106 ECONOMIC ESSAYS IN HONOR OF JOHN BATES CLARK 
of X to expand while the fall in the price of Y would, since its 
supply curve is negative, cause the quantity of Y to expand also. 
But since Y’s negative elasticity is unity while X's positive 
elasticity is now .9, this would mean that the quantity of Y 
would tend to increase more rapidly than that of X, and hence its 
marginal productivity would continue to fall and that of X 
would continue to rise, so that the supply of Y would be con- 
tinuously increasing faster than X, and there would tend to be a 
cumulative increase in the remuneration of X and a correspond- 
ing fall in that of Y. Under these elasticities it might be thought 
that there would not be stable equilibrium. But the outcome 
depends on the type of productivity equation which is assumed, 
for its partial derivatives furnish the demand curves for the 
factors whose intersections with the supply curves determine the 
point of equilibrium. 
If, however, the negative elasticity of the one were equal to 
the ultimate positive elasticity of the other, after the initial 
alteration in productivities developed, there would be no further 
alteration of the equilib- 
rium since the increase in 
quantity would be the 
same for both. 
If the final positive elas- 
ticity were to be higher 
than the negative elas- 
ticity, then there would 
be a counteracting force 
tending to bring the rel- 
ative returns nearer even 
to the original level than 
that which would result 
from equal elasticities. 
Where both supply 
curves are negatively in- 
clined (Figure 21) there are further possibilities of unstable equi- 
librium. Thus, if the supply curve of one factor X is to shift 
to the left, so that less will be offered at the same price as before, 
then the increase in payment to X will cause its supply to con- 
tract while that of Y will expand. This will in turn mean a still 
oreater increase in the marginal productivity of X and a further