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 débris 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.