A step further, and we come to instances of Symbiosis, where the commingled masses of protoplasm of host and invader continue this harmonious action during life. Clearly there are resemblances between these latter cases and successful grafts, and between both and successful sexual unions where the resulting embryo-cell gives rises to a vigorous and healthy plant; and the more these resemblances are examined in the light of what we know of symbiosis the more they support our contention.

Such considerations as the foregoing suggest, then, that life consists in the regular and progressive building up and breaking down of the complex protoplasm molecules, and is necessarily accompanied by the influx of the indispensable food-elements in certain combinations and atom-complexes for assimilation, and by the combustion of some of the debris of the shattered molecules, which combine with the oxygen in respiration and so afford explosions which raise the temperature and enhance the lability of existing molecules, and act as stimuli to the shattering of further molecules. The results of these rhythmical buildings up (assimilation) and shatterings (dis-assimilation) of the protoplasm molecules are the growth of the protoplasm, with further intercalations of water and new food-supplies, etc., on the one hand, and the formation of metabolic products (proteids, cellulose, sugars, fats, etc.), some of which are again used up, others respired, others deposited as stores, cell-walls, etc., on the other.

That the building-up process depends on the action of molecular forces comparable to those by which a growing crystal goes on selecting atom-complexes of its particular kind from the solution around seems highly probable, and this being the case we can understand how under certain circumstances substitutive selections may occur. That is to say, just as a crystal will sometimes build up into its structure atom-complexes of a kind different from its normal molecules, so, given the proper conditions, a protoplasmic molecular unit will build up into its structure atom-complexes somewhat different from those it had hitherto taken up - i.e. assimilated - with consequent modifications of its behaviour. If this occurs, the modes of further building up and breaking down will be affected by the subsequent action of these slightly modified protoplasm units, and it may well be that the whole significance of variation turns on this. Whether the resulting variation makes for the welfare or otherwise of the organism will then be decided by the struggle for existence, and the natural selection which ensues. Such a view also implies that the energy concerned is primarily what is usually termed chemical energy, and that every compound entering into the protoplasm carries in a supply of this, available in various ways.

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 - 40C., 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.

Or again, in Clover, we may have the hypha of a Botrytis with its protoplasm unable to do more than penetrate into the cellulose walls and diffuse a poison into the adjacent cells, being utterly incapable of directly facing, or mingling with the living protoplasm of such cells, whereas the protoplasm of another organism - e.g. Rhizobium - will penetrate directly into the cells, live in them for weeks or months without injury - nay even with advantage to their life. And hundreds of similar cases can be selected.