Nature Of Life
( Originally Published 1909 )
MORE puzzling than the riddle propounded by the fabled Sphynx is the problem suggested by the title of this chapter. "What is life?" is a question which has been asked by the scholars of all ages and man today is no nearer a final answer than were the philosophers in the earliest centuries of the historic era. Modern science has vastly extended knowledge of the phenomena which are called vital but has failed to tell what life is. Thus according to Herbert Spencer's so-called proximate definition, which of the many definitions of life has attracted most attention, life is the continuous adjustment of internal relations to external relations. This definition Spencer has explained as follows :
"All vital actions, considered not separately but in their ensemble, have for their final purpose the balancing of certain outer processes by certain inner processes. There are unceasing external forces tending to bring the matter of which organic bodies consist, into that state of stable equilibrium displayed by inorganic bodies; there are internal forces by which this tendency is constantly antagonized, and the perpetual changes which constitute Life may be regarded as incidental to the maintenance of the antagonism. To preserve the erect posture, for instance, we see that certain weights have to be neutralized by certain strains : each limb or other organ, gravitating to the Earth and pulling down the parts to which it is attached, has to be preserved in position by the tension of sundry muscles; or in other words, the group of forces which would if allowed bring the body to the ground, has to be counterbalanced by another group of forces. Again, to keep up the temperature at a particular point, the external process of radiation and absorption of heat by the surrounding medium, must be met by a corresponding internal process of chemical combination, whereby more heat may be evolved; to which add, that if from atmospheric changes the loss becomes greater or less, the production must be-come greater or less. And similarly throughout the organic actions in general.
"When we contemplate the lower kinds of life, we see that the correspondences thus maintained are direct and simple; as in a plant, the vitality of which mainly consists in osmotic and chemical actions responding to the co-existence of light, heat, water and carbonic acid around it. But in animals, and especially in the higher orders of them, the correspondences become extremely complex. Materials for growth and repair not being, like those which plants require, everywhere present, but being widely dispersed and under special forms, have to be found, to be secured, and to be reduced to a fit state for assimilation. Hence the need for locomotion; hence the need for the senses; hence the need for. prehensile and destructive appliances; hence the need for an elaborate digestive apparatus.
"Observe, however, that these successive complications are essentially nothing but aids to the maintenance of the organic balance in its integrity, in opposition to those physical, chemical and other agencies which tend to over turn it. And observe, moreover, that while these successive complications subserve this fundamental adaptation of inner to outer actions, they are themselves nothing else but further adaptations of inner to outer actions. For what are those movements by which a predatory creature pursues its prey, or by which its prey seeks to escape, but certain changes in the organism fitted to meet certain changes in its environment? What is that compound operation which constitutes the perception of a piece of food, but a particular correlation of nervous modifications, answering to a particular correlation of physical proper-ties? What is that process by which food when swallowed is reduced to a fit form for assimilation,. but a set of mechanical and chemical actions which distinguish the food? Whence it becomes manifest, that while Life in its simplest form is the correspondence of certain inner physico-chemical actions with certain outer physico-chemical actions, each. advance to a higher form of life consists in a better preservation of this primary correspondence by the establishment of other correspondences.
"Divesting this conception of all superfluities and reducing it to its most abstract shape, we see that Life is definable as the continuous adjustment of internal relations to external relations."
It is clear that this definition in the last analysis is but a general statement of the fundamental vital relations existing between living matter and lifeless phenomena. It does not really define life. Other authors have done no better, in fact their definitions have been less inclusive. But aside from attempts at forming abstract definitions of life there are some significant considerations regarding living matter which have been important in the development of knowledge of the differences between the living and the lifeless and of the working of the mechanism of life.
The living and the lifeless present a fundamental contrast. "If the development of the conception of life be followed back," says Verworn, "when mankind had no presentiments of all the occupations that accompany a highly developed culture, when he was unacquainted with fire, when he did not know how to make even the most primitive tools, the conclusion is reached that the conception sprang from the combination of a number of simple phenomena, which early man discovered by self-observation, especially those phenomena that are associated with evident movements, such as locomotion, breathing, nutrition, the heartbeat and others. In fact, it is not difficult to analyze into their primary constituents the complex occupations of our present life, and to recognise that its diversity is produced by various combinations of a few elementary phenomena, such as nutrition, respiration, growth, reproduction, movement and the production of heat."
It must be remembered, however, that such a conception of life is limited to the vital phenomena of human, beings, while the field of life is far greater. Animals and plants likewise exhibit vital phenomena, and it may be asked whether these latter are the same as or different from the phenomena that prevail among men. It is evident that all living organisms must be included in the sphere of physiological investigation, the flower and the worm equally with man. Hence the first duty of physiology is to mark out the field of the living, to determine what is living and what is not living an undertaking that is more difficult than it appears.
The conception of life has not always been the same. It has experienced fundamental changes in the course of the development of the human species. Formed first with respect to mankind, it was early extended to other objects. With primitive races, the conception was much wider than 2t present, and they termed living what is no longer regarded as such. With them stars, fire, wind and waves were beings endowed with life and mind, and they were personified in the image of man. The remains of these ideas are still found in the mythology of the classic and modern races. In the course of time the distinction between living and lifeless has been made constantly sharper, but even today a child regards a steam engine as a living animal. The child is guided more or less consciously by the same criterion as the primitive races, who, from the fact of motion, considered as living the dancing flame of a fire or a moving wave. In fact, of all vital phenomena, motion is that which gives most strongly the impression of living.
It may be said that only primitive races and children are misled by the criterion of motion, and that the civilized and adult man, who is versed in a knowledge of life, is capable of deciding easily in any given case between the living and the lifeless. But this is not always true. For example, are dried grains living or lifeless? Is a lentil that has lain unchanged in a chest for years living? Scientific men themselves are not agreed upon this point. The lentil, when dry, does not show phenomena of life, but, if placed in moist earth, it can at any moment be induced to do so. It then sprouts and grows into a plant.
The decision between the living and the lifeless becomes, however, much more difficult with objects that are not commonly seen in daily life e.g., certain microscopic things. Long observation and very detailed investigation are frequently required in order to determine whether certain bodies that are found in a liquid by microscopic examination are living or not. If a drop of the dregs be taken from a bottle of weissbeer and examined with the microscope, it will be found that the liquid contains innumerable small pale globules, often clinging together in groups of two or three, completely at rest so long as they are observed, and showing no trace of movement or other change. Very similar small globules may be observed with a microscope in a drop of milk. The two kinds of globules can be distinguished from one another by strong magnifying powers only.
No trace of vital phenomena can be found in either by the most patient and continued microscopic examination, yet the two objects are as widely different as a living organism and a lifeless substance; for the globules from the beer are the so-called yeast-cells, the active agent in the fermentation of the beer, and are fully developed, unicellular, living organisms, while the globules from the milk are lifeless droplets of fat, which, by their abundant presence and their reflection of light from all sides, give to the milk its white color. The manifestation of motion, which is often ascribed to an internal source because no external source is directly visible, thus frequently misleads to the assumption of life.
Hence, under certain circumstances it is not at all easy to distinguish the living from the lifeless, and it is accordingly clear that the first duty of physiology must be to inquire after the criteria of such a distinction i.e., mentally to circumscribe the subject matter, life, in relation to nonliving nature.
The distinction between living and lifeless matter is made still more complicated by the fact that the living substance of the human body, or of any animal or plant, is only the transformed lifeless matter of the food which has been taken into the body and has there assumed, for a time, the living state. Lifeless matter in the shape of food is continually streaming into all living things on the one hand and passing out again as waste on the other. In its journey through the organism some of this matter enters into the living state and lingers for a time as part of the bodily substance. But sooner or later it dies, and is then for the most part cast out of the body (tho a part may be retained within it, either as an accumulation of waste material, or to serve some useful purpose). Matter may thus pass from the lifeless into the living state and back again to the lifeless, over and over in never-ending cycles. A living plant or animal is like a whirlpool into which, and out of which, matter is constantly streaming, while the whirlpool maintains its characteristic form and individuality.
"To put the matter in the most general shape," says Huxley, "the body of the organism is a sort of focus to which certain material particles converge, in which they move for a time, and from which they are afterward expelled in new combinations. The parallel between a whirlpool in a stream and a living being, which has often been drawn, is just as it is striking. The whirlpool is permanent, but the particles of water which constitute it are incessantly changing. Those which enter it on the one side are whirled around and temporarily constitute a part of its individuality ; and as they leave it on the other side, their places are made good by newcomers.
"Those who have seen the .wonderful whirlpool, three miles below the Falls of Niagara, will not have forgotten the heaped-up wave which tumbles and tosses, a very embodiment of restless energy, where the swift stream hurrying from the Falls is compelled to make a sudden turn toward Lake Ontario. However changeful in the contour of its crest, this wave has been visible, approximately in the same place and with the same general form, for centuries past. Seen from a mile off, it would appear to be a stationary hillock of water. Viewed closely it is a typical expression of the conflicting impulses generated by a swift rush of material particles.
"Now, with all our appliances, we cannot get with a good many miles, so to speak, of the living organism. If we could, we should see that it was nothing but the constant form of a similar turmoil of material molecules, which are constantly flowing into the organism on the one side and streaming out on the other."
What are the distinctive properties of living matter as contrasted with lifeless matter? What are the elementary vital phenomena? The composition of living substance has long been supposed to hold the key to the mysteries of life manifestations. The ancients naïvely believed that they were able to explain the substance of living bodies by the intermixture of certain materials. Thus, Hippocrates believed that the normal human body consists of blood, phlegm and bile; which are mixed together in certain proportions. In the middle ages, when people endeavored to solve the riddle of nature by the great power of alchemy, they thought that they were upon the track of the secret of living substance. How strong this delusion was is shown by the many attempts of the middle ages to produce living substance artificially.
The ardent expectation with which the medieval alchemist in the somber dusk of his laboratory, surrounded by skilled workers and strange apparatus, hoped every moment to see the homunculus arise complete from the retorts or crucibles is a very characteristic feature of the developmental stage of science during these centuries. But, however proud moderns may be of the advancement of science, they have no right to look with scorn upon those attempts of the middle ages, when it is realized that from that time even to the most recent period the attempts have been continued to produce artificially not man himself, but the simplest forms of living substance. Yet all these attempts resemble the endeavor of a man to put together a complicated clock-work without knowing its essential parts.
"However simple the problem of the artificial production of living substance appeared fo the middle ages," says Verworn, "the progress of sober thought and critical investigation has shown constantly how far we are yet from a knowledge of the intimate composition of such substance. How is it possible to produce chemically a substance the chemical composition of which is not at all known? Modern research has been directed, therefore, more and more toward an examination of the composition of living substance. It has penetrated deeply, and continues to penetrate, into the morphological, physical and chemical relations, and the intimate structure of living matter."
Altho this accumulated knowledge concerning the composition of living matter does not distinguish between the living and the lifeless, it has given many interesting facts concerning the properties of living matter. In its physical properties living matter in animal and plant tissues behaves like a liquid. The idea that vital phenomena can be assodated with a solid substratum only is not only unjustified, but even untenable. Not only is it unsupported on any acceptable ground, but it even contradicts facts that may easily be observed. It is quite impossible to understand how protoplasm in the more or less stiff condition of a framework or network can be capable of streaming and flowing as can be observed so easily in certain plant-cells and in Amoeba. It is impossible for a solid network to flow in such a manner that the individual particles of its mass mix continually with one another, as may be seen so clearly in Amoeba. If at first sight the theory of the solid consistency may not be incompatible with the behavior of cells ,that possess a constant form, it is absolutely so with the phenomena exhibited by naked protoplasmic masses. Hence various investigators, especially Berthold and Bütschli, have recently defended strongly the idea of the liquid nature of the cell-contents, and no investigator who is familiar with the phenomena need hesitate to accept this view.
Living matter has a greater density, than water. That some animals and plants float is due to the accumulation in their tissues of certain lifeless matters such as fat and gasses, which diminish their specific gravity.
In chemical structure the differences between living and lifeless matter, altho not distinctive, are of importance. Only twelve elements are found constantly in living matter: carbon, nitrogen, sulphur, hydrogen, oxygen, phosphorus, chlorine, potassium, sodium, magnesium, calcium and iron. A few others, silicon, fluorine, bromine, iodine, aluminium and manganese are found occasionally. But all these elements are also found in the lifeless matter composing the air, water and the surface of the earth.
Since the chemical analysis of living substance has shown that no constituents but these organic elements are to be found in the organism, the important fact follows that an elementary vital substance exists no more than a specific vital force. The conception of a "vital ether," a "spiritus animalis," a "vital matter," etc., with which the earlier physiology so freely dealt, have, therefore, in harmony with the advanced development which analytical chemistry has undergone at the present time, completely disappeared from the present theory of life; living sub-stance is composed of no different chemical materials from those occurring within lifeless bodies.
Nevertheless, one fact deserves mention, viz., that the few general organic elements are not scattered irregularly here and there through the natural system of elements, but they occupy a definite position, being remarkable as elements having very low atomic weights. "Hence," writes Verworn, "the conclusion may with great probability be drawn that in the evolution of the elements the organic elements arose by condensation very early, and therefore existed in the very early stages of the development of our planetary system, at a time when other elements, such as the heavy metals, had not yet been formed."
Following the discovery that living matter contains no distinctive chemical elements, physiological chemists turned their attention to the search for specific chemical compounds. Here the investigation is most difficult. It is not possible to apply the methods of chemistry to living substance without killing it. Every chemical reagent that comes in contact with it disturbs it and changes it, and what is left for investigation is no longer living substance, but a corpse a substance that has wholly different properties. Hence ideas upon the chemistry of the living object can be obtained only by deductions from chemical discoveries in the dead object, deductions the correctness of which can be proved experimentally in the living object only in rare cases.
This alone is responsible for the excessively slow advance of the knowledge of the chemistry of the vital process. It is evident that the greatest foresight is necessary in applying results obtained upon the dead object to conditions in the living, and it must constantly be borne in mind that the chemical relations of the latter are to be distinguished sharply from those of the former.
Altho there is no fundamental difference between the elements composing living and those composing lifeless substance, in other words, altho no special vital element exists in the organic world, some of the elements in living substance form unique compounds which characterize it only, and are never found in lifeless substance. Thus, there exist in the former, besides chemical compounds that occur also in the latter, specific organic complexes of atoms. Many of these organic compounds, especially those that are of special importance to living substance, possess so complicated a constitution that thus far chemistry has not succeeded in obtaining an insight into the spatial relations of the atoms in their molecules, altho the percentage composition of the molecules is known to a greater extent.
There are especially three chief groups of chemical bodies and their transformation-products, by the presence of which living substance is distinguished from lifeless substance; these are proteids, fats and carbohydrates. Of these only the proteids and their derivatives have been demonstrated with certainty as common to all cells; hence they must be set apart among the organic constituents of living matter as the essential or general substances, in contrast to all special substances.
It is not, however, the mere presence of proteids which is characteristic of living matter. White of egg (albumen) contains an abundance of a typical proteid and yet is absolutely lifeless. Living matter does not simply contain proteids, but has the power to manufacture them out of other substances ; and this is a property of living matter exclusively. While the other organic compounds carbohydrates, fats and some simpler substances have not been found in all living matter, it is a significant fact that they are derived from proteids and are necessary to the construction of proteids.
But even the presence of the so-called organic compounds proteids, carbohydrates and fats is an uncertain distinction between the living and the lifeless, for chemistry is rapidly breaking down the supposed barrier between the organic and the inorganic. Each year records new synthetic combinations of compounds supposed to be produced by living matter. Who knows when even the proteid molecule may yield its secret to some synthetic chemist?
Thus the conclusion is reached, strange at first sight, that the matter constituting the living world is identical with that which forms the inorganic world. "And not less true," says Huxley, "is it that, remarkable as are the powers, or, in other words, as are the forces which are exerted by living beings, yet all these forces are either identical with those which exist in the inorganic world, or they are convertible into them. I mean in just the same sense as the researches of physical philosophers have shown that heat is convertible into electricity, that electricity is convertible into magnetism, magnetism into mechanical force or chemical force, and any one of them into the other, each being measurable in terms of the other even so, I say that great law is applicable to the living world."
The composition of living matter as known in modern chemistry then does not satisfactorily distinguish between living and lifeless matter, and the question, what is the characteristic difference? again forces itself to the center of attention. The generally accepted answer is that this difference is to be found in certain powers or properties of living matter. These are the power of waste, repair and growth and the power of reproduction. "Living matter," to quote from Sedgwick and Wilson, "is continually wasting away by a kind of internal combustion, but continually repairs the waste by the process of growth. Moreover, this growth is of a characteristic kind, differing absolutely from the so called growth of lifeless things. Crystals and other lifeless bodies grow, if at all, by accretion, or the addition of new particles to the outside. Living matter grows from within by intussusception, or taking in new particles, and fitting them into the interstices between those already present, throughout the whole mass. And lastly, living matter not only thus repairs its own waste, but also gives rise by reproduction to new masses of living matter which become detached from the parent mass and enter forthwith upon an independent existence.
"We may perceive how extraordinary these properties are by supposing a locomotive engine to possess like powers: to carry on a process of self-repair in order to compensate for wear, to grow and increase in size, detaching from itself at intervals pieces of brass or iron endowed with the power of growing up step by step into other locomotives capable of running themselves, and of reproducing new locomotives in their turn. Precisely these things are done by every living thing, and nothing in any degree comparable with them takes place in the lifeless world."
In connection with the foregoing it should be noted that some modern authors, notably Verworn, hold that even metabolism (the power of waste, repair and growth) does not constitute 'a fundamental contrast between living organisms and inorganic bodies. For example, there are rare cases known to chemists in which simple chemical compounds under certain conditions undergo a regular succession of destructions and constructions, always with the gain or loss of substances which correspond in principle to the continued streaming of matter through the constructive and destructive changes in the metabolism of living substances. Metabolism, then, cannot be said to be absolutely distinctive, as a general principle, of living matter, but practically the metabolism, of proteid which is found in nature only in organic bodies, is sufficiently distinctive of living matter. The discovery of apparently analogous processes in inorganic substances simply suggests that the chemical changes concerned in the powers or activities of living matter are subject to the same fundamental laws as govern change in lifeless matter.
Certain critical physiologists have pointed out that reproduction is not absolutely distinctive. It is true that in higher organisms in which reproduction involves development of eggs, seeds or germs, reproduction is absolutely incomparable with any process in lifeless matter, but many microscopic organisms reproduce by simple division with-out development, and such simple reproduction is not far removed from similar changes which may occur in lifeless matter. But here again these are really only suggestions of similarity between living and lifeless processes. To most present-day biologists the known facts of reproduction, like those of metabolism, are not in any scientific sense paralleled by processes occurring in inorganic matter.
Finally, even the method of growth by intussusception of particles has been said to offer no criterion of distinction, for liquids (and living matter is liquid) grow by intussusception. However, this strikes many biologists as a mere quibble with words, for growth by intussusception in organisms involves other changes e.g., foods not proteids may be the material for growth of plant proteid, and plant proteid may in turn serve as food for growth and give rise to animal proteid.
Such critical comparisons suggesting that metabolism, growth and reproduction are not entirely absent from life-less matter are interesting, but in the present state of scientific knowledge the processes involved in waste, repair, growth and reproduction are to be considered strikingly characteristic of living matter. These are probably complications of simple chemico-physical processes, some of which suggest similarity between living and lifeless matter, but the very complexity of the metabolic and reproductive processes in living forms is distinctive. A parallel case is that of proteid of which, as has been stated, the most distinctive feature is the complexity of its molecule.