124 ECONOMIC PRINCIPLES

was for the purpose of providing an answer to this question
that it was necessary to give attention to marginal productivity
 when dealing with masses of men who, as individuals,
 could not be dealt with, since they formed
indistinguishable parts of a mass of work-people, that is to
say, parts indistinguishable for the purpose of assigning a
distinct part, or a distinct share of the value, of the product
to the work of the individuals in question, by any other
method than the division of the value of the product by the
number of those engaged in producing it, The value so
divided would need to be disentangled from the productive
contributions of other classes of workers, of capital, etc., and
the preceding discussion is designed to afford a means for
handling some of the obvious difficulties which this problem
presents,
We pass now to the consideration of the features which
call for attention in reference to the supply-price of labour.
This term is used to denote that price which will suffice to
evoke a volume of supply adequate to the need at that price.
Generally speaking, with a change in the supply needed,
there will be a change in the corresponding supply-price. It
may also be noted that this price is generally only one of
the features which serve to influence the volume of labour
available. Hours of labour and conditions of employment,
for example, may be such as to either add to or detract from
the attraction of a given price offered.
As affecting the supply of labour, we need to distinguish
clearly between the two kinds of problems we may have to
consider. The supply-price may have reference to a supply,
attracted to a given place and industry from other places,
and from such other industries as can supply labour suitable
for the ends in view. It may, on the other hand, be used in
reference to the training of boys to a particular trade rather
than to any of the other trades among which they are practically
 able to choose ; or even to the stimulation of a general
increase in population by increase of births, resulting from the
encouragement of marriages among young people due to
generous remuneration of labour, whether the labour be
self-employed, or hired out to a master. In the first of the