128 ECONOMIC PRINCIPLES
intelligent the selection of the most promising openings by
or on behalf of the adolescent, the more surely and rapidly
will the adjustment be approximated to. But the point
aimed at does not remain fixed, its changes cannot be
precisely foreseen, and thus the complete adjustment is not
secured. In the degree in which heedlessness, or the
pressure of circumstances, compel the selection of trades
without relation to the fitness of the individual to the work
to be done, a waste of power results, in the sense that ability
to perform much-needed work is possessed by those engaged
on work to which they are not specially fitted, and in which
they can be readily replaced.
Taking industry as a whole, it is conceivable that, even
if the supply of labour were duly distributed between the
different trades, so that neither the workmen would gain,
nor the value of their product be increased, by any redistribution
 as between trades and localities, yet the marginal
productivity of labour might be at a level not corresponding
with the wage-rate which would suffice to maintain the
labour supply without increase or decrease of abundance.
We must now give attention to the reaction of supply and
demand on each other as related to the problem thus
suggested, that is, we need to consider the influence of
economic conditions on the movements of population, on
births, marriages, and deaths.
The question of whether a given level of wages will
suffice to maintain the supply of labour, introduces the
consideration of the standard of living among the recipients
of the wages. In order to reduce the problem to one which
shall deal with but one point of prime importance at a time,
we may suppose that the conception of a given level of
wages includes, not one uniform level, but such a set of rates
as, in their deviations from one another, do not set up
movements of labour from trade to trade or from place to
place. A change in the level will, therefore, imply a change
of the advantages of each trade which cannot be adjusted
by a simple movement between trades. We thus concentrate
attention on a feature of great importance, but one liable to
be confused by the co-existence of many trades, with varying