THE question 'whether life exists in any other worlds than our own' is one in which very many persons feel an interest, and about which much has been said and written; but if the man of ordinary education has any ideas on the subject, they are generally mistaken; and even scientists are prone to regard it too exclusively in the light of their particular science, and thus to conceive and propagate fallacies which might be easily avoided.
Of course no absolute answer to the question can be given until perchance some hitherto undreamt-of means shall be discovered for the observation of distant worlds. We can hardly forward the matter by mere speculation on general grounds, such as the law of probabilities, or the relative position of worlds in the universe. Nevertheless there is one method by which we can at least guard ourselves against erroneous speculation, and prepare the way for discovery when the opportunity comes; and that method is, to find out the conditions on which terrestrial life depends, and then to search other worlds and find if possible whether they provide similar or parallel conditions.
Though the ultimate nature of life is as yet unknown to us, its secrets are being gradually unraveled by research; and it becomes more and more apparent that the phenomena of life are but special and intricate developments of physical action. The most prominent and perhaps most fundamental characteristic of life is what may be called the energy traffic, or the function of trading in energy; and the phenomena of assimilation, growth, movement, etc., are the outward and visible signs of this traffic. Living substance possesses in the highest degree the property of absorbing radiant energy, as heat or light, storing it in a potential form, and subsequently expending it in active forms such as motion, mechanical work, heat and electricity.
Actions of this kind are not unknown in the inorganic world. For example, the atmosphere and ocean absorb the energy of light and heat from the sun, store it temporarily and convert it subsequently into the energy of wind and wave, lightning and thunder. But in living substance there exists a more finely coordinated energy-trading system, evolved out of the chemical capacities of a small number of elements acting under the physical conditions which prevail on our planet.
The energy traffic depends, no doubt, on causes at present unknown to us; and some biologists are wont to personify these causes as a