The manuscript of Sir William Osler's lectures on the "Evolution of Modern Medicine, " delivered at Yale University in April, 1913, on the Silliman Foundation, was immediately turned in to the Yale University Press for publication. Duly set in type, proofs in galley form had been submitted to him and despite countless interruptions he had already corrected and revised a number of the galleys when the great war came. But with the war on, he threw himself with energy and devotion into the military and public duties which devolved upon him and so never completed his proof-reading and intended alterations. The careful corrections which Sir William made in the earlier galleys show that the lectures were dictated, in the first instance, as loose memoranda for oral delivery rather than as finished compositions for the eye, while maintaining throughout the logical continuity and the engaging con moto which were so characteristic of his literary style. In revising the lectures for publication, therefore, the editors have merely endeavored to carry out, with care and befitting reverence, the indications supplied in the earlier galleys by Sir William himself. In supplying dates and references which were lacking, his preferences as to editions and readings have been borne in mind. The slight alterations made, the adaptation of the text to the eye, detract nothing from the original freshness of the work.