Contrast of living and dead—Eating and being eaten—Trace matter upwards and life downwards—Colloids—Cells—Protoplasm—Monera—Composition of protoplasm—Essential qualities of life—Nutrition and sensation—Motion—Reproduction—Spontaneous generation—Organic compounds—Polar conditions of life. Polarity having been established as the universal law of the inorganic world, we have now to pass to the organic, or world of life. At first sight there seems to be a great gulf fixed between the living and the dead which no bridge can span. But first impressions are very apt to deceive us, and when things are traced up to their origins we often find them getting nearer and nearer until it is difficult to say where one begins and the other ends. Take for instance such an antithesis as ‘eating or being eaten.’ If a hunter meets a grizzly bear in the Rocky Mountains, one would say that no distinction can be sharper than whether the bear eats the man, or the man the bear. In the one case there is a man, and in the other a bear, less in the world. But look through a microscope at a glass of water, and you may see two specks of jelly-like substance swimming in it. They are living creatures, for they eat and grow, and thrust out and retract processes of their formless mass, which serve as temporary legs and arms for seizing food and for voluntary motion. In short, they are[77] each what may be called strictly individual am?b?, forming separate units of the animated creation as much as the man and the bear. But if the two happen to come in contact, what happens? The two slimy masses involve one another and coalesce, and the resulting am?ba swims away merrily as two gentlemen rolled into one.
Now in his case what became of their individualities: did am?ba A eat am?ba B, or vice versa, and is the resulting am?ba a survival of A or of B, or of both or neither of them? And what becomes of the antithesis of ‘eating or being eaten’ which was so clear and distinct in the highly specialised forms of life, and is so evanescent in the simpler forms? This illustration may serve to teach us how necessary it is to trace things up to their origins, before expressing too trenchant and confident opinions as to their nature and relations.
In the case of the organic and inorganic worlds the proper course obviously is, not to draw conclusions from extreme and highly specialised instances, but to follow life downwards to its simplest and most primitive form, and matter upwards to the form which approaches most nearly to this form of life. Following matter upwards, we find a regular progression from the simple to the complex. Take the diamond, which is one of the simplest of substances, being merely the crystallised form of a single ultimate element, carbon. It is extremely hard and extremely stable. Ascending to compounds of two, three, or more elements, we get substances which are more complex and less stable; and at last we arrive at combinations which involve many elements and are extremely complex. Among these latter substances are some, called colloids, which[78] are neither solid, like crystals, nor fluid, like liquids, but in an intermediate state, like jelly or the white of an egg, in which the molecules have great mobility and are at a considerable distance apart, so that water can penetrate their mass. These colloids are for the most part very complicated compounds of various elements based on a nucleus of carbon, which, from its atom having four poles with strong mutual attractions, is eminently qualified for forming what may be called the inner skeleton of these complex combinations. Colloids of this description form the last stage of the ascending line from inorganic matter to organic life.
Next let us trace life downwards towards matter. There is a constant succession from the more to the less complex and differentiated: from man, through mammals, reptiles, fishes, and a long chain of more simple forms, until at its end we come to the two last links, which are the same for all animals, all plants, and all forms of animated existence. The last link but one is the cell, the last of all is protoplasm.
Protoplasm, or, as Huxley calls it, ‘the physical basis of life,’ is a colourless jelly-like substance, absolutely homogeneous, without parts or structure, in fact a mere microscopic speck of jelly.
The cell is the first step in the specialisation of protoplasm, the outer layer of which, in contact with the surrounding environment, becoming hardened so as to form an enclosing cell-wall, while a portion of the enclosed protoplasm condenses into a nucleus, in which a further condensation makes what is called the nucleolus or second smaller nucleus. This constitutes the nucleated cell, whose repeated subdivision into other similar cells in geometrical progression furnishes the[79] raw material out of which all the varied structures of the world of life are built up. Plants and animals, bones, muscles, and organs of sense, are all composed of modified cells, hardened, flattened, or otherwise altered, as the case may require. If we trace life up to its origin in the individual instead of in the species, we arrive at the same result. All plants and animals, whether of the lowest or highest forms, fish, reptile, bird, mammal, man, begin their individual existence as a speck of protoplasm, passing into a nucleated cell, which contains in it the whole principle of its subsequent evolution into the mature and completed form.
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[80] 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 animalcul?, 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[81] 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.[82] For instance, when am?b?, 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 am?ba, it will or will not shoot out a finger to seize it. If the am?ba is hungry it probably will; if it is enjoying a siesta after a full meal, it probably will not.
[83]
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[84] matter and of energy; but we can discern no analogy between the non-reproductive nitrog............