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. We may, therefore, conclude that _Variation_ depends fundamentally on alterations in the structure or mode of building up and disintegration of the protoplasmic molecular unit, brought about either by direct modifying action of the inorganic environment--nutrition, temperature, oxygen supply, light, etc., etc.--or by the mingling with it of other protoplasm, the molecules of which since they have already a slightly different composition, configuration, mode of breaking down and building up, etc., affect its molecules by supplying them with altered nutritive atom-complexes, by competing with them for oxygen, etc., etc. Once these molecules are affected, we must assume that long sequences of other chemical and molecular changes will be also modified; and although we have no conception of _how_ these changes bring about changes in form, that they do so is only a conclusion of the same order as that which we hold regarding the much simpler changes concerned in the formation of crystals. That such variations may be of every degree as regards profundity, permanence, kind, etc., may well be imagined; and there is nothing surprising in our being able to induce them more easily by the action of external factors _in the readily accessible cell-protoplasm_ than in the _less exposed nuclear-protoplasm_; because the latter is only accessible through the former, or through the agency of _other nuclear protoplasm already modified_. On these and similar phenomena depend the relative permanency and transmissibility of the variations. Our measure of the latter only begins when the effects referred to have become manifest in large masses of cells, because only then do they become appreciable to our senses. Further, variations thus induced may be of advantage to the continued life of the plant, or in all degrees disadvantageous or threatening to its existence. These latter variations are _Disease_, and if their interference with the normal rhythmical play of the building up and breaking down of the protoplasm molecules proceeds beyond certain limits, life ceases, and we have death supervening on disease. NOTES TO CHAPTER XXX. It appears probable that calcium is not always needed by living cells, and may not enter into the composition of protoplasm; on the other hand traces of iron are perhaps necessary.