Alfred Marshall, Principles of Economics
Book V, Chapter IV, Paragraph 3 ("The Principle of Substitution")

3. At the beginning of his undertaking, and at every successive stage, the
alert business man strives so to modify his
arrangements as to obtain better results with a given expenditure, or
equal results with a less expenditure. In other words, he
ceaselessly applies the principle of substitution, with the purpose of
increasing his profits; and, in so doing, he seldom fails
to increase the total efficiency of work, the total power over nature
which man derives from organization and knowledge.

Every locality has incidents of its own which affect in various ways the
methods of arrangement of every class of business
that is carried on in it: and even in the same place and the same trade no
two persons pursuing the same aims will adopt
exactly the same routes. The tendency to variation is a chief cause of
progress; and the abler are the undertakers in any trade
the greater will this tendency be. In some trades, as for instance
cotton-spinning, the possible variations are confined within
narrow limits; no one can hold his own at all who does not use machinery,
and very nearly the latest machinery, for every
part of the work. But in others, as for instance in some branches of the
wood and metal trades, in farming, and in
shopkeeping, there can be great variations. For instance, of two
manufacturers in the same trade, one will perhaps have a
larger wages bill and the other heavier charges on account of
machinery; of two retail dealers one will have a larger capital
locked up in stock and the other will spend more on advertisements and
other means of building up the immaterial capital of
a profitable trade connection. And in minor details the variations are
numberless.

Each man's actions are influenced by his special opportunities and
resources, as well as by his temperament and his
associations: but each, taking account of his own means, will push the
investment of capital in his business in each several
direction until what appears in his judgment to be the outer limit, or
margin, of profitableness is reached; that is, until there
seems to him no good reason for thinking that the gains resulting from any
further investment in that particular direction
would compensate him for his outlay. The margin of profitableness, even in
regard to one and the same branch or sub-branch
of industry, is not to be regarded as a mere point on any one fixed line
of possible investment; but as a boundary line of
irregular shape cutting one after another every possible line of
investment.