Before giving a succinct account of the discovery of paludal miasma and of its natural history, I ought in the first place to state that I have not had the opportunity of reading or studying the great original treatise of Professor Salisbury. I am acquainted with it only through a resume published in the _American Journal of the Medical Sciences_ for the year 1866, new series, vol. li. p. 51. At the beginning of my investigations I was engaged in a microscopic examination of the water and mud of swampy shores and of the marshes, also with a comparison of their microphytes with those which might exist in the urine of patients affected with intermittent fevers. Nearly three months passed without my being able to find the least agreement, the least connection. Having lost nearly all hope of being able to attain the end which I had proposed, I took some of the slime from the marshes and from the masses of kelp and Confervæ from the sea shores, where intermittent fevers are endemic, and placed them in saucers under the ordinary glass desiccators exposed on a balcony, open for twenty-four hours, the most of the time under the action of the burning rays of the sun. With the evaporated water deposited within the desiccators, I proceeded to an examination, drop by drop. I at length found that which I had sought so long, but always in vain. The parasite of intermittent fever, which I have termed Limnophysalis hyalina, and which has been observed before me by Drs. J. Lemaire and Gratiolet (_Comptes Rendus Hebdomadaires de l'Academie des Sciences_, Paris, 1867, pp. 317 and 318) and B. Cauvet (_Archives de Medecine Navale_, November, 1876), is a fungus which is developed directly from the mycelium, each individual of which possesses one or several filaments, which are simple or dichotomous, with double outlines, extremely fine, plainly marked, hyaline, and pointed. Under favorable conditions, that is, with moisture, heat, and the presence of vegetable matter in decomposition, the filaments of mycelium increase in length. From these long filaments springs the fungus. The sporangia, or more exactly the conidia, are composed of unilocular vesicles, perfectly colorless and transparent, which generally rise from one or both sides of the filaments of the mycelium, beginning as from little buds or eyes; very often several (two to three) sporangia occur placed one upon the other, at least on one side of the mycelium.