That organic change in matter is the most universal and distinctive evidence of life has been declared by physiological chemists and chemical physiologists times without number during the last thirty years and their utterances are translated by Herr Duehring into his own clear and elegant language. But to define life as an organic change of matter is simply to define life as life, for organic change of matter, or change of matter with plastic creative arrangement is a statement which must itself be explained by life, and the explanation in its turn by the difference between organic and inorganic, that is between that which is alive and that which is not alive. So that with this explanation we do not get at the problem.
Organic change, as such, is frequently found where life does not exist. There are whole series of processes in chemistry, which by the proper combination of the elements, produce again their own conditions, so that thereby a certain body is the creator of a process. Thus in the manufacture of sulphuric acid by the burning of sulphur, there is created in this process sulphuric dioxide SO2, and if one add steam and nitric acid thereto, the sulphuric dioxide takes up the water and the oxygen and becomes H2 SO4. Nitric acid gives off oxygen and becomes nitric oxide, this nitric oxide simultaneously takes up new oxygen from the atmosphere and is transformed into a higher oxide of nitrogen and from this acid sulphuric dioxide is again given off and made by the same process, so that, theoretically, an infinitely small amount of nitric acid should be effective to transform an unlimited quantity of sulphuric dioxide, oxygen and water into sulphuric acid. Change in matter regularly occurs through the passing of fluids through dead organic and inorganic membranes as in the artificial cells of Traube. It therefore appears that there is no progress by the way of organic change for the quality of organic change which was to explain life must itself be explained by life. We must therefore seek it elsewhere.
Life is a mode of existence of protoplasm and consists essentially in the constant renewal of the chemical constituents of this substance. Protoplasm is here understood in the modern chemical sense and comprises under this name all substances analogous to the white of an egg, otherwise called protein substances. The name is not satisfactory, for the ordinary white of egg plays the least active role of all transformed substances, since it only serves as mere nourishment for the yolk, for the self-developing germ. As long however as so little is known of the chemical constituents of protoplasm the name is better than any other because more inclusive.
Whenever we discover life we also find it bound up with protoplasm, and when we find a piece of protoplasm not in solution there we find also life, without exception. Doubtless the presence of other chemical constituents is necessary to a living body, to produce the various differentiations of these elements of life. They are not necessary to life in itself, hence they enter as food and become transformed into protoplasm. The lowest forms of life with which we are acquainted are nothing but simple pieces of protoplasm and yet they have all the appearance of living objects.
But in what consist these signs of life which are common to all living objects? In this, that the protoplasm takes from its surroundings other matter suitable to itself and assimilates it while other former portions of the body become decomposed and are thrown off. Other things, not living bodies, decompose or make combinations, but cease thereby to be what they were. The rock worn by atmospheric action is no longer rock, the metal which becomes oxidised goes off in rust. But what causes the destruction of dead bodies is the essential of the existence of living protoplasm. From the very moment when the unbroken interchange in the constituents of protoplasm ceases, the continual interchange of receiving and throwing off, from that moment the protoplasmic substance itself ceases, becomes decomposed, that is, dies.
Life, the mode of existence of protoplasmic substance, therefore consists in this, that at one and the same moment it is itself and something else, and this is not the result of a process to which it is compelled by external agency, since this may happen also with objects which are dead. On the contrary life, which is change of matter, is consequent upon nourishment and throwing off, is a self-fulfilling process inherent in its medium, protoplasm, without which it cannot exist. Hence, it follows that if chemistry should ever discover how to make protoplasm artificially, this protoplasm must show some signs of life, even if very insignificant. It is, of course, doubtful if chemistry will discover the proper food for this protoplasm at the same time as the protoplasm.
Through the changes in matter produced by nourishment and throwing off, as actual functions of the protoplasm, and through its own plasticity, proceed all the other most simple factors of life, sensibility which consists in the interchange between the protoplasm and its food, contractibility which shows itself at a very low stage in the consumption of food, possibility of growth which is shown in the lowest stages of development by splitting, and internal motion without which neither the consumption nor assimilation of food is possible.
Our definition of life is, of course, very incomplete since in order to include all the widely differing manifestations of life it must confine itself to the most universal and simple. Definitions are of little scientific worth. In order to determine what life is we must examine all forms of its manifestation from the lowest to the highest. For ordinary use such definitions are very convenient and in a certain sense indispensable, and they can do no harm as long as their inevitable deficiencies are not forgotten.
(The remainder of this section simply teases Herr Duehring.)