only grow by apposition, or the deposit of fresh solid matter at their surface; while the monera grow, like all cells, by intussusception, or the taking of new matter into their interior. But this difference is easily explained by their difference in consistency, the crystal being solid and the plasm semi-fluid. Moreover, the difference is not absolute; there are intermediary stages between apposition and intussusception. A colloid globule suspended in a salt solution in which it is not dissolved may grow by intussusception.
It was once the custom to restrict sensation and movement to animals, but they are now recognized to be present in nearly all living matter. They are, in fact, not altogether lacking in crystals, as the molecules move in crystallization in definite directions, and unite according to fixed laws; they must, therefore, also possess sensation, as we could not otherwise understand the attraction of the homogeneous particles. We find in crystallization, as in every chemical process, certain movements which are unintelligible without sensation—unconscious sensation, of course. In this respect, also, then, the growth of all bodies follows the same laws (cf. chapters xiii. and xv.).
The growth of a crystal is restricted like the growth of a moneron or of any cell. If the limit is passed and the conditions remain favorable to growth, we find an instance of that excessive or transgressive growth which we call reproduction in the case of living individuals. But we find just the same kind of extension in the inorganic crystal. Every crystal grows in a supersaturated medium only up to a definite size, which is determined by its chemical-molecular constitution. When this limit is reached a number of small crystals appear on the large one. Ostwald, who has made a thorough comparison of the process of growth in crystals and monera, especially notices the striking analogy between a bacterium (a plasmophagous moneron) growing and multiplying in its nutritive fluid and a crystal in its matrix. When the water slowly evaporates from a supersaturated solution of Glauber-salt, not only does a crystal slowly grow in it, but several young crystals appear on it. The analogy with the bacterium multiplying in its nutritive fluid can even be followed as far as its permanent forms or "spores." This quiescent form is assumed by the bacterium if its supply of food is exhausted; if fresh food is added, the multiplication by cleavage begins again. In the same way the crystals of Glauber-salt begin to decay when the solution is evaporated; they lose their crystal water, but not their power of multiplication. Even the amorphous powder of the salt causes again the formation of new watery crystals when put in a supersaturated solution. But the powder loses this property when it is heated, just as the dormant forms (or spores) of the bacteria lose their power of germination.
The exhaustive comparison of the growth of crystals and monera (as the simplest forms of unnucleated cells) is important, because it shows the possibility of tracing the vital function of reproduction—which had usually been regarded as a quite special "wonder of life"—to purely physical conditions. The division of the growing individual into several young ones must necessarily take place when the natural limit of growth has been passed, and when the chemical composition of the growing body and the cohesion of its molecules allow no further enlargement by the assumption of new matter. In order to illustrate the limit of this transgressive growth by a simple physical example, Ostwald imagines a ball placed in a small flat basin, built up high on one side. The ball is in a state of equilibrium in the basin; when it is lightly pushed aside it always returns to its original position. But when the push goes beyond a certain point, and the ball is thrust over the side of the basin, the balance is lost; the ball does not return, but falls to the ground. The crystal behaves just in the same way in a supersaturated solution when it exercises its power of forming new crystals; and it is just the same with the bacterium growing in a nutritive fluid when it passes the limit of its volume of growth, and divides into two individuals.
As we can find no morphological and little physiological difference between the living and non-living, we must look upon metabolism as the chief characteristic of organic life. This process causes the conversion of food into plasm; it is determined by the vital force itself, and is the formation of new living matter. It thus effects the nutrition and growth of the living being, and therefore its reproduction, which is merely transgressive growth. As I shall describe this metabolism fully in the tenth chapter, I will do no more here than emphasize the fact that this vital process also has analogies in inorganic chemistry, in the curious process of catalysis, especially that form of it which we call fermentation.
The distinguished chemist Berzelius discovered in 1810 the remarkable fact that certain bodies, by their mere presence, apart from their chemical affinity, set other bodies in decomposition or composition without being themselves affected. Thus, for instance, sulphuric acid changes the starch in sugar without undergoing any alteration itself. Finely ground platinum brought in contact with hydrogen-superoxide divides it into hydrogen and oxygen. Berzelius called this process catalysis; Mitscherlich, who discovered the cause of it to be the peculiar surface-action of many bodies, gave it the name of "contact-action." It was afterwards discovered that catalysis of this kind is very general, and that a special form of it—fermentation—plays an important part in the life of organisms.
This special form of contact-action which we call fermentation is always effected by catalytic bodies of the albuminoid class, and, in fact, of the group of non-coagulable proteins which are known as peptones. They have—in however small a quantity—the capacity to throw into decomposition large masses of organic matter (in the form of yeast, putrid matter, etc.) without themselves taking part in the decomposition. When these ferments are free and unorganized they are called enzyma, in opposition to organized ferments (bacteria, yeast-fungi, etc.); though the catalytic action of the latter also consists essentially in the production of enzyma. The recent investigations of Verworn, Hofmeister, Ostwald, etc., have shown that these catalyses play everywhere an important part in the life of the plasm. Many recent chemists and physiologists are of opinion that plasm is a colloid catalysator, and that all the varied activities of life are connected with this fundamental vital chemistry. Thus Franz Hofmeister (1901) says in his excellent work on The Chemical Organization of the Cell:
The belief that the agents of the chemical transformation in the cell are catalysators of a colloid nature is in complete accord with other facts that have been directly ascertained. What else are the chemists' ferments but colloid catalysators? The idea that the ferments are the essential chemical agency in the cell is calculated to meet the difficulty which arises from the smallness of the cell in appreciating its chemical processes. However large we suppose the colloid ferment molecules to be, there is room for millions of them in the smallest cell.
In the same way Ostwald attributes the greatest significance to catalysis in connection with the vital processes, and seeks to explain them on his theory of energy by reference to the duration of chemical processes. In the discourse "On Catalysis" that he delivered at Hamburg in 1901 he says:
We must recognize the enzyma as catalysators that arise in the organism during the life of the cells, and by their action relieve the living being of the greater part of its duties. Not only are digestion and assimilation controlled by enzyma from first to last, but the fundamental vital action of most organisms, the production of the necessary chemical energy by combustion at the expense of the oxygen in the air, takes place with the explicit co-operation of enzyma, and would be impossible without them. Free oxygen is, as is well known, a very inert body at the temperature of the living body, and the maintenance of life would be impossible without some acceleration of its rate of reaction.
In his further observations on catalysis and metabolism he says that they are both equally subject to the physico-chemical laws of energy.
Max Verworn has given us a very searching analysis of the molecular process in the catalytic aspect of metabolism in his Biogen Hypothesis (1903), "a critical and experimental study of the processes in living matter." He simplifies the catalytic theory of the enzyma by tracing all the phenomena of life to the catalytic metabolism of one single chemical compound, the plasm, and regards its active molecules, the biogens, as the ultimate chemical factors of the vital process. While the enzyma hypothesis assumes that there are in each cell a great number of different enzyma which are all co-ordinated, and each of which only performs its little special work, the biogen hypothesis deduces all the vital phenomena from one compound, the biogenetic plasm; and thus the biogen molecules, which increase by division into