When we come to enquire into the causes of disease, it appears at first an obvious and easy plan to subdivide them into groups of factors which interfere with the normal physiology of the plant. Scientific experience shows, however, that the easy and the obvious are here, as elsewhere in nature, only apparent, for disease, like health, is an extremely complex phenomenon, involving many reactions and interactions between the plant and its environment. If we agree that a living plant in a state of health is not a fixed and unaltering thing, but is ever varying and undergoing adaptive changes as its life works out its labyrinthine course through the vicissitudes of the also ever-varying environment, then we cannot escape the conviction that a diseased plant, so long as it lives, is also varying in response to the environment. The principal difference between the two cases is, that whereas the normal healthy plant varies more or less regularly and rhythmically about a mean, the diseased one is tending to vary too suddenly or too far in some particular directions from the mean; the healthy plant may, for our present purposes, be roughly likened to a properly balanced top spinning regularly and well, whereas the diseased one is lurching here, or wobbling there, to the great danger of its stability. For we must recognise at the outset that disease is but variation in directions dangerous to the life of the plant. Health consists in variation also, but not in such dangerous grooves. That the passage from health to disease is gradual and ill-defined in many cases will readily be seen. In fact we cannot completely define disease. Mere abnormality of form, colour, size, etc., is not necessarily a sign of disease, in the usual sense of the word, otherwise the striking variations of our cultivated plants would suggest gloomy thoughts indeed, whereas we have reason to believe that many cultivated varieties are more healthy--in the sense of resisting dangerous exigencies of the environment--than the stocks they came from. Strictly speaking, no two buds on a fruit-tree are alike, and the shoots they produce vary in position, exposure, number, and vigour of leaves, and so forth. The minute variations here referred to are not seen by the ordinary observer, but those who bud, graft and multiply by cuttings on a large scale know that such bud-variations are important, quite apart from more extensive "sports" which occasionally occur. On the other hand, we have reason to believe that many species have died out gradually as the environment altered. These plants died because they did not vary sufficiently, or did not vary in the right directions; they became diseased with respect to the then prevailing conditions of normal physiology or health. Disease, therefore, may be said to be variation of functions in directions, or to extents, which threaten the life of the plant, the normal in all cases being the state of the plant characteristic of the species.