near a town.  For most people would prefer to move out into the country where sites can be obtained at little more than an agricultural rent.  The country covers so large an area relatively to the towns that the supply of rural sites would be still very plentiful as compared with the demand.  Their rents would not, therefore, rise by very much, although the rents of the housing sites in towns would fall heavily.  Of course, there are other factors to be taken into account before we could pronounce upon the effect on aggregate rents.  Central sites for shops might, for instance, fetch a higher rental than before.  The purpose of this discussion is not to generalize but to show the danger of generalizing about rents in the aggregate, or land as a whole.

Sec.4. The Margin of Transference.  The last illustration may serve, however, to remind us of an obvious fact which we must now take into account.  The same piece of land may be used for a variety of purposes.  It may have been used for growing corn, and later it may be devoted to the building of houses, or, as at Slough, to a repair depot for motor vehicles.  It need hardly be said that the land will, as a general rule, be put to the use in which its value is greatest; or to speak more strictly, in which the biggest rent, or the biggest selling price can be obtained.  But the notion of the differential advantages which a piece of land possesses over the marginal land becomes decidedly more complicated when we take account of this variety of uses.  Let us turn our attention, for instance, to the sites used for shop and office purposes, and consider what we can regard as the marginal site in this connection.  Clearly it will not be the marginal land of which we spoke above, which it only just paid to cultivate, and which yielded no rent at all.  For this will probably be agricultural land in an out-of-the-way district, where no one would dream of setting up an office or a shop.  Any site upon which a sane man would contemplate setting up a shop will certainly possess value for other purposes, such as house-building.  Hence the marginal site for shopkeeping purposes will not be like our marginal farm, a site which yields no rent.