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 in the case of a field where the balance of argument to the farmer in favor of pasture is overwhelming.  The position of the margin of transference between different uses may, in other words, be somewhat out of place from the social point of view, and it may be desirable by appeals and propaganda, even conceivably by the devices of State subsidy and compulsion, to push it forwards or backwards in greater or less degree.  But it will be necessarily a matter of degree, and nothing could be more foolish than to speak as though there was, or could be, some ideal method of cultivation equally applicable to all lands, without regard to their climatic and other conditions.  Needless to say, none of the agricultural experts who sometimes deplore the decline of arable farming are guilty of such foolishness.  But the sense of the diversity of nature which is very vivid to them may sometimes be lacking in people who live in towns, and a firm grasp of the marginal notion may serve best to keep the latter from forgetting it.