Moreover, Nature herself shows us that such pests are kept in check in her domain by the struggle for existence entailed by innumerable barriers and competitors. As matter of experience also it is found that rotation of crops, planting forests of mixed species, and breaking up large areas of cultivation into plantations, fields, etc., of different species afford natural and often efficient checks to the ravages of fungus and insect pests. Over and over again it has been found that a fungus or an insect which is merely endemic so long as it is isolated in the forest, where its host is separated from other plants of the same species by other plants which it cannot attack, becomes epidemic when let loose on the continuous acres so beloved of the planter. And the same reasoning applies to the success of such pests on open areas from which the birds or other enemies of the pest have been driven. True, we cannot always trace the tangled skein of inter-relationships between one organism and another in Nature: the recognition of the principle of natural selection and the struggle for existence is too recent, and our studies of natural history as yet too imperfect to lay all the factors clear, but no observant and thoughtful man can avoid the truth of the general principle here laid down. The history of all great planting enterprises teaches us that he who undertakes to cultivate any plant continuously in open culture over large areas must run the risk of epidemics. NOTES TO CHAPTER XVII. The principal literature, now very voluminous, on this subject is contained in the publications of the U.S. Department of Agriculture from 1890 onwards. See especially _Bulletins_, Nos. 3, 6, and 9; _Farmers' Bulletin_, No. 91, 1899; and _The Journal of Mycology_ during the same period. See also Lodeman, _The Spraying of Plants_, London, 1896. A summary of the principal processes will be found in Massee, _Text-Book of Plant Diseases_, pp. 31-47. With regard to the history of the subject, which still needs writing, the reader should not overlook Roberts, "On the Therapeutical Action of Sulphur," _St. George's Hospital Reports_, date unknown, but subsequent to the following: Berkeley, _Introduction to Cryptogamic Botany_, 1857, p. 277, with references. These are, I believe, with the references to steeping of wheat in De Bary, _Unters. über d. Brandpilze_, Berlin, 1853, among the first attempts to utilise such remedies. Further facts will be found in the pages of the _Gardeners' Chronicle_, especially since 1890, and in _Zeitsch. f. Pflanzen-krankheiten_ since 1891. CHAPTER XVIII. VARIATION AND DISEASE.