_Predisposition and immunity--Pathological conditions vary--Hardy varieties--"Disease-proof" varieties--Disease dodging--Thick skins--Indian wheats, etc. Cell-contents vary--Citrus, Cinchona, Almonds, etc. Double ideals in selection--Cultivation of pest and host-plant--Variations of fungi--Bacteria--Specialised races--Difficulties--Experiment only will solve the problems._ The numerous and often expensive failures in the application of any prophylactic treatment, have proved an acute stimulus to the research for other ways of combating the ravages of plant diseases. It is a matter of every-day experience that particular varieties of cultivated plants may suffer less from a given disease than others in the same district; also that one and the same species may suffer badly in one country and not in another--_e.g._ the Larch in the lowlands of Europe as contrasted with the same tree in its Alpine home, and the various species of American Vines in Europe. These matters, in the hands of astute observers, are turning the attention of cultivators and experts to new aspects of the question of plant diseases, namely, the possible existence of immunity, and the breeding of disease-proof varieties; and the existence on the part of the host plant of predispositions to disease which may depend on some factors in the plant or in the environment over which it is possible to exercise control, or which, if known, can be avoided. The matter is complicated by the recent demonstration of the fact that parasites also vary and can adapt themselves to altered conditions, as is shown by the history of the coffee-leaf disease (_Hemileia_) in Ceylon, and by Eriksson's results with Wheat-rusts (_Puccinia_) and various experiments with _Coleosporium_ and other Uredineae; but there are good grounds for concluding that hybridisation, grafting, and selection of varieties may do much towards the establishment of races which will resist particular diseases, as shown by Millardet's experiments with Vines, and the results obtained by Cobb and others with Wheat. The great difficulty with so-called "disease-proof varieties" is to test them under similar conditions in different countries, and for a sufficient period of time. A particular race of Wheat may behave very differently in Norfolk, Devonshire, and Northumberland, and the recent introduction of the purely experimental method in this connection is a marked advance. However rough the experiments may of necessity have to be, it is only by such means that data can be gradually accumulated.