A steam-engine may afford us a third illustration. Here fresh material is continually being introduced into the engine there to suffer change. Part is supplied as fuel to the fire there to maintain the temperature of the engine; so far the illustration is analogous to that of the gas flame. But the engine carries us a step further, for part of the material supplied to it is water, which is converted into steam by the heat of the fire, and from the expansion of the steam the energy sought from the machine is derived. Here again we have change in the material with development of energy; but there is not only work done in the subject, there is work done by it. But the living organism differs from artificial machines in that, of itself and by itself, it is continuously drawing into itself non-living matter, converting it into an integral part of the organism, and so endowing it with the qualities of life. And from this non-living matter it derives fresh energy for the carrying on of the life of the organism. The engine and the butterfly gas flame do not give us, any more than the river, a complete picture of the living organism. The form of the river is imposed upon it from without; the river is defined by its bed, by the contour of the country through which it flows. The form and size of the flame are equally defined by exterior conditions; they are imposed upon it by the shape of the burner and the pressure of the gas passing through it. The form of the engine is as its designer has made it. But the form of the living organism is imposed upon it from within; and, as far as we can tell, is inherent in it. Here is the wonder and mystery of life: the power of the living organism to assimilate dead matter, to give it life and bring it into the law and unity of the organism itself. But it cannot do this indiscriminately; it is not able thus to convert every dead material; it is restricted, narrowly restricted, in its action. "One of the chief characteristics of living matter is found in the continuous range of chemical reactions which take place between living cells and their inorganic surroundings. Without cease certain substances are taken up and disappear in the endless round of chemical reactions in the cell. Other substances which have been produced by the chemical reactions in living matter pass out of the cell and reappear in inorganic nature as waste products of the life process. The whole complex of these chemical transformations is generally called _Metabolism_. Inorganic matter contrasts strikingly with living substance. However long a crystal or a piece of metal is kept in observation, there is no change of the substance, and the molecules remain the same and in the same number. For living matter the continuous change of substances is an indispensable condition of existence. To stop the supply of food material for a certain time is sufficient to cause a serious lesion of the life process or even the death of the cell.