It is true that insects are exceedingly destructive to plants, but as far as I am able to obtain information, they appear to be so chiefly by their voracious propensities; they consume the structure of the plant in its entity, and do not primarily interfere with its vitality. The instance of the vibriones, before-mentioned, seems at first to be an exception {154} to the rule, but this is rather apparent, than real; and it may be made to apply more as a confirmation, than an obstacle to the vegetable theory: for if we may fairly compare the diseases of animals with those of plants, the existence of entozoa in the latter, would be considered an essential point to be substantiated. Having now considered the question as to the infeasibility of supposing that chemical fermentation is the basis upon which a theory of diseases can be sustained, and having shewn that life is inseparable from infection, and miasmatic generation;--having explained the phenomena of the dispersion of diseases by comparison with the dispersion of plants, and finally, having demonstrated that the physiology and pathology of plants bear so close a relation to each other, and that their epidemic affections depend upon minute organic germs, I submit to the judgment of my readers, whether there is not much reasonableness in the application of the facts to the inference--that living germs are the cause of epidemic disease in man and animals. * * * * * {155} CHAPTER IV. RESULTS IN PROOF OF THE TENABLENESS OF THE PROPOSITION. -------- SECTION I. OBSERVATIONS ON SOME OF THE LAWS OF EPIDEMIC DISEASES. The results obtained by comparing certain facts connected with Epidemic Affections of animals, with analogous affections in plants, afford, from the few instances I shall here notice, a very strong presumption, that analogous causes operate in the production of these affections. I have already quoted from Hecker, to shew that previously to, and during the Epidemics of the Middle Ages, the minuter forms of animal and vegetable life appeared to be called into existence, much more abundantly than usual; that famines prevailed in consequence of failure of cereal crops, no doubt depending then, as now, upon the various forms of fungiferous growth. I cannot refrain quoting here, a passage or two from our old friend Virgil; for he confirms not only the fact of peculiar showers in {156} connexion with diseases, but he also refers to the rust of corn, thus: 150. "Mox et frumentis labor additus; ut mala culmos Esset rubigo ... ... Intereunt segetes." _Georg. 1._ Then: 311. "Quid tempestates autumni et sidera dicam? . . . . . . 322. "Saepe etiam[61] immensum coelo venit agmen aquarum Et foedam glomerant tempestatem imbribus atris Collectae ex alto nubes." _Georg. 1._