In this paper I intend to confine myself to one aspect of the manysided changes which came over English political economy between the publication of John Stuart Mill's Principles of Political Economy (1848) and of Alfred Marshall's Principles of Economics (1890). During this period great theoretical advances were made by W. S. Jevons and Alfred Marshall, advances which were paralleled in Lausanne and Vienna, and pushed further on the American continent by Professors J. B. Clark and Irving Fisher. I shall not attempt to summarize these contributions to pure theory, nor to estimate how wide was the gulf between the theory of Mill and the theory of Marshall (but in parenthesis I cannot refrain from suggesting that the real change in the theory of value was relatively small, while the change in the theory of distribution which derived from it was revolutionary). It is more appropriate at this first annual meeting of the Economic History Association to discuss the change in the status of economic theory in the corpus of economics as a result, mainly, of the influence of the historical school. Emphasis will be placed on the terms of peace rather than on the war of the methods, for it is my purpose to emphasize the possibilities of cooperation rather than to revive old antagonisms. I want to discuss the influence of some economic historians on economics. I shall make frequent reference to William Ashley, first professor of political economy in the University of Toronto and first professor of economic history in Harvard University and, I think, on the American continent. If I give him undue prominence this can surely be forgiven a Toronto man on this occasion.