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.
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.
The criticisms and summary of facts on which the hypothesis regarding protoplasm here adopted is based are developed at length in Kassowitz, Allgemeinc Biologie,
Wien, 1899, B. I. and II., where the collected literature may be found, and the reader introduced to the huge mass of controversial writings put forward since Darwin and associated with the names of Weismann and others.
It will probably be noticed that I have employed the term molecular unit of protoplasm, and have not discussed the question of organised structure in the latter: this is because it seems clear to me that living protoplasm as such does not possess "organised structure" in the true sense of that term - it is, rather, busy preparing and making "organised structure," and a molecular constitution would have to be ascribed to all "physiological units" of the nature of micellae, pangens, ids, etc., as truly as to the structural units of a starch-grain or cell-wall, or even of a crystal. In this connection, the student will find the necessary points of view put forward in Pfeffer, Physiology, pp. 37 - 83.