94 ECONOMIC ESSAYS IN HONOR OF JOHN BATES CLARK 
of Y, so that its marginal productivity would rise still further 
and that of Y would decline yet more. This in turn would 
stimulate X to decrease at twice the rate of Y and would lead 
to another increase in X’s marginal productivity. There would 
thus be a cumulative process. Here as in all these cases the 
point of equilibrium would depend on the type of productivity 
equation assumed. Its partial derivatives furnish the demand 
curves for the factors which must be thought of as equations to be 
solved simultaneously with the supply curves under discussion. 
When, however, the negative elasticities are less than the 
bositive elasticities, as in Figure 13 with X as —.5 and Y as +1.0, 
then though the initial 
increase to both would 
cause the supply of X to 
contract and that of Y to 
expand, there would not 
be the same after effect. 
In the first place, there 
would not be the same 
relative differences in the 
supplies of the factors 
created as would have been 
the case had X's elasticity 
been —1.0 rather than 
—.5. Secondly, the supply 
of Y would now decrease 
from the amount B at 
twice the rate at which that of X would increase from C. Hence, 
there would be something of a readjustment of marginal produc- 
tivities, with Y rising from the lowly station to which the move- 
ment in opposite directions had consigned it while that of X 
would be lowered from its high estate. The final equilibrium 
(i.e., P5 for Y and P4 for X) then would be one which would be 
distinctly more favorable to Y than when the elasticities were 
plus and minus 1.0 respectively. 
Finally, what is the situation when both supply curves are 
negative? If they are equal, then an advance in the return 
paid to each unit, will cause equal proportionate reductions in 
the quantity offered and hence will not throw the relative 
marginal productivities of the two factors out of line with each 
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