It has long been my earnest desire to address my honoured colleagues, especially those with whom I feel myself connected by congeniality of sentiment, in order to impress on them a subject in which science is deeply interested, and which, according to the direct evidence of Nature herself, is one of the most exalted and important that can be submitted to the researches of the learned. I allude to the investigation of Epidemic Diseases, on a scale commensurate with the extent of our exertions in other departments, and worthy of the age in which we live. It is, with justice, required of medical men, since their sole business is with life, that they should regard it in a right point of view. They are expected to have a perception of life, as it exists individually and collectively: in the former, to bear in mind the general system of creation; in the latter, to demonstrate the connexion and signification of the individual phenomena,—to discern the one by the aid of the other, and thus to penetrate, with becoming reverence, into the sanctuary of cosmical and microcosmical science. This expectation is not extravagant, and the truth of the principles which the medical explorer of nature deduces from it, is so obvious, that it seems scarcely possible that any doubts should be entertained on the subject. Yet we may ask, Has medical science as it exists in our days, with all the splendour which surrounds it, with all the perfection of which it boasts, satisfied this demand? This question we are obliged to answer in the negative. Let us consider only the doctrine of diseases, which has been cultivated since the commencement of scientific study. It has grown up amid the illumination of knowledge and the gloom of ignorance; it has been nurtured by the storms of centuries; its monuments of ancient and modern times cannot be numbered, and it speaks clearly to the initiated, in the languages of all civilized nations. Yet, hitherto, it has given an account only of individual diseases, so far as the human mind can discern their nature. In this it has succeeded admirably, and its success becomes every year greater and more extensive.