TRANSACTIONS OF SECTION F. 



693 



Suppose the extra produce is a function involving several variables (or para- 

 meters) like Land, Ability, Opportunity. Say /,.= F(A, o, « . . .), where F is a form 

 common to the community, and x, a, a> denote the quality of land, ability, and 

 opportunity peculiar to the individual. If the extra produce is F(\ o «) — F(o, o, o) ; 

 is F(A, 0, o) — F(o, o, o) the ' economic rent ' of land ; F(o ao) — F(o, o, o) the rent of 

 ability ; F(o ou) — F(o, o, o) the extra produce due to opportunity ? (ibid., pp. 16, 17.) 

 If so, the three parts do not make up the whole. 



(4.) Anyway, to call the third extra produce Interest is very unhappy. Its 

 aflBnities are evidently with Rent. (Cf. Sidgwick ' Pol. Econ.,' Book ii., ch. 7, § 4.) 



(5.) I should not have complained about the use of a term, b\it that it is connected 

 with Mr. Webb's main contention against Prof. Walker, to which I am imable to 



attach significance : that • this, not the " rent of ability " is the real keystone of the 

 arch' (ib. p. 17). From the point of view here taken (above, p. 673) this search 

 for the ' keystone ' among the factors of distribution is nearly ;is hopeless as the 

 speculation of the ancients about the real vj> or doivn. 



(j) Determinatkness of Economic Equilibrium. — To investigate the possi- 

 bility of there being more than one rate of exchange at which the supply is just 

 carried off by the demand, let us consider the most favourable or, at least, the most 

 familiar case in which there may be two scales of production, and therefore two series 

 of terms on which tlie producer is willing to deal — two supply curves. Let the two 

 branches in Fig. 7 represent such a double supply-curve, the ordinate denoting price, 

 and the abscissa the quantity of product which is offered at that ])rice by a certain indi- 

 vidual. Now, the essential idea or leading property of an individual Supply-curve is 

 that it represents the quantity which the individual will prefer to offer at any given 

 price. Hence the descending part of the branches, dotted in the figure, cannot form 



