This article examines the sequence of ideas which led Marshall to conclude that economic growth is organic in nature and that its analysis must be based on a biological not a mechanical analogue. At the outset of his career in the early 1870s, when he was attempting to reconcile increasing returns with atomic competition, Marshall came up against the problem of irreversibility; he had to discard his long-period curves because they were interdependent and irreversible. Volume I of the Principles (1890), particularly Book V, was an analysis of mechanical equilibrium, and it was to be followed by Volume I1 on dynamics. When Marshall replied to a critic of the Principles in the Economic Journalin 1898, he gave an illuminating account of his methodology and declared that 'the Mecca of the economist is economic biology rather than economic dynamics'. He failed to produce Volume I1 of his Principles because by the end of the 1890s he had changed his mind on important theoretical matters. He would have had to write a treatise on economic biology, and he must have felt that this was a task for his successors, not for him.Two outstanding critics of standard economics, Kenneth Boulding and Nicholas Georgescu-Roegen, have argued powerfully that the mechanical analogue should be replaced by a n evolutionary approach (Boulding, 198 1 ;Georgescu-Roegen, 1971). In recent years the challenge has evoked a growing response; a significant number of economists are exploring economic biology and there are signs that cliometricians are beginning to question the dominant influence of mechanical models on their work.' The patron saint of this movement is Alfred Marshall, whose famous declaration provides a motto: 'The Mecca of the economist is economic biology rather than economic dynamics' (1898: 43). As we celebrate the centenary of the first edition of Marshall's Principles (1890)' it is fitting to explore Marshall's case for economic biology and to assess the contribution which he made to advance the cause. 1 Sigiienza (1982: 1560) gives a list of contemporary economists in the English-speaking world interested in the biological method. For economic biology and economic history see Mokyr (1989).
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