It does not concern us in the present connection how life first came into existence on this planet. It is sufficient that we know from experience that life does exist here; and in whatsoever way it was first generated here, in that same way we may consider that it could have been generated on another planet. Nor need any question trouble us as to the precise line of demarkation to be drawn between inorganic and organic substances, or amongst the latter, between plants and animals. These are important subjects for discussion, but they do not affect us here, for we are essentially concerned with the highest form of organism, the one furthest from these two dividing lines. It suffices that living organisms do exist here, and exist under well-defined conditions. Wanting these conditions, they perish. We can, to a varying degree, determine the physical conditions prevailing upon the heavenly bodies, and we can ascertain whether these physical conditions would be favourable, unfavourable, or fatal to the living organism. What is a living organism? A living organism is such that, though it is continually changing its substance, its identity, as a whole, remains essentially the same. This definition is incomplete, but it gives us a first essential approximation, it indicates the continuance of the whole, with the unceasing change of the details. Were this definition complete, a river would furnish us with a perfect example of a living organism, because, while the river remains, the individual drops of water are continually changing. There is then something more in the living organism than the continuity of the whole, with the change of the details. An analogy, given by Max Verworn, carries us a step further. He likens life to a flame, and takes a gas flame with its butterfly shape as a particularly appropriate illustration. Here the shape of the flame remains constant, even in its details. Immediately above the burner, at the base of the flame, there is a completely dark space; surrounding this, a bluish zone that is faintly luminous; and beyond this again, the broad spread of the two wings that are brightly luminous. The flame, like the river, preserves its identity of form, while its constituent details--the gases that feed it--are in continual change. But there is not only a change of material in the flame; there is a change of condition. Everywhere the gas from the burner is entering into energetic combination with the oxygen of the air, with evolution of light and heat. There is change in the constituent particles as well as change of the constituent particles; there is more than the mere flux of material through the form; there is change of the material, and in the process of that change energy is developed.