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. In a letter to one of the editors, Osler described these lectures as "an aeroplane flight over the progress of medicine through the ages." They are, in effect, a sweeping panoramic survey of the whole vast field, covering wide areas at a rapid pace, yet with an extraordinary variety of detail. The slow, painful character of the evolution of medicine from the fearsome, superstitious mental complex of primitive man, with his amulets, healing gods and disease demons, to the ideal of a clear-eyed rationalism is traced with faith and a serene sense of continuity. The author saw clearly and felt deeply that the men who have made an idea or discovery viable and valuable to humanity are the deserving men; he has made the great names shine out, without any depreciation of the important work of lesser men and without cluttering up his narrative with the tedious prehistory of great discoveries or with shrill claims to priority. Of his skill in differentiating the sundry "strains" of medicine, there is specific witness in each section. Osler's wide culture and control of the best available literature of his subject permitted him to range the ampler aether of Greek medicine or the earth-fettered schools of today with equal mastery; there is no quickset of pedantry between the author and the reader. The illustrations (which he had doubtless planned as fully for the last as for the earlier chapters) are as he left them; save that, lacking legends, these have been supplied and a few which could not be identified have with regret been omitted.