may be well to add, to the extent of its comparative disadvantage. For there may be many such marginal sites, some of which will fetch low rents, and others very high rents indeed; the same site being often of great potential utility for a large variety of occupations. Between any two occupations there will thus usually be a _margin of transference_, which we must conceive not as a point, but as an irregular line, upon or near to which there will be many pieces of land, differing greatly in the rents which they fetch. These variations of rent will correspond to the differences between the advantages or derived utilities which the sites possess for _both_ the occupations in question. The position of such margins of transference will of course alter as industrial conditions change, and, when they alter, the rents of sites which are not near any margin of transference will be affected also. Thus an increased demand for the products of any particular industry will make it profitable for that industry to offer higher rents, and thus draw land away from other occupations. This will have the effect of raising, though possibly to a very slight extent, the rents of sites which still remain in other uses; for there will be fewer of them available; and their derived utilities will consequently be increased. But here, as everywhere, it is upon the margin that our attention should be focussed, because it is round about the margin (wherever it is found) that the changes are taking place which really matter for society. When Mr. Mallaby-Deeley buys an estate in Covent Garden from the Duke of Bedford, the transaction hardly deserves the degree of public interest it excites. Nothing has happened which is of material consequence to anyone except the two gentlemen concerned; the various sites are still used for the various purposes for which they were used before; nothing has occurred that really matters. But when houses are pulled down for the erection of a cinema, or when a field is diverted from tillage to pasture, something has happened which affects for good or ill the interests of the whole community. Conversion from tillage to pasture represents, indeed, a tendency which has been very marked in Great Britain during the last generation, and has aroused misgivings in many public-spirited observers. Possibly for a variety of reasons, these misgivings may be justified; certainly the problem is well worthy of attention. But when in this way the issue is raised of tillage versus pasture, it is essential, if we are to discuss it rationally, that we should envisage it clearly as applying only to a limited portion of agricultural land, to the portion which lies somewhere near the margin of transference, as things are now, between the two forms of agriculture. It might be socially desirable to bring under the plow a field which the farmer finds it only _slightly_ more profitable to lease under grass; but this would be highly improbable