[Developments Stemming from the Marshall-Wicksell Apparatus]

The modern theory of consumers' behavior developed almost wholly during the last quarter of a century so far as the doctrine is concerned that is actually used and taught by the sector of the economics profession that is primarily interested in theory in this sense. But what was and is being developed consists in methods and results, mainly associated with the work of Fisher, Pareto, Barone, Johnson, and, if we do not mind adding a paper that remained practically unknown for a decade or more after its publication, Slutsky. This means that the fundamental ideas were present before the close of the First World War, not in the form of embryonic suggestions only, but well worked out, mainly by authors of international reputation, in forms accessible, so we should think, to every professional theorist. They had only to sink in and to be clarified, amplified, applied, and occasionally straightened out in the process. But little had to be added to them that was fundamentally new. The situation was much as it was in the automobile industry: in spite of all the improvements and new gadgets, a modern motor car is still much the same kind of thing as the motor car of 1914. ¹ Exactly the same holds for what may still be called the new theory of production. And the concept of elasticity of substitution illustrates well not only what has been done but also what could be done in this field under the given circumstances. ²