If the principles I have announced be correct, the extreme impropriety--not to use a stronger phrase--of carrying on excavations or other extensive works on the muddy banks of rivers, in marshy or swampy forests, during the summer months, must be obvious to all reflecting persons. No work should be done in such places, or in ponds, after the month of April, for it is warm dry weather that sets malaria afloat. But if this ferment--which we may strictly call malaria, as producing a malarious condition of the air--be, as I apprehend it is, the cause of fever, why should not medical men direct their attention more earnestly to the question in how far such a fermentation of the blood may be met by the employment of substances known to resist and counteract fermentation? Are physicians agreed on the nature of fevers, and the best means of curing them?[69] [69] I have known many persons sickly from the effects of intermitting fever or malaria from a residence in warm climates, and who have suffered and perished from an injudicious treatment. Ill-formed or incomplete agues are extremely common, even in the south of England, in London especially. They show themselves under a variety of forms, and with much severity, in the cases of those who, having once visited a true malarious climate, are ever afterwards more or less liable to a return of the disease. Let men reflect; simple truths travel slowly, yet are truths notwithstanding. The death of the well-known M. Soyer was evidently caused by his wholly misunderstanding the nature of his complaint, which, in fact, was a fever originally caught in the Crimea. Nothing can be more interesting, in a natural history point of view, than to watch the results upon large bodies of water, of attempts, more or less successful, to complete their drainage. Thus during the operations carried on for this purpose at Haarlem, there sprung up in the dry places of the more elevated parts an extraordinary quantity of plants and herbs, which were not seen in the country before they flowered and sent millions of seeds with their diminutive rocket, silky tails into the air. They were too minute to be seen upon grass, but the footpaths were covered with them, and a current of wind might carry them to distant regions, as the sand is carried from the coast of Africa into the track of the Brazilian packets, to such an extent as to make it uncomfortable to walk on deck. It is by no means, therefore, improbable that those errant seeds came from a foreign land, the native produce of other countries. Continuing my observations into the transit of seeds, I have found them to be the cause of shallow canals in England being full of heretofore unknown water-plants, to the extent of impeding navigation.