88 ECONOMIC ESSAYS IN HONOR OF JOHN BATES CLARK 
tend to increase in any such ratio and if its supply remained 
constant, then the produce jointly attributable to labor and 
capital would decrease. Within this joint product, the relative 
productivity of these two factors would be the same but their 
absolute shares would shrink and this would bring the unit return 
for each down toward the P; and P points which originally 
prevailed. 
Secondly, it is of course virtually inconceivable that the supply 
curves of two factors or even of one would be thus infinitely 
elastic. The natural forces of resistance to labor and to saving 
would tend to cause them to turn upward after a time. And 
when this happened the approach to an equilibrium would be 
hastened. Irrespective of changes in marginal productivity, the 
upward movement of the supply curves would at some time inter- 
cept the new returns. This would be hastened, of course, by the 
failure of a third factor to expand commensurately and would 
be complicated, as we 
shall see, if the upward 
tilt of the supply curve of 
either X or Y began 
earlier or sloped more 
sharply than that of the 
other factor. 
A third illustration 
which may be chosen is 
that where both elastic- 
ities are equal. In Figure 
8, both X and Y are given 
unit positive elasticity 
and are given a common 
point of origin. They are 
both therefore represented 
by the curve of S in which the quantity A is offered for the 
return P. 
A word should be added here concerning the scale on which 
quantities of two differing factors are drawn, since it may well be 
asked how it is possible to represent hours of labor and physical 
units of capital upon the same scale. The author makes no effort 
to prove, as Cairnes sought to do, that both factors can be 
reduced to common and commensurate units of disutility, for each 
F1ag. 8