We are not without instances of diseases, observing this peculiarity which attaches to plants; but their specific characters have hardly been sufficiently considered in reference to climate and situation, together with diet and local influences, to afford us accurate data for comparison. It has, however, been remarked, in every country where Epidemics have prevailed, that some districts or tracts of country, though supposed to possess all the qualities favourable to the development of the diseases, have nevertheless been entirely or nearly free from them. The following passage on the course of the Cholera gives an example of this peculiarity. "Whenever the malady deviated, so to speak, from its normal direction, and passed towards the west, it seemed incapable of propagating itself; and _died away spontaneously, even in places which appeared to be well fitted for its reception_.--The rich fertile and densely peopled countries to the right of the Dneiper, enjoyed an equal freedom from attack, which can only be explained by the fact that they were situated _beyond the line of the disease_." With this I close the subject of the diffusion of plants and diseases, though it would require a volume of itself, to record all that has been noticed. I have endeavoured to select such instances as shall mark distinctly the features which point to comparison without overloading the enquiry. * * * * * {96} SECTION IV. THE RELATION BETWEEN EPIDEMIC AND ENDEMIC DISEASES. Epidemic diseases, which multiply their germs in any climate, and under apparently the most varying conditions of temperature and hygrometric and electrical states of atmosphere, offer many points of contrast with Endemic affections, and many of relationship. The latter are traceable to a certain extent, to geological and geographical positions of the localities where they are observed to prevail, in combination with atmospheric vicissitudes and peculiarities, as well as to extent of cultivation of the soil: it has been remarked that the sickly island (as it is called) of St. Lucia has certain salubrious parts, but these are where sulphur abounds; this geological peculiarity has been deemed sufficient to account for the absence of endemic affections in these parts, and with much force of reason; for in the neighbourhoods where sulphur or sulphurous acid, a compound of sulphur, is an element prevalent in the soil or atmosphere, vegetation and the ague disappear together. Now ague, and other endemic fevers, doubtless originate from some allied, if not identical cause; for the localities in which they appear have so many {97} features in common, that we are constrained to acknowledge that endemic fevers have some relations and analogies, though not yet unravelled.