low organisms live and multiply in chemical solutions which contain no protoplasm, but only inorganic compounds containing the requisite atoms for making protoplasm, or when a plant not only chemically decomposes carbonic dioxide, exhaling the oxygen and depositing the carbon in its stem and leaves, but also from this and other elements drawn from the soil or air manufactures the living protoplasm which courses through its channels, the result is that life has manufactured life out of non-living materials. If we take sensation, this, in its last analysis, is change, or molecular motion, induced in a body by the action of its environment. Here there is a certain analogy between living and non-living matter, for the latter does respond to changes in the surrounding environment, as in the case of heat, electricity, and otherwise; but living matter is far more sensitive, the changes are far more frequent and complex, and in certain cases they are accompanied by a sensation of what is called consciousness, which in the higher organisms rises into a perception of voluntary effort or free-will as a factor in the transformation of energies. Thus it happens that in the case of dead matter the changes produced by a change of conditions follow fixed laws and can be predicted and calculated, while those of living matter are apparently uncertain and capricious. We can tell how much an iron bar will expand with heat; but we cannot say whether, if a particle of food is brought within reach of an amœba, it will or will not shoot out a finger to seize it. If the amœba is hungry it probably will; if it is enjoying a siesta after a full meal, it probably will not. The case of sensation includes that of motion, which is after all only sensation applied in the liberation of energy of position which has by some chemical process become stored up, either in the living mass, or in some special organ of it, such as muscle. Iron, for instance, moves when it expands by heat or is attracted by a magnet; but it moves, like the planets, by fixed and calculable laws: while living matter moves, as might be expected from the variable character of its sensation, in a manner which often cannot be calculated. There are cases, however, of reflex or involuntary motion, where, even in the highest living organisms, sensation and motion seem to follow change of environment, in a fixed and invariable sequence, as in shrinking from pain, touching or galvanising a nerve; and it may be that the apparent spontaneousness and variability of living motion is only the result of the almost infinitely greater complexity and mobility of the elements of living matter.