Popular Science Monthly/Volume 30/April 1887/On the True Aim of Physiology
By Professor W. PREYER,
OF THE UNIVERSITY OF JENA.
FOR a long while I have felt the desire to answer in a popular treatise the question, What ways and aims ought physiology to pursue? Most naturalists consider the explanation of all phenomena, including those of living bodies, only satisfactory if mechanical—that is to say, if, in strict logical sequence, it is based upon the principles of modern physics as taught by Galileo nearly three centuries ago. Thus, G. Kirchhoff considers the highest aim which the natural sciences have to strive for to be the discovery of the "forces" existing in Nature and of the condition of "matter"; in other words, "the explanation of all natural phenomena by means of mechanical laws." The fact that besides the forces, which mechanics has to deal with, there exist, too, chemical forces independent of the former, is illustrated by an hypothesis: "The same particles of matter, which at a greater distance affect each other only through gravitation, manifest, when placed in proper proximity, molecular forces, which appear in their protean forms, now as forces of elasticity, of cohesion and adhesion, now as forces of chemical affinity." The proof to what extent chemical affinity is a molecular force dependent on the unequal proximity of bodies affecting each other chemically, is wanting. It also remains an open question whether at greater distances masses act upon each other only through gravitation. But, lest it should be inferred that inorganic nature only must be explained mechanically. Professor Kirchhoff, in accord with many naturalists, adds: "We must confess that at present we possess but little knowledge of the condition of matter as well as of the forces through which its particles act upon each other, and that our comprehension of natural phenomena, even of those connected with inorganic bodies, is as yet very imperfect. The same may be said with respect to the much more complex processes which take place in plants and in animate bodies. In either case a true conception can not be formed so long as the mechanical theory has not been satisfactorily demonstrated. This goal never will be reached by the natural sciences, but the mere fact of its having been recognized as such gives a certain satisfaction, and in approaching it we experience that highest enjoyment which the investigation of the phenomena of Nature affords."
I am unable to share that satisfaction, since I do not recognize such a goal as the true one, nor does the approach to it afford that high enjoyment, because of our progress being constantly impeded by facts. The processes in a living body, even in mere protoplasm, can not possibly be entirely based on mechanics, nor can we meet the difficulty by substituting for "mechanics" the highest physics and chemistry, including consummate molecular mechanics. For physics treats only of phenomena dependent on motion—that is, changes in space, the forces, the causes of motion, and the causes of changes in motion, as formulated by human intelligence, are its proper domain. Chemistry, which ascribes to Nature one single force—that of affinity—because it does not need any other, deals with substances alone, with the various elements of matter. Processes which can not be attributed to either changes of forces, i. e., change of one force into another, or to changes of substances, i. e., the transformation of the latter through separation and combination, are not physical and not chemical; consequently, neither physics nor chemistry is engaged in their investigation. Such processes, however, take place in animate bodies—the feeling of hunger, for instance, which the most thorough physical and chemical inquiry fails to explain, though its necessary conditions and consequences may be ascertained.
During the fertilization and division of the ovum, during the differentiation of embryonic forms, during the gradual transformation of fetal and aimless into functionary movements of muscles and nerves, and during the perfection of the organs of sense and of the nerve-centers after birth, a series of processes occur which are entirely outside the domain of both the physicist and the chemist. They are not called upon to deal with the problems of heredity and psychogenesis, as they do not present themselves in the physical and chemical world.
For the very reason that there is one world only, all dualistic systems are untenable. The mechanical theory asserts, though illogically, that, by means of its one-sided principles, all things will in time be understood. The dualistic or vitalistic theory starts by assuming the existence of a contrast in the world, the forces in the living body being different from those in the crystal, those in the brain not the same as those in the material of which the brain is composed, those in young protoplasm not the same as in the old. Now it devolves upon physiology to show how to eliminate this "vitalism" which encumbers its path. Physiological inquiry must attach more importance to the conception of evolution. Morphology, in applying the method of evolution in all its departments, has gained a higher repute than ever. The anatomical phylogenetic history of man, in connection with his embryological and later history of development, may indeed furnish an explanation of the marvelous fitness and natural evolution of the human body.
It is, therefore, surprising that in that domain of the science treating of living bodies, in physiology, or the science of the functions, it hitherto has not been applied at all, or only occasionally and reluctantly. The neglect is due partly to the erroneous opinion that not the physiological function, but only its substratum the bodily organ, is capable of evolution. The organ, that much is certain, develops—that is, it passes through a series of transformations before assuming its final form. Any part of every organism will furnish the proof. But what determines the final form during phylogenetic evolution? I answer, Function. With its assertion begins the differentiation of the substratum of primitive beings. It is not the organ from which function derives its origin, but just the reverse. The functions create their organs, or, to use a better definition, necessity determines the organic form, which hence becomes hereditary, and ultimately in the embryo of the higher animals in structure at least precedes function.
When the embryo of the land-salamander, many months previous to the normal time of its entry into the world, is taken out of the egg and kept in water well supplied with oxygen, neither too warm nor too cold, nor too dark, and amply fed with small living water-animals, and if care is taken that the creature can not get out of the water, its organism will change. It has to inhale the oxygen dissolved in the water, not that of the atmosphere, like its parents breathing with lungs. Its lungs therefore remain undeveloped, but by way of compensation strong gills appear at each side of the head. The originally very feeble function of respiration through gills, in conformity with the increased demands of the growing body, creates a new organ, or calls forth one possessed by its remote ancestors. The animal, moreover, feels the necessity to swim, not to creep, like its terrestrial parents. Its four extremities, therefore, become mere rudimentary appendages, while, on the other hand, a vigorous rudder-tail develops. The function of swimming calls forth fins, new organs which the parents lack. Thus a substantially new animal is produced, which elsewhere does not exist, and which shows how through the development of new functions new organs are formed, or, as it were, resuscitated.
This principle applies not only to particular cases with artificially created conditions but to all functions. All of them precede the organs devoted to their exclusive service. All of them originate through competition for the necessaries of life. At first a simple want is easily satisfied by simple means, but gradually the organism is called upon to meet numerous demands requiring complex contrivances through differentiation.
Instruments, apparatus, engines, are tools or organs invented by human beings, because the wants of better food, better air, better water, or the necessity of saving space and time, or of having means of communication, protection etc., become more and more pressing among them. They have, as it were, become part of the organism. Such newly invented artificial organs as the spectacles, the watch, the shoe, have all a long history of evolution.
The kitchen, with all its large and small utensils for the cooking and mixing and chemical preparation of the raw produce of the animal and vegetable kingdoms, may be looked upon as one single digestive apparatus, an ante-stomach whose history of evolution, covering thousands of years, distinctly shows the increasing demands for digestibility and relish, the need to diminish the work of the teeth and to ease the process of mastication. The starting-point is here the function of taking and assimilating food.
In this sense it is not only permissible but necessary to speak of an evolution of physiological functions. No organic structure develops without having an activity, a necessity, to intensify this activity for its cause. The cause of this increase in activity or differentiation is simply functional evolution. It is the principle of all organic growth, of all morphological evolution, and, wherever it decreases or ceases, the latter at once retrogrades. Without function, no organic formation; increase of function, organic differentiation; cessation of function, organic retrogression.
In applying this principle to the physical and mental activities of the healthy and the sick human being, physiological inquiry must of necessity be carried on in two directions. Since it is necessary to know not only the nature of functions, but also how they have evolved, the evolving living body, the embryo, must be physiologically examined. That is one direction. To compare the complex functions of man, the most complex of all beings, with the less perfected functions of animals—and of plants too—is the other task. Physiology must be comparative only, said, in 1826, Johannes Müller. The genetic and the comparative science of functions go together. Both, as yet incipient, must by-and-by become the basis of the biology of the future.
Each single function of man must step by step be followed back to its first manifestation in the living ovum, in the individual life, and in that line of animals which nearest approach his ancestors, and hence further up to the merely living protoplasm which is neither animal nor vegetable. It then may dawn on us whence are derived the high and lower functions—e.g., speaking and seeing, as well as breathing and walking, and how they have become as they are.
But if we continue to inquire without comparing, we can not arrive at such knowledge; but by merely observing in which way in one instance a function is performing, we may, with great expenditure of labor and ingenuity, time and material sacrifices, find only how it may be or may have been, not how it is and has been. The preference since Galvani's time for the frog as an object of physiological inquiry, the too frequent use of dogs, rabbits, and Guinea-pigs, already called the domestic animals of physiologists; and the levity with which the discoveries made on these few animals, so widely differing from man, have sometimes been applied to the latter, have caused many errors. It is gratifying that at least some of the younger investigators choose also other objects of inquiry, but they should not be the exception. By no means can it be urged that it is too difficult to procure the material, and that in zoölogical gardens physiological laboratories can not well be established. Not every physiologist will, like the great Harvey, get a deer-park placed at his disposal by his king; but the forests and fields, the lakes and rivers of Germany yield abundant material to the investigator, and with our modern means of communication living animals may be quickly dispatched from zoölogical gardens to physiological institutes. The latter, however, avail themselves too little of their opportunities.
All the working material that can be obtained in this way, however, is but a very small fraction of what the sea affords, and he who earnestly desires to further comparative physiology without laboriously hunting for objects for examination must go to the sea-shore. But in order better to approach the end in view without waste of time, laboratories for physiologists ought to be constructed on the coast, especially in places where varieties abound. The aquaria which are being erected in increasing numbers in Great Britain and France are admirably adapted to the purpose. The zoölogical station at Naples, however, which owes its existence to the tireless energy of Professor Anton Dohrn, has as yet no equal. Its scientific achievements, its international character, its admirable organization, its favorable location, make it, for purposes of physiological and morphological investigations, especially desirable. Plaving enjoyed the privilege of working there during one winter, I am able to speak from experience of the eminent services which the high-minded founder and manager of the Naples Institute, the first of its kind, has rendered to the furtherance of the true physiology. He, the first to accept functional change as the principle of morphological and philogenetic inquiry, has also been the first to recognize the advantages of the physiological investigations of marine animals.
It must frequently pain a naturalist to witness how untold living treasure coming from the sea up to the surface in aquaria is allowed to return to the dark deep as unworthy of observation and examination. And yet by experimenting on the superabundant and to the physiologist irresistible wonders of the sea, more light may be thrown on the relations of the phenomena of life, on the becoming and being of the higher and the highest, and also the mental functions, than is to be obtained by limiting ourselves, as has been the custom, to a few animals of our own geographical environment. When, for instance, we observe how star-fishes, generally supposed to be capable of reflex movements only, will free themselves from fetters and difficult situations, like highly intelligent beings, in an amazingly dexterous manner and with the nicest adaptiveness, how, with the precision and alacrity of expert gymnasts they will vault from a piece of driftwood on to the solid rock, or, while freely suspended, will change the dorsal attitude into the normal one, remove entangled obstacles with their long spikes braced against each other as circumstances require, like long-armed or short-armed levers, and otherwise accomplish unexpected feats, we have the proof not only that the prevailing views in regard to the lack of intelligence of such low organisms of the animal kingdom are erroneous, but also that their mental functions may be highly developed without such intricate development of the nervous system as is possessed by the higher organized beings of less psychic capability. I have witnessed how a South-Sea Islander was unable to take off a coat which had been put on him in the regular way. It did not occur to him to stretch one arm backward. The star-fish, however, easily frees itself in the best possible manner from rings, firmly knotted threadings, wrappings, and incumbrances, with which it has not previously come in contact. Such observations must necessarily influence the principles of inquiry. A large brain is required not for one single intellectual act but for a multiplicity. I have found that when many of the tiny ganglionic cells of the Echinodermata remain in organic connection with only one spike, they are capable of doing more work both as to quantity and quality than a smaller number will accomplish under the same conditions. Hence it would appear that also with the higher animals, and with man, the greater intelligence depends on the greater number of ganglionic cells and their combined action rather than on the relatively larger brain. In this way the inquiry into the movements of marine animals directly leads up to the physiology of the brain. Through comparison with that of the animal only is human psychic activity to be understood, for it is the last and highest link of a long chain of evolution whose gradations can only be recognized by the aid of philogeny and physiology—i.e., through the comparison and the history of evolution of functions.
The most attractive problems of the future lie in this direction, and, as soon as the labors in this field have borne more fruit, the different views which now oppose each other will become reconciled. But in other departments of science, too, the perception is dawning that it is of far greater significance to ascertain by comparison the becoming, the growing, the evolution, than to describe the phenomenon by itself just as it happens to present itself to the observer whenever he thinks fit to observe.
In 1861 one of the foremost chemists of the period declared, "The relations of a body to what it has been and to what it may become are the essential part of chemistry" (Kékulé). Instead of "chemistry," we might just as well say "morphology, or history of evolution." The same principle applies to physics, to astronomy, geology, and in a certain sense even to the science of languages. For physics, too, deals with the relations of a conglomeration of forces or of a body to its own past and future. Its ideal is to predict the future of a body, and to estimate its past from its present appearance. Astronomy, in this respect, surpasses all sciences, because its prophecies are being verified with most precision. Geology is essentially the history of the evolution of the globe; comparative philology endeavors, as it were, to find out, from the relations of living languages to the dead and living ones, the pedigree of each idiom, just as the zoölogist is trying to discover the origin of present animals from their relations to the fossils and to each other. Everywhere thinking naturalists are tacitly or confessedly influenced by ideas closely resembling the conception of evolution. All are anxious to ascertain the past and future conditions from existing ones, which is the very essence of evolution. The nature of the transition of one condition into another, its laws, its velocity, its consequences, all these differ in the special departments, not the general fact of the change of condition itself. When, at the same time, the sunbeams warm the human body and delight the eye with glowing colors, when they dye the young plant green and the sensitive glass plate violet; when they move the radiometer, make the telephone resound, allure millions of tiny winged insects into the air, banish millions of other beings which shun the light under ground and to the depths of the waters, close night-blowing flowers while opening others bedecked with dew at the dawn of day—it is ever the one immense sun who, with the same life-giving and life-destroying rays, is working such different wonders. In like manner it is, with all natural sciences, the evolution theory which is producing the different conceptions of Nature. Every one is anxious to comprehend the true sequence of phenomena. All stand firmly on the impregnable basis of the "principle of sufficient reason," which states that every change must be preceded by a change and be followed by another. But when we ask. Which was the first, which the following? difference of opinion will arise: Is an animal, which has little capability, endowed with a simple organism, because it has as yet not been differentiated, or because it has retrograded in its descent from more highly developed beings? Such questions sometimes are extremely difficult to answer, and in these cases it devolves upon physiology to decide or at least to pronounce its weighty judgments. For whenever an organ retrogrades, function has disappeared much sooner than the rudimentary organ; but when, on the other hand, an organ is continuing to develop, function has appeared much sooner than the perfected organ. It would then have to be ascertained whether the organ in question is still possessed of a function, or has already lost it. If, for example, a perfected eye shows little or no sensitiveness to light, it must be in a state of retrogression; if it is very imperfect, and yet extremely sensitive to light, it is developing, while an eye of very simple structure which is not sensitive to light can only have become so through retrogression.
Embryonic eyes, of course, are in a state of progressive but individual development, while here we are only speaking of phyletic evolution. The inorganic sciences have similar questions to answer, though with them the conception of sensitiveness to light has a different meaning, and merely signifies receptibility, the capability of assimilation or of disintegration, without an admixture of feeling and sensitiveness in a physiological sense. But, to continue the illustration, the mere question whether decomposition through light is less felt by the bromide of silver of a photographic plate than by the protoplasm in the leaves of a tree which derive their green color from that light—this question is embarrassing to those who assign to animals alone the sense of feeling, for, since the highly excitable protoplasm of nerveless animals is likewise sensitive to light, and not to be distinguished from that of many plants, they must decide whether the former specifically differs from the latter, or whether both are equally incapable of feeling. In the former case they are called upon to point out the specific difference, which they can not do; in the other they must state where, proceeding from the lower to the higher organisms in the chain of animal creation, the inability of feeling ceases and the sense of feeling begins, which is equally impossible.
Thus, it is in accordance with facts to assume that there is no well-developed dividing line between beings capable and incapable of feeling, but that all matter is endowed with a certain sense of feeling, which, however, only with a definite and an extremely complex arrangement and vibration of the molecules will develop into feeling. The simple bodies, the dead elements, therefore, although partly very easily changed through slight influences, are, in spite of their dim sense of feeling, not able to feel perceptibly, but as soon as they become part of the ganglionic cell of the brain, or only of the living protoplasm (through assimilation of food), they, combined with others, will by indescribably complex vibrations cause feeling to arise like lightning whenever an impression is made on them.
Every physiological explanation must, above all, be in perfect accord with morphological, mechanical, and chemical facts; on that all physiologists lay the greatest stress, but I do not understand why, regardless of physiological facts, morphologists, physicists, and chemists should be allowed to declare their explanations and principles to be the only true ones, or even the only possible ones. It has been demonstrated that matter must have other fundamental properties besides those ascribed to it by physicists and chemists. The axiom of mechanics, "Matter is dead!" will soon become obsolete, since a sense of feeling is inherent in all matter. This supposition does not make the least alteration in the imposing structure of the physical and chemical sciences, because in their formulas the new factor is merely an infinitesimal quantity in proportion to the rest; but the imperceptible is not the less real than the perceptible, because of its imperceptibility.
No one can hear a single leaflet tremble in the wind, but during a storm the roaring of the forest, caused by many leaves rustling together, may reach awe-inspiring power. Similarly, each molecule of matter may feel imperceptibly little when vibrating by itself, while, together with many particles feeling likewise imperceptibly, it may co-operate in manifesting feeling, which, like lightning, arises and vanishes.
Through this conception, through acknowledging evolution and the sense of feeling, the whole of Nature may be brought in harmonious connection.