_Death_, on the contrary, is the cessation of these rhythmical processes of building up and breaking down of the protoplasm molecules. It does not imply the cessation of chemical changes of other kinds, but that these rhythmical constructions of the complex and labile protoplasm molecules breaking down on stimulation to bodies partly re-assimilable, partly combustible in respiration, and partly excretory, etc., have ceased, and that further chemical changes in the material are thenceforth simpler and different in kind and degree, eventually leading to total disintegration so that no units are left capable of restoring the rhythm. If these ideas are correct, we may define _Disease_ as dangerous disturbances in the regularity, or interference with the completeness or range of the molecular activities constituting normal Life--_i.e._ Health--and it is evident that every degree of transition may be realised between the two extremes. Now, if we further assume, as I think we must do, that a considerable range or "play" must exist in the molecular activities of the protoplasm constituting life, we obtain a sort of expression of what we mean by limits of variation. The fact that life can go on in a given plant at temperatures between from 1°-5° and 35°-40° C., or in lights of different intensity, or within considerable ranges of water supply, concentration of salts, partial pressure of oxygen, etc., implies that the molecular activities of the protoplasm are of the normal _kind_ all the time, though they may differ in rapidity, and even in _quantitative_ and _qualitative_ respects within certain limits; and the meaning of the _optimum_ temperature, illumination, oxygen pressure, etc., is, from this point of view, not that the molecular activities differ in kind from those nearer the minima and maxima, so much as that they are running at the best rates for the welfare of the plant--_i.e._ for permanent health. If we transcend the cardinal points limiting the range of this play, however, and we get variations in the _kind_ as well as _rates_ of molecular constructions and disruptions, then we pass by imperceptible gradations into ill-health--_i.e._ _Disease_. And similarly in relation to other protoplasm. That of the right kind of pollen grain from another plant of its own species, stimulates the contents of the ovule to produce a vigorous embryo and healthy seedling: that of a similar pollen grain in its own flower either does no positive harm, but has a feebler effect, or it may act like a poison. That of another pollen grain again may refuse to unite at all; while that of a fungus hypha--_e.g._ of _Sclerotinia_ on _Vaccinium_--may run down the style as does the pollen tube and produce death and destruction throughout the ovule.