The Mechanistic Conception of Life

These observations may serve as examples of the way in which the analysis of the vital phenomena of certain animals shows tropisms to be elements of these phenomena. Many observations of a similar nature are found in the papers of Georges Bohn, Parker, Radl,^ and myself. Under the influence of the theory of natural selection the view has been accepted by many zoologists and psychologists that everything which an animal does is for its best interest. The exact doctrine of heredity, founded by Mendel and advanced to the position of a systematic science in 1900, reduces this idea to its proper value. It is only true that species possessing tropisms which would make reproduction and preservation of the species impossible must die out.

Galvanotropism illustrates this fact in a striking manner. If a galvanic current is passed through a trough filled Tvith water, and animals are placed in this trough, it can be observed that an orientation in relation to the direction of the current takes place in many of the animals, and that they move in the direction either of the positive or of the negative current. This phenomenon we call galvanotropism. In galvanotropism the current lines or the current curves play the same role as the light rays in heliotropism. At those points where the current curves enter the cells^ a collection of ions takes place which influences the chemical reactions. The number of species which show typical galvanotropic reactions is not so great as the number of those showing typical heliotropism. In my opinion this difference is the result of the physical difference in the action of light and of the electric current. Light acts essentially upon the free surface of the animal, while the electric current affects all the cells and nerves. Thus the action of the current upon the skin becomes complicated and modified by its simultaneous effect upon the nerve branches and upon the central nervous system. The result is thus much more complicated than that of the action of light where essentially only the effect upon the skin and retina is involved. For this reason, a distinct galvanotropism is found more often in organisms with a simple structure, as, for instance, in unicellular organisms, than in vertebrates, although it is also demonstrable in the latter.

Galvanotropism is, however, purely a laboratory product. With the exception of a few individuals, which have in recent years fallen into the hands of physiologists who happened to be working on galvanotropism, no animal has ever had the chance to come under the influence of an electric current. And yet galvanotropism is a remarkably common reaction among animals. A more direct contradiction of the view that the 1 Or where the movement of the ions withm the cell is retarded.

reactions of animals are determined by their needs or by natural selection could hardly be found. One might be led to suppose that galvanotropism and heliotropism are not comparable. They are, however, as a matter of fact, phenomena of the same category with the exception of the aforementioned fact, that light acts generally only upon the surface of the skin, while the electric current influences all the cells of the body. As already mentioned, the disturbing complications arising from this latter circumstance disappear for the most part when we work with unicellular organisms, and we should expect that galvanic and heliotropic reactions would more nearly resemble one another in this case, provided that we work with organisms possessing both forms of sensitiveness. And this expectation is fulfilled. The algae of the species Volvox show heliotropism and galvanotropism. The investigations made by Holmes and myself upon heliotropism, as well as those of Bancroft upon the galvanotropism of these organisms indicate that the mechanism of these reactions in Volvox is the same and the degree of determinism of the heliotropic and galvanotropic reactions in Volvox is equally great.

Claparede raises the objection that the galvanotropic reactions are purely compulsory, while the heliotropic reactions are governed by the ''interest of the animal. "^ Such a view, however, is not supported by the facts. The reason whjheliotropism may occasionally, as we have seen, be of use, while galvanotropism has no biological significance, is because the electric current does not exist in nature. It can, however, be shown also that heliotropism is just as useless to many animals as galvanotropism. For instance, I pointed out twenty years ago that some varieties of animals which do not live in the light at all, for instance, the larvae of the goat moth, which live mider the bark of trees, may show positive heliotropism. I found, moreover, that the crab, Cuma Rathkii, which lives in the mud of

1 Claparede, "Les tropismes devant la psychologic," Journ. /. Psychologie und Neurologie, XIII, 150, 1908. the harbor at Kiel, when brought into the Ught and removed from the mud shows positive heUotropism. It is, therefore, just as incorrect to assert that the hehotropic reactions are governed by the bio- ^ ' logical interests of the animal as that this is true for galvanotropism. We must, therefore, free ourselves at once from the overvaluation of natural selection and accept the consequences of Mendel's theory of heredity, according to which the animal is to be looked upon as an aggregate of independent hereditary qualities.

The attempt has been made to prove that organisms are attuned to a certain intensity of light and so regulate their heliotropism that they invariably reach that intensity of light which is best suited to their well-being. I believe that this is also a suggestion forced upon the investigators by the extreme application of the theor}' of natural selection. I have made experiments upon a large number of animals, but, with a clear Fig. 21. — Arrangement to prove that positively hello tropic animals move toward the source of light even if by so doing they go from the simlight into the shade. W W is & window through which smihght S falls into the room. By a piece of board d e the sunlight 5 is prevented from striking the region 6 c of a table near the window and this part of the table is in the shade. Only the dayUght D can reach this part of the table.

A test-tube a c is put on this table at right angles to the plane of the window. At the beginning of the experiment the animals (e.g., the winged aphides) are all at a. The animals move at once toward the window, but instead of remaining at b they keep on moving from the direct svmhght into the shade toward the sovu"ce of light until they all reach the end of the tube c near the window (in the shade) where they remain permanently. arrangement of the physical conditions of the experiment, I have never found a single indication of such an adaptation. In every case it has been sho\\Ti that positively heliotropic animals are positive to any intensity of light above the threshold. Thus winged plant lice or wingless larvae of Chrysorrhoea or copepods, which have been made heliotropic by acids, go toward the light whether the source of light is the direct sunlight or reflected light from the sky or weak lamp light, provided that the (threshold) value of the intensity of light required for the reaction is exceeded. Indeed, I have been able to show that positively heliotropic animals also move toward the source of light even if the arrangement is such that by so doing they go from the light into the shadow.^ I have never observed a *' selection" of a suitable intensity of light.

What probably lies behind these interpretations of the "selection of a suitable intensity of light" is the fact that under certain conditions reaction products formed by the photochemical action of light may inhibit the positive heliotropism. I found a very clear instance of this sort in the newly hatched larvae of Balanus perforatus, which are positively heliotropic. If they are placed in the light of a quartz mercury lamp (of Heraus), which is very rich in ultra-violet rays, the positively heliotropic larvae soon become negatively heliotropic. For these experiments the larvae should be placed only in a very shallow depth of sea-water.

Even in a strong light which is not so rich in ultra-violet rays as the light of the mercury lamp, it is sometimes possible to cause positively heliotropic animals to become negatively heliotropic. This is the case, for instance, with the larvae of Polygordius. But it would be wrong in this case to speak of an adaptation of the animal to a certain intensity of light. 1 Quite often without even stopping for a moment. In animals sensitive to differences (see next chapter) a stopping occurs in this experiment in the passing from the light into the shadow, but they go, nevertheless, immediately on in the direction of the source of light. The reader will find a further account of this experiment in my book on The Dynamics of Living Matter.

In my opinion it is merely a case where a metabolic product either alters the photochemical action or so influences the central nervous system that the excitation of the retina by the light weakens the tonus of the muscles, instead of strengthening it. Some of the other mistakes have perhaps also arisen because the writers worked with complicated experimental conditions instead of with simple ones, for instance, because they used a hollow prism filled with ink in order to produce a gradual decrease in the light intensity. In the semidarkness thus produced, the intensity of light often remains beneath or near the threshold of stimulation, and the writers fall victims to that class of errors which we have already pointed out in speaking of the influence of lesser intensities of light.

Heliotropic phenomena are determined by the relative rates of chemical reactions occurring simultaneously in symmetrical surface elements of an animal. There is a second class of phenomena which is determined by a sudden change in the rate of chemical reactions in the same surface elements. Reactions to a sudden change in the intensity of light are sho^vn most clearly in marine tube-worms, whose gills are exposed to light. If the intensity of the light in the aquarium is suddenly diminished the worms withdraw quickly into their tubes. A sudden increase in the intensity of light has no such effect. With other forms, for instance, with planarians, a sudden decrease in the intensity of the light causes a decrease in movement. Such animals gather chiefly in parts of the space where the intensity of light is relatively small. I have designated such reactions as the expression of sensitiveness to changes in the intensity of a •stimulus (''Unterschiedsempfindlichkeit") differential sensibility, in order to distinguish them from tropisms.^

1 Loeb, " Ueber die Umwandlung positiv heliotropischer Tiere, u.s.w.," Pflugers Archiv, 1893. See also the recent investigations of Georges Bohn, La naissance de V intelligence, Paris, 1909; "Les essais et les erreurs ctiez les etioles de mer," Bull. It is hardly necessary to point out here that the effects of rapid changes in intensity, when they are very marked, can easily complicate and entirely obscure the heliotropic phenomena. In Hypotricha and other infusoria this differential sensibility is very pronounced in response to sudden touch or sudden alteration of the chemical medium, and like the tubeworms they thereupon draw back very quickly. Since their locomotor organs are not symmetrical, but are arranged in a peculiar unsymmetrical manner, they do not, after the next progressive movement, return to the former direction of movement, but deviate sideways from it, and it is therefore easy to understand that such animals do not furnish the best material for demonstrating the laws of heliotropism, especially since they possess only a slight photochemical sensitiveness. But Jennings^ has with special preference used observations on such organisms to argue against the theory of tropisms. Just as the action of a constant current in muscles and nerves is different from that of an intermittent current, so we find an analogous case in the action of light. If we wish to trace all animal reactions back to physico-chemical laws we must take into consideration besides the tropisms not only the facts of the differential sensibility but also all other facts which exert an influence upon the reactions. The influence of that mechanism which we call "associative memory" also belongs in this category, but we cannot discuss this further at this place. The reader is referred to my book^ as well as to the more recent works of Bohn, La naissance de V intelligence^ and La nouvelle psychologie animale.^ Let us bear in mind that "ideas" also

2 Comparative Physiology of the Brain and Comparative Psychology, New York and London, 1900. * Paris, " Bibliotheque de philosophic contemporaine," 1911. can act, much as acids do for the hehotropism of certain animals, namely, to increase the sensitiveness to certain stimuli, and thus can lead to tropism-like movements or actions directed toward a goal. Besides light and the electric current, the force of gravity also has an orienting influence upon a number of animals. The majority of such animals are forced to turn their heads away from the center of the earth and to creep upward. It was uncertain for a long time how the orientation of cells in relation to the center of gravity of the earth could influence the rate of the chemical reactions within, but it has been suggested that an enlargement or shifting of the reacting surfaces formed the essential connecting link. If it is assumed that in such geotropically sensitive cells two phases (for instance, two fluid substances which are not at all, or not easily, miscible, or one solid and one fluid substance) of different specific gravities are present, which react upon one another, a reaction takes place at the surfaces of contact. Every enlargement of the latter increases the mass of reacting molecules. A shifting of the surfaces would act in the same manner. Finally, a third possibility remains which could perhaps be realized in plant roots and stems. If in the geotropically sensitive elements two masses of different specific gravity are present, only one of which reacts to the flowing sap in the center or the periphery of the stem, the cells of the upper side of a stem which is laid horizontally will acquire a different rate of reaction from those of the lower side, because in the former the specifically heavier substances are directed toward the center of the stem, while in the latter the specifically lighter ones are directed toward the center. Consequently, one side will grow faster than the other, hence the geo tropic bending.^ In the frog's egg, we can actually demonstrate directly the existence of two substances of different

specific gravity and can study their behavior, since in this case they are of different color. In animals it has been observed that orientation toward the center of gravity of the earth often becomes less compulsory when the inner ear has been removed. Mach first pointed out the possibility that the otoliths are responsible for this. He believed that they might press upon the end-organs of the sensory nerves and every change of pressure might cause a correction of the position of the animal. It is generally assumed that this view has been verified by experiment but I cannot entirely agree with it although I once described experiments which seemed to support Mach's otolith theory. I had found that when the otoliths of the inner ear of the shark are scraped out with a sharp spoon the normal orientation of the animal suffers; but if the otoliths are simply washed out from the inner ear by a weak current of sea-water the orientation does not so easily suffer.

In the latter case, it is doubtful whether all the otolith powder has been removed from the ear. The problem was solved by experiments on flounders, which have only a single large otolith that can easily be removed from the ear. E. P. Lyon carried out these experiments, which showed that no disturbance of the orientation resulted from this operation. We may conclude, therefore, that in my experiments of scraping out the otoliths a disturbance of the orientation occurred, because in so doing the nerve endings in the ears were injured. We have, therefore, no right to maintain that the orientation of animals in relation to the center of gravity of the earth is regulated by the pressure of the otoliths upon the nerve endings, but that this regulation takes place in the nerve endings themselves, and probably, indeed, as a result of the existence there of two different phases of different specific gravity which react upon one another. Through the change of orientation of the cells in relation to the center of gravity of the earth, the two

phases undergo a shifting by means of which a change in the rate of reaction is brought about according to one of the ways described above. Since then I have looked through the hterature on the function of the otoliths or statoliths, and have reached the conclusion that all writers who assert that the removal of the otoliths disturbs the geotropic orientation of animals have been victims of the same fallacy as myself. They have injured or removed the nerve endings. In the only case in which a removal of the otoliths without tearing or other injury of the nerve endings can be justifiably assumed, no disturbance of the orientation occurred.

While in my own work I have aimed to trace the complex reactions of animals back to simpler reactions like those of plants and finally to physico-chemical laws, the opposite tendency has lately been gaining influence. Some botanists, namely, Haberlandt, Nemec, and F. Darwin, endeavor to show that the relatively simpler reactions of plants may be traced back to the more complex relations found in animals. Instead of deriving the tropic reactions of plants as directly as possible from the law of mass action or the law of Bunsen and Roscoe, they try to show that ''sense-organs" exist in the cells of plants and France even attributes to the latter a ''soul" and "intelligence." I believe that in order to be consistent, these writers ought to base the law of mass action upon the assumption of the existence of sense-organs, souls, and intelligence in the molecules and ions. It is probably unnecessary to emphasize the fact that it is better for the progress of science to derive the more complex phenomena from simpler components than to do the contrary. For all "explanation" consists solely in the presentation of a phenomenon as an unequivocal function of the variables by which it is determined, and if in nature we find a function of two variables, it does not, in my opinion, tend toward progress to assert that this is a case of functions of more than two variables, without furnishing sufficient proof for this assertion.

These writers explain the geotropic reactions of plants by saying that in certain cells starch grains are present which serve the purpose of the otoliths in animals. These starch grains are believed to press upon the sense-organs or nerve endings in the plant cells concerned and the ''pressure-sense" of the plant is then supposed to give rise to the geotropic curvature. I have no opposition to offer to the assumption that the starch grains change their position with a change in the position of the cells, and I am also willing to pass over for the present the view that the starch grains form one of the two phases in the cell. But I see no necessity for assuming besides this the existence of intracellular sense-organs which perceive the pressure of the starch grains. This is, in my opinion, an unnecessary complication of simple relations.

The progress of natural science depends upon the discovery of rationalistic elements or simple natural laws. We find that there are two classes of investigators in biology, grouped according to their attitude toward such simple laws or rationalistic elements. One seems to aim at the denial of the existence of such simple laws and every new case which does not fall at once imder such a law offers an opportunity for them to point out the inadequacy of the latter. The other group of investigators aims to discover and not to disprove laws. When such investigators have discovered a simple law which is generally applicable, they know that an apparent exception does not necessarily overthrow the law, but that possibly an opportunity is offered for a new discovery and an extension of the old law. Mendel's laws have been brilliantly confirmed in a number of cases. In some cases of apparent deviations (from these laws), however, it has not always been possible at once to recognize the cause. One group of investigators has recognized that these deviations do not indicate the incorrectness of Mendel's laws, but that they are merely the

result of secondary and often minor complications; the latter investigators have from this standpoint made further fruitful discoveries. The role of the other group of investigators in this case has consisted, primarily, in an attempt to minimize the importance of Mendel's laws and thus to retard the progress of science. The case is similar in the realm of tropisms. Tropisms and tropism-like reactions are elements which pave the way for a rationalistic conception of the psychological reactions of animals and I believe, therefore, that it is in the interest of the progress of science to develop further the theory of animal tropisms. The fact that in an electric current the same animal often moves differently from what it does under the influence of light finds its explanation for the observer conversant with physical chemistry in the fact that the electric current causes changes in the concentration of ions within, as well as upon the surface, while the chemical action of light is essentially limited to the surface. Certain A\Titers, however, leave this difference in the action of the two agents out of consideration and make use of the difference in the behavior of certain organisms in response to light and to the electric current, to assert that it is not permissible to speak of tropisms as being governed by general laws; in other words, they say that tropisms are without significance. Animals in general are symmetrically built and the motor elements of the right and left sides of the body usually act symmetrically. Consequently the heliotropic orientation, for instance, comes about as we have already described. There are animals, however, which move sideways, for instance, certain crabs, such as the fiddler crab. Holmes has found that these crustaceans also go sideways toward the light. Jemiings draws from this fact the following conclusions: ''The symmetrical position is an incident of the reaction, not its essence."

indicate that the role ascribed to symmetry has no importance for the theory of tropisms. I am, however, inclined to draw another conclusion, namely, that in the fiddler crabs in the first place there is an entirely different connection between the retina and the locomotor muscles from that in other crustaceans, and that, secondly, there is a special peculiarity in regard to the function of the two retinae whereby they do not act like symmetrical surface elements. I believe that a new discovery may be made here.^

These data may suffice to explain my point of view. To me it is a question of making the facts of psychology accessible to analysis by means of physical chemistry. In this way it is already possible to reduce a set of reactions, namely, the tropisms to simple rationalistic relations. Many animals, because their body structure is not only morphologically, but also chemically, symmetrical, are obliged to orient their bodies in a certain way in relation to certain centers of force, as, for instance, the course of light, an electric current, the center of gravity of the earth, or chemical substances. This orientation is automatically regulated according to the law of mass action. The application of the law of mass action to this set of reactions is thus made possible. I consider it unnecessary to give up the term *' comparative psychology," but I am of the opinion that the contents of comparative psychology will under the influence of the above-mentioned endeavors be different from the contents of speculative psychology. But I believe also that the further development of this subject will fall more to the lot of biologists trained in phj^sical chemistry than to the specialists in psychology or zoology, for it is in general hardly

1 From which. I expect, furthermore, that they will only confirm still more the laws of heliotropism. This expectation is based upon analogous relations in the pleiironectids, which I cannot, however, discuss further here. However, probably no one will maintain that the existence of the pleiironectids invalidates all laws in regard to the symmetrical body structure. to be expected that zoologists and psychologists who lack a physico-chemical training will feel attracted to the subject of tropisms.

In closing let me add a few remarks concerning the possible application of the investigations of tropisms. I believe that the investigation of the conditions which produce tropisms may be of importance for psychiatry. If we can call forth in an animal otherwise indifferent to light by means of an acid a heliotropism which drives it irresistibly into a flame; if the same thing can be brought about by means of a secretion of the reproductive glands, we have given, I believe, a group of facts, within which the analogies necessary for psychiatry can be called forth experimentally and can be investigated.

These experiments may also attain a similar value for ethics. The highest manifestation of ethics, namely, the condition that human beings are willing to sacrifice their lives for an idea is comprehensible neither from the utilitarian standpoint nor from that of the categorical imperative. It might be possible that under the influence of certain ideas chemical changes, for instance, internal secretions within the body, are produced which increase the sensitiveness to certain stimuli to such an unusual degree that such people become slaves to certain stimuli just as the copepods become slaves to the light when carbon dioxide is added to the water. Since Pawlow and his pupils have succeeded in causing the secretion of saliva in the dog by means of optic and acoustic signals, it no longer seems strange to us that what the philosopher terms an ''idea'* is a process which can cause chemical changes in the body.

1. The understanding of complicated phenomena depends upon an analysis by which they are resolved into their simple elementary components. If we ask what the elementary components are in the physiology of the central nervous system, our attention is directed to a class of processes which are called reflexes. A reflex is a reaction which is caused by an external stimulus, and which results in a coordinated movement, the closing of the eyelid, for example, when the conjuctiva is touched by a foreign body, or the narrowing of the pupil under the influence of light. In each of these cases, changes in the sensory nerve endings are produced which bring about a change of condition in the nerves. This change travels to the central nervous system, passes from there to the motor nerves, and terminates in the muscle-fibers, producing there a contraction. This passage from the stimulated part to the central nervous system, and back again to the peripheral muscles, is called a reflex. There has been a growing tendency in physiologj^ to make reflexes the basis of the analysis of the functions of the central nervous system, and consequently much importance has been attached to the processes underlying them and the mechanisms necessary for reflex.

The name reflex suggests a comparison between the spinal cord and a mirror. Sensory stimuli were supposed to be reflected from the spinal cord to the muscles; destruction of the spinal cord would, according to this, make the reflex impossible, 1 Reprinted from Loeb, J., Comparative Physiology of the Brain and Comparative Psychology (1899). By courtesy of G. P. Putnam's Sons of New York and London. just as the destruction of the mirror might prevent the reflection of Hght. This comparison, however, of the reflex process in the central nervous system with the reflection of light has, long since, become meaningless, and at present few physiologists in using the term reflex think of its original significance. In- stead of this, another feature in the conception of the term reflex has gained prominence, namely, the purposeful character of many reflex movements. The closing of the eyelid and the narrowing of the pupil are eminentlj' purposeful, for the cornea is thereby protected from hurtful contact with foreign bodies, and the retina from the injurious effects of strong light. Another striking characteristic in such reflexes has also been emphasized. The movements which are produced are so well planned and coordinated that it seems as though some intelligence were at work either in devising or in carrying them out. The fact, however, that even a decapitated frog will brush with its foot a drop of acetic acid from its skin, suggests that some other explanation is necessary. A prominent psychologist has maintained that reflexes are to be considered as the mechanical effects of acts of volition of past generations.^ The ganglioncell seems the only place where such mechanical effects could be stored up. It has therefore been considered the most essential element of the reflex mechanism, the nerve-fibers being regarded, and probably correctly, merely as conductors.

Both the authors who emphasize the purposefulness of the reflex act and those who see in it only a physical process have invariably looked upon the ganglion-cell as the principal bearer of the structures for the complex coordinated movements in reflex action. I should have been as little inclined as any other physiologist to doubt the correctness of this conception had not the establishment of the identity of the reactions of animals and plants to light proved the untenability of this view and at the

1 A statement for which no trace of experimental proof exists. same time offered a different conception of reflexes. The flight of the moth into the flame is a typical reflex process. The light stimulates the peripheral sense-organs, the stimulus passes to the central nervous system, and from there to the muscles of the wings, and the moth is caused to fly into the flame. This reflex process agrees in every point ^vith the heliotropic effects of light on plant organs. Since plants possess no nerves and no ganglia, this identity of animal with plant heliotropism can force but one inference — these heliotropic effects must depend upon conditions which are common to both animals and plants. At the end of my book on heUotropism^ I expressed this view in the following words: ''We have seen that, in the case of animals which possess nerves, the movements of orientation toward light are governed by exactly the same external conditions, and depend in the same way upon the external form of the body, as in the case of plants which possess no nerves. These heliotropic phenomena, consequently, cannot depend upon specific qualities of the central nervous system." On the other hand, the objection has been raised that destruction of the ganglion-cells interrupts the reflex process. This argument, however, is not sound, for the nervous reflex arc in higher animals forms the only protoplasmic bridge between the sensory organs of the surface of the body and the muscles. If we destroy the ganglion-cells or the central nervous system, we interrupt the continuity of the protoplasmic conduction between the surface of the body and the muscles, and a reflex is no longer possible. Since the axis cylinders of the nerves and the ganglion-cells are nothing more than protoplasmic formations, we are justified in seeking in them only general protoplasmic qualities, unless we find that the phenomena cannot be explained by means of the latter alone.