On the dissemination of fungi, the reader will find Fulton, "Dispersal of the Spores of Fungi by the Agency of Insects," _Ann. Bot._, Vol. III., 1889, p. 207, and Sturgis, "On Some Aspects of Vegetable Pathology and the Conditions which Influence the Dissemination of Plant Diseases," _Botanical Gazette_, Vol. XXV., 1898, p. 187, both useful papers. Further information will be found in Zopf, _Die Pilze_, Breslau, 1890, pp. 79-95 and 228, and Wagner, "Ueber die Verbreitung der Pilze durch Schnecken," in _Zeitschr. f. Pflanzen Krankh._, 1896, p. 144. The estimates as to losses due to epidemics are taken from Watt, _Agricultural Ledger_, Calcutta, 1895, p. 71; Balfour, _The Agricultural Pests of India_, London, 1887, pp. 13-15; Eriksson and Henning, _Die Getreideroste_; the publications of the U.S. Department of Agriculture, _The Kew Bulletin_, and elsewhere. The reader will find further examples in Massee, _Text-Book of Plant Diseases_, 1899, pp. 47-51. Both these subjects are well worth further attention, and I know of no complete account of them. CHAPTER XVI. THE FACTORS OF AN EPIDEMIC. _Illustrations afforded by the potato disease--The larch disease--The phylloxera of the vine._ When we come to enquire into what circumstances bring about those severe and apparently sudden attacks on our crops, orchards, gardens, and forests by hosts of some particular parasite, bringing about all the dreaded features of an epidemic disease, we soon discover the existence of a series of complex problems of intertwined relationships between one organism and another, and between both and the non-living environment, which fully justify the caution already given against concluding that any cause of disease can be a single agent working alone. The statement of prophecy that a particular insect or fungus need not be feared, because it is found to do so little harm in particular cases or districts examined, will thus be seen to be a dangerous one: any pest may become epidemic if the conditions favour it!