The dysentery which occurred in the army till we came to the shores of the Mediterranean, and for some time after, was clearly the dysentery of India; but, afterwards, we witnessed a different disease. I must confess, that, having come to so certain a conclusion, I was not ready to give up an opinion which appeared to me to rest on very sure grounds; and it was not till after much doubt, hesitation, and careful observation, that I became convinced, in Alexandria, that, with the change of country and climate, we had a different disease. This is one proof how improper, and how unsafe, it is for the practitioner of one climate to sit down and describe the diseases of another. They only who have studied the same diseases, in various and opposite climates, can fully comprehend the extreme absurdity as well as fallacy of this. From reasoning of any kind, we are incompetent to decide on the identity of disease. Reasoning from analogy here always deceives. In many of the symptoms diseases may agree; but, from thence to infer their identity is taking a very narrow view. The comparison will be found to hold good only in some points, and that we have satisfied ourselves with an imperfect outline. Between diseases, as they occur in Europe and in Asia, there are just as many shades of difference as between the plants of those opposite regions, or in the colour of the inhabitants. The dysentery of India, or what I shall term the tropical dysentery, is not the disease which is described by Cullen under that name. The dysentery of Cullen, as it is faithfully described by Sydenham, Pringle, Monro, and Sir George Baker, is a frequent disease on the continent of Europe; and it has ever been a most destructive disease in our armies there. The dysentery of Europe I believe to be generally a disease of the intestinal canal. I have already said that the tropical dysentery proceeds from a different cause; but the diseases likewise differ much in their symptoms. The “pyrexia contagiosa” certainly forms no part of the tropical dysentery; and the torminia and tenesmus occur so rarely, that, in a definition of this disease, I presume they cannot be admitted. Spasmodic affection of the intestinal canal, I believe, rarely occurs in the tropical dysentery. I have ever experienced difficulty in distinguishing dysentery from diarrhæa, and am inclined to think, that, in Cullen’s definition of diarrhæa, is described tropical dysentery. The diarrhæa biliosa, as well as his seventh species, the diarrhæa hepatirrhea, perhaps, ought to be included in a definition of tropical dysentery.