THE PROBLEM OF WAGES 127

gradual depletion by natural causes, may be treated, also, as
a quasi-rent, but a negative rent in this case.
In both cases, under-supply and over-supply, the problem
presented may refer to even more temporary conditions than
those here contemplated. It may be that circumstances
preclude the recruiting of labour from other localities or
trades, a sudden demand, too sudden to be met in that way,
arising. Or, similarly, relief by removal to other trades or
localities may, for the purposes of some problems, practically
not be contemplated. The treatment of such cases will
follow along lines sufficiently indicated by what has been
said in reference to the cases actually considered.
In what has preceded, reference has been made to the
adjustment of the supply of labour, as between different
trades, by influences affecting the choice of trades by young
men just entering on life. A few years may make a considerable
 difference in the supply, even of highly skilled
labour, if strong inducements exist to select one branch of
work rather than another at the moment when choice is
least hampered. Later, a sacrifice of acquired skill must be
made by a workman who seeks to change his trade, and
such changes are therefore hindered, quite apart from any
Customs, or union rules, requiring definite apprenticeship,
perhaps before a definite age.
Though individuals be not free to choose from a wide
fange of employments, the ability to choose among a small
Qumber may have important effects in changing the distribution
 of labour from one generation to the next. These
changes of supply of labour in different industries affect the
relation of the earnings of labour in one employment to
those in another. The employments in which earnings are
high, relative to the ability demanded and the costliness
and length of the training required, will attract a larger
relative share of the new supplies of labour than those in
¥hich the contrary holds. Earnings and ability tend to get
Proportioned the one to the other in such redistribution.
But industry is in constant change, and thus the complete
adjustment of earnings to ability may not be secured, though
it may be constantly approached. The more active and