This list would set forth theoretically in one column of figures various amounts of exertion and therefore of production； and in a parallel column the prices which must be paid to induce the available workers to put forth these amounts of exertion.(4*)But this simple method of treating the supply of work of any kind， and consequently the supply of goods made by that work， assumes that the number of those who are qualified for it is fixed； and that assumption can be made only for short periods of time.The total numbers of the people change under the action of many causes.Of these causes only some are economic； but among them the average earnings of labour take a prominent place；though their influence on the growth of numbers is fitful and irregular.
But the distribution of the population between different trades is more subject to the influence of economic causes.In the long run the supply of labour in any trade is adapted more or less closely to the demand for it： thoughtful parents bring up their children to the most advantageous occupations to which they have access； that is to those that offer the best reward， in wages and other advantages， in return for labour that is not too severe in quantity or character， and for skill that is not too hard to be acquired.This adjustment between demand and supply can however never be perfect； fluctuations of demand may make it much greater or much less for a while， even for many years， than would have been just sufficient to induce parents to select for their children that trade rather than some other of the same class.Although therefore the reward to be had for any kind of work at any time does stand in some relation to the difficulty of acquiring the necessary skill combined with the exertion， the disagreeableness， the waste of leisure， etc.involved in the work itself； yet this correspondence is liable to great disturbances.
The study of these disturbances is a difficult task； and it will occupy us much in later stages of our work.But the present Book is mainly descriptive and raises few difficult problems.
NOTES：
1.Labour is classed as economic when it is "undergone partly or wholly with a view to some good other than the pleasure directly derived from it." See p.65 and footnote.Such labour with the head as does not tend directly or indirectly to promote material production， as for instance the work of the schoolboy at his tasks， is left out of account， so long as we are confining our attention to production in the ordinary sense of the term.From some points of view， but not from all， the phrase Land， Labour， Capital would be more symmetrical if labour were interpreted to mean the labourers， i.e.mankind.See Walras， 蒫onomie Politique Pure， Le鏾n 17， and Prof.Fisher， Economic Journal， VI， p.529.
2.We have seen (p.124) that， if a person makes the whole of his purchases at the price which he would be just willing to pay for his last purchases， he gains a surplus of satisfaction on his earlier purchases； since he gets them for less than he would have paid rather than go without them.So， if the price paid to him for doing any work is an adequate reward for that part which he does most unwillingly； and if， as generally happens， the same payment is given for that part of the work which he does less unwillingly and at less real cost to himself； then from that part he obtains a producer's surplus.Some difficulties connected with this notion are considered in Appendix K.
The labourer's unwillingness to sell his labour for less than its normal price resembles the unwillingness of manufacturers to spoil their market by pushing goods for sale at a low price； even though， so far as the particular transaction is concerned， they would rather take the low price than let their works stand idle.
3.Theory of Political Economy， Ch.V.This doctrine has been emphasized and developed in much detail by Austrian and American economists.
4.See above III， iii， section 4.
