s so much more suitable for shop purposes that no
alternative use would be worth considering, will exceed the rent paid
for one of these marginal sites by, roughly speaking, the extra
advantage it possesses for shop purposes. Or will fall short of it, it
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