Protoplasm is, therefore, evidently the nearest approach of life to matter; and if life ever originated from atomic and molecular combinations, it was in this form. To suppose that any more complex form of life, however humble, could originate from chemical combinations, would be a violation of the law of evolution, which shows a uniform development from the simple to the complex, and never a sudden jump passing at a bound over intermediate grades. To understand life, therefore, we must understand protoplasm; for protoplasm, closely as it approximates to colloid matter, is thoroughly alive. A whole family, the Monera, consist simply of a living globule of jelly, which has not even begun to be differentiated. Every molecule, as in a crystal, is of homogeneous chemical composition and an epitome of the whole mass. There are no special parts, no organs told off for particular functions, and yet all life-functions—nutrition, reproduction, sensation, and movement—are performed, but each by the whole body. The jelly-speck becomes a mouth to swallow, and turning inside out, a stomach to digest. It shoots out tongues of jelly to move and feel with, and presently withdraws them.
With these attributes it is impossible to deny to protoplasm the full attributes of life, or to doubt that, like the atom in the material world, it is the primary element of organic or living existence. Given the atom, we can trace up, step by step, the whole evolution of matter; so given the protoplasm, we can trace up the evolution of life by progressive stages to its highest development—man. To understand life, therefore, we must begin by trying to understand protoplasm.
What is protoplasm? In its substance it is a nitrogenous carbon compound, differing only from other similar compounds of the albuminous family of colloid by the extremely complex composition of its atoms. It consists of five elements, and its average composition is said by chemists to be 52·55 per cent. carbon, 21·23 oxygen, 15·17 nitrogen, 6·7 hydrogen, 1·2 sulphur. Its peculiar qualities, therefore, including life, are not the result of any new and peculiar atom added to the known chemical compounds of the same family, but of the manner of grouping and motions of these well-known material elements. It has in a remarkable degree the faculty of absorbing water, so that its molecules seem to float in it in a condition of semi-fluid aggregation, which seems to be necessary for the complex molecular movements which are the cause or accompaniment of life. Thus, many seeds and animalculæ, if perfectly dry, may remain apparently as dead and as unchanging as crystals, for years, or even, as in the case of the mummy wheat, for centuries, to revive into life when moistened.
But in addition to those material qualities in which protoplasm seems to differ only from a whole group of similar compounds of the type of glycerine, by the greater complexity and mobility of its molecules, it has developed the new and peculiar element which is called life. Life in its essence is manifested by the faculties of nutrition, sensation, movement, and reproduction.
As regards nutrition there is this essential difference between living and non-living matter. The latter, if it feeds and grows at all, does so only by taking on fresh molecules of its own substance on its outer surface, as in the case of a small nucleus-crystal of ice in freezing water. If it feeds on foreign matter and throughout its mass, it does so only in the way of chemical combination, forming a new product. Living matter, on the other hand, feeds internally, and works up foreign substances, by the process we call digestion, into molecules like its own, which it assimilates, rejecting as waste any surplus or foreign matter which it cannot incorporate. It thus grows and decays as assimilation or waste preponderates, remaining always itself. The distinction will be clear if we consider what happens when water rusts iron. In a certain sense the iron may be said to eat the oxygen, reject the hydrogen, and grow, or increase in weight by what it feeds on; but the result is not a bigger piece of iron, but a new substance, rust, or oxide of iron. That living matter should feed internally is not so wonderful, for its semi-fluid condition may well enable foreign molecules to penetrate its mass and come in contact with its own interior molecules; but it is an experience different from anything known in the inorganic world that it should be able to manufacture molecules of protoplasm like its own out of these foreign molecules, and thus grow by assimilation. For instance, when amœbæ, bacteria, and other low organisms live and multiply in chemical solutions which contain no protoplasm, but only inorganic compounds containing the requisite atoms for making protoplasm, or when a plant not only chemically decomposes carbonic dioxide, exhaling the oxygen and depositing the carbon in its stem and leaves, but also from this and other elements drawn from the soil or air manufactures the living protoplasm which courses through its channels, the result is that life has manufactured life out of non-living materials.
If we take sensation, this, in its last analysis, is change, or molecular motion, induced in a body by the action of its environment. Here there is a certain analogy between living and non-living matter, for the latter does respond to changes in the surrounding environment, as in the case of heat, electricity, and otherwise; but living matter is far more sensitive, the changes are far more frequent and complex, and in certain cases they are accompanied by a sensation of what is called consciousness, which in the higher organisms rises into a perception of voluntary effort or free-will as a factor in the transformation of energies. Thus it happens that in the case of dead matter the changes produced by a change of conditions follow fixed laws and can be predicted and calculated, while those of living matter are apparently uncertain and capricious. We can tell how much an iron bar will expand with heat; but we cannot say whether, if a particle of food is brought within reach of an amœba, it will or will not shoot out a finger to seize it. If the amœba is hungry it probably will; if it is enjoying a siesta after a full meal, it probably will not.
The case of sensation includes that of motion, which is after all only sensation applied in the liberation of energy of position which has by some chemical process become stored up, either in the living mass, or in some special organ of it, such as muscle. Iron, for instance, moves when it expands by heat or is attracted by a magnet; but it moves, like the planets, by fixed and calculable laws: while living matter moves, as might be expected from the variable character of its sensation, in a manner which often cannot be calculated. There are cases, however, of reflex or involuntary motion, where, even in the highest living organisms, sensation and motion seem to follow change of environment, in a fixed and invariable sequence, as in shrinking from pain, touching or galvanising a nerve; and it may be that the apparent spontaneousness and variability of living motion is only the result of the almost infinitely greater complexity and mobility of the elements of living matter.
Reproduction remains, which is the faculty most characteristic of life, and which distinguishes most sharply the organic from the inorganic world. In the inorganic there is no known process by which dead matter reproduces itself, as the cell does when it contracts in the middle and splits up into two cells, which in their turn propagate an endless number of similar cells, increasing in geometrical progression until they supply the raw material from which all the countless varieties of living organisms are built up, which, in their turn, repeat the process and reproduce themselves in offspring. This is the real mystery of life; we can partly see or suspect how its other faculties might arise from an extension of the known qualities and laws of matter and of energy; but we can discern no analogy between the non-reproductive nitrogenous carbon compound, which makes so near an approach to protoplasm in its chemical composition, and the reproductive protoplasm, which is fertile, increases and multiplies, and replenishes the earth. Can the gap be bridged over: can protoplasm be manufactured out of chemical elements? It is done every day by plants which make protoplasm out of inorganic elements, and by the lowest forms of life which live and multiply in chemical solutions. It is done also in the life-history of all individuals whose primitive cell or ovum makes thousands or millions of other cells, each containing within its enclosing membrane as much protoplasm as there was in the unit from which they started. But in all these instances there was the living principle to start with, existing in the primitive speck of protoplasm, from which the rest were developed. Can this primitive speck be created; or, in other words, can protoplasm be artificially manufactured by chemical processes?
The answer must be, No; not by any process now known. The similarity of chemical composition, and the increasing conviction of the universality of natural law and of evolution, have led to a very general belief that such a spontaneous generation of life must be possible, and numerous experiments have been made to produce it. For a time the balance seemed to be very evenly held between the supporters and opponents of spontaneous generation. In fact, starting from the assumption, which at first was common to both sides, that heat equal to the boiling point of water destroyed all life organisms, spontaneous generation had the best of it: for it was clearly proved that living organisms did appear in infusions contained in vessels which had been hermetically sealed, after being subjected to this, or even a higher degree of heat. But subsequent and more careful experiments have shown that the germs or spores of bacteria and other animalculæ, which are generally floating in the air, can, when dry, withstand a greater degree of heat, and that when the experiments are made in optically pure air no life ever appears and the infusions never putrefy. On questions of this sort all who are not themselves expert experimentalists must be guided by authority, and we may be content to accept the dictum of Huxley that biogenesis, or all life from previous life, was ‘victorious along the whole line.’ But in doing so we must accept Huxley’s caution, ‘that with organic chemistry, molecular physics, and physiology yet in their infancy, and every day making prodigious strides, it would be the height of presumption for any man to say that the conditions under which matter assumes the qualities called vital, may not some day be artificially brought together.’