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A TEEATISE ON MONEY

BK. II

Equivalent Substitution and the Method of Limits asthe only valid methods of approximation—other thanthe direct estimation of the money-incomes of personswho are on general grounds deemed similar.

That, in spite of so many practical and theoreticaldifficulties, useful comparisons of purchasing powercan often be made in actual practice is due to the factthat, even when the relative, as well as the absolute,prices of individual commodities are fluctuating, theaverage character of typical expenditure and of typicaltastes and the average level of real-incomes do notshift, as a rule, rapidly or widely as between communi-ties not much removed from one another in time orplace ; so that the problem, arising out of the shiftingcomposition of consumption and out of changes intaste and environment, is not acute. For example,the evidence indicates, according to Dr. Bowley, thatthe changes in taste and habit in England between1904 and 1927 as tested by general statistics of work-ing-class consumption were comparatively slight.