Popular Science Monthly/Volume 79/August 1911/The Significance of Tropisms for Psychology

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1538793Popular Science Monthly Volume 79 August 1911 — The Significance of Tropisms for Psychology1911Jacques Loeb




AUGUST, 1911





THE scientific analysis of psychical phenomena must, I believe, aim to explain these phenomena according to laws of physical chemistry. I know very well that many people would hold that even a complete physico-chemical analysis of all psychic phenomena must still leave the "truly psychical" unexplained. I do not agree with such an opinion, but as we are to-day still very far from the ideal of a complete physico-chemical analysis of psychical phenomena, there is nothing to be gained by quarrelling about just how much scientific illumination and satisfaction we shall attain when that goal is once reached. On the second point, however, a general agreement may be reached, namely, first, that we must undertake and carry out a physico-chemical analysis of psychical phenomena; and, second, that for such an analysis the same principles of investigation are required as for the physicochemical analysis of the very much simpler processes in inanimate nature.

Twenty-two years ago I came to the conclusion that what we call "will" in many lower animals is nothing but the phenomena of tropisms well known in plants, especially through the work of Sachs. In a series of articles, of which the first two appeared[2] in January, 1888, I have tried to establish this view, and I will now summarize the facts briefly, and try to do away with some of the difficulties which zoologists and psychologists have experienced in applying my theories. To my mind the essential value of my theory lies in the preparation it gives for the application of the law of mass action (and other physicochemical laws) to phenomena which usually form the material for psychological speculation. To make possible a better understanding of my lecture, let me mention briefly how I came to hold the views set forth here.

The writings of the metaphysicians on the will in nature led me to an experimental analysis of the nature of will. When in my first years at the university Munk's investigations on the cerebral cortex fell into my hands I believed that here was a starting point toward my goal. Munk stated that he had succeeded in proving that every memory image in a dog's brain is localized in a particular cell or group of cells and that any one of these memory images can be extirpated at will. Five years of experiments later with extirpations in the cerebral cortex proved to me without doubt that Munk had become the victim of an error and that the method of cerebral operations can really give only data concerning the nerve connections in the central nervous system but teach practically nothing about the dynamics of brain processes.

A better way seemed to lie in the comparative psychology of the lower animals in which the memory apparatus is developed but slightly or not at all. It seemed to me that some day it must become possible to trace the apparently random movements of animals back to general laws, just as definitely as it has been done for the movements of the planets, and that the word "animal will" is only the expression of our ignorance of the forces which prescribe for animals the direction of their apparently spontaneous movements just as unequivocally as gravity prescribes the movements of the planets. For if a savage could directly observe the movement of the planets and should begin to ponder over it, he would probably come to the conclusion that a "will action" guides the movements of the planets, just as a chance observer is inclined to assume that "will" causes animals to move in a given direction.

The scientific solution of the problem of will seemed to consist in finding the forces which unequivocally determine the movements of animals, and in discovering the laws according to which these forces act. Experimentally, the solution of the problem of will must take the form of forcing, by external means, any number of individuals of a given kind of animal to move in a definite direction by means of their locomotor apparatus. Only if this succeeds have we the right to assume that we know the force which under certain conditions seems to a layman to be the will of the animal. But if only a part of the number moves in this definite direction and another does not, then we have not succeeded in finding the force which in a given case determines unequivocally the direction of movement.

One other point should be observed. If a sparrow flies down from a roof to a seed lying in the street, we speak of an act of will, but if a dead sparrow falls from the roof upon the seed this does not appear to us to be an act of will. In the latter case purely physical forces are concerned, while in the former chemical reactions are also taking place in the sense organs, nerves and muscles of the animal. We speak of an act of will, only when this latter complex, that is, the natural movement of locomotion, plays its part also, and it is only with this sort of reactions that we have to deal in the psychology of the will.


Some experiments on winged plant lice may serve as an introduction to these methods of definitely prescribing to animals the direction of their progressive movements.

In order to obtain the material, potted rose-bushes or cinerarias thickly infected with plant lice are brought into a room and placed in front of a closed window. If the plants are allowed to dry out, the aphids, previously wingless, change into winged insects. After this metamorphosis the animals leave the plants, fly to the window and there creep upward on the glass. They can then be easily collected by holding a test-tube underneath and touching one animal at a time from above with a pen or scalpal; the animals then drop into the test-tube. In this way a sufficiently large number, perhaps twenty-five or fifty suitable subjects for the experiment, may be quickly obtained. With these animals it may be demonstrated that the direction of their movement toward the light is definitely determined—provided that the animals are healthy and that the light is not too weak. The experiment is arranged so that only a single source of light, e. g., artificial light, is used.

The animals place themselves with their heads toward the source of light and move in as direct a line as the imperfectness of their locomotor apparatus allows, toward it. If they are in a test-tube they go as far toward the source of light as their prison allows. When they reach that end of the test-tube which is directed toward the source of light, they remain there, stationary, in a closely crowded mass. If the test-tube is turned around 180° the animals again go straight toward the source of light until the interference of the glass stops their further progressive movements.[3] It can be demonstrated in these animals that the direction of their progressive movement is just as unequivocally governed by the source of light as the direction of the movement of the planets is determined by the force of gravity.

The theory of the compulsory movements of aphids under the influence of light is as follows: Two factors govern the progressive movements of the animals under these conditions; one is the symmetrical structure of the animal, and the second is the photochemical action of light. We will consider the two separately. In regard to the photochemical action of light, we know to-day that a great many chemical reactions of organic bodies are accelerated by light. Especially is this true of oxidations.[4] The mass of facts presented to us here concerning this relatively young branch of physical chemistry is already so great that we have the right to assume that the determining action of light upon animals and plants is in its last analysis due to the fact that the rate of certain chemical reactions in the cells of the retina or of other photosensitive regions of the organisms is modified by light; with increasing intensity of light the rate of certain chemical reactions, for instance, oxidation, increases according to a definite law, namely, the law of Bunsen and Eoscoe.

The second factor is the symmetrical structure of the animal. As expressed in the gross anatomy of the animal this is shown by the well-known fact that the right and left halves of the body are symmetrical. But it is my belief that such a symmetry exists in a chemical sense as well as in an anatomical sense—by which I mean that symmetrical regions of the body are chemically identical and have the same metabolism, while non-symmetrical regions of the body are chemically different, and in general have a quantitatively or qualitatively different metabolism. In order to illustrate this difference it is only necessary to point out that the two retinas, which are certainly symmetrical, have an identical metabolism, while a region of the skin which is not symmetrical with the retina has a different metabolism. The individual points on the retina are also chemically unlike. The observations upon visual purple, the differences in the color-sensitiveness of the fovea centralis, and the peripheral parts of the retina indicate that the points of symmetry of the two retinas are chemically like, but the nonsymmetrical points chemically unlike.

Now if an unequal amount of light falls upon the two retinas, the photo-chemical reactions in the retina which receives more light will also be more accelerated than in the other. The same thing naturally holds true for every other pair of symmetrical photosensitive surface elements. For it should be mentioned just here that photochemical substances are not only found in the eyes, but also in other places on the outer surface of many animals. In planarians, as my experiments and those of Parker have shown, not only the eyes, but also other places on the skin, are photosensitive. But if more light falls upon one retina than upon the other, the chemical reactions, for instance, the organic oxidation, will also be more accelerated in one retina than in the other, and accordingly more intense chemical changes will take place in one optic nerve than in the other. S. S. Maxwell and C. D. Snyder have demonstrated, independently, that the rate of the nerve impulse has a temperature coefficient of the order of magnitude which is characteristic for chemical reactions. According to this we must conclude that when two retinas (or other points of symmetry) are illuminated with unequal intensity, chemical processes, also of unequal intensity, take place in the two optic nerves (or the sensory nerves of the two points). This inequality of chemical processes passes from the sensory to the motor nerves and eventually into the muscles connected with them. We conclude from this that with equal illumination of both retinas the symmetrical groups of muscles of both halves of the body will receive equal chemical stimuli and thus reach equal states of contraction, while when the rate of reaction is unequal, the symmetrical muscles on one side of the body come into stronger action than those on the other side. The result of such an inequality of action of symmetrical muscles of both halves of the body is a change in the direction of movement on the part of the animal.

This change in the direction of movement can result either in a turning of the head toward the source of light and the accompanying movement of the whole animal toward the source of light, or in a turning of the head in the opposite direction and the accompanying movement of the whole animal in the opposite direction. In order to show that the choice between the two possibilities has to do with purely physico-chemical conditions, we should have to discuss, one by one, a whole series of topics upon the physiology of the central nervous system. It may suffice to call to mind briefly first that the structure of the central nervous system is segmental and that the head segments generally determine[5] the behavior of the other segments with their accessory parts; and secondly that chemical processes in any single element can cause an increase in the tonus of certain muscle groups as well as causing just the opposite effect under other conditions.

In the winged aphids the relations are as follows: Suppose that a single source of light is present and that the light strikes the animal from one side. As a consequence the activity of those muscles which turn the head or body of the animal towards the source of light will be increased.[6] As a result the head, and with it the whole body of the animal, is turned toward the source of light. As soon as this happens, the two retinas become illuminated equally. There is therefore no longer an)' cause for the animal to turn in one direction or the other. It is thus automatically guided toward the source of light. In this instance the will of the animal which determines the direction of its movement is light, just as it is gravity in the case of a falling stone or the movement of a planet. Only the action of gravity upon the direction of movement of the falling stone is direct, while the action of light upon the direction of movement of the aphids is indirect, inasmuch as the animal only by means of an acceleration of photochemical reactions is caused to move in a definite direction.

We will now designate as positively heliotropic those animals which are forced to turn the head or the parts of the body which are foremost during locomotion toward the source of light, and as negatively heliotropic those animals which are oriented in the opposite direction.

The aphids serve here only as an example. The same phenomena of positive heliotropism may be demonstrated with equal precision in a great many animals, vertebrates as well as invertebrates, for instance in young fishes. We can not, of course, give an account of all these cases here. The reader who is interested in them must look into the voluminuous literature upon this subject.[7]


The winged aphids can serve as an example, because in their case the above-mentioned requirement is fulfilled, namely, that all individuals, without exception, move toward the light. For mechanistic science it is a methodological postulate that the same law applies without exception, or that a sufficient reason must be given in case of an exception. But it was soon found, as might be expected, that not all organisms in their natural condition are equally suitable for these experiments. Many animals show no heliotropism at all; many show only a slight reaction, while others show it to as pronounced a degree as do the winged aphids. The problem therefore presented itself of making artificially heliotropic those animals which show no positive heliotropism. Such attempts give us a broad insight into the mechanism of acts of will. If small crustaceans of a fresh-water pond or lake are taken with a plankton net at noon-time or in the afternoon and placed in an aquarium which is illuminated from one side only, it is generally found that these animals move about in the vessel pretty much at random and distribute themselves irregularly. Some seem to go more toward the

lighted side, others in the opposite direction, and the majority perhaps pay no attention to the light.

This condition changes instantly if we add to the water some acid, preferably carbonic acid, which easily penetrates the cells. This is done by slowly adding to every 50 c.cm. of the fresh water a few cubic centimeters of water charged with carbondioxide. If the correct amount of carbonic acid is added all the individuals become actively positively heliotropic in a few moments and they move in as straight a line as the primitiveness of their swimming movements permits, toward the source of light, and remain there closely crowded together on the lighted side of the vessel. If the vessel is turned around 180°, they go directly back again to the lighted side of the vessel. Every other acid acts like carbonic acid and alcohol acts in the same manner, only more feebly and much more slowly. Animals which were previously indifferent to light become, under carbonic acid treatment, complete slaves of the light.[8]

How does the acid produce this result? We will assume that it acts as a sensitizer. The light produces chemical changes, for instance, oxidation on the surface of the animal, especially in the eye, as was suggested in the case of the aphids. The mass of photochemical substance which is acted upon by the light is often relatively small, so that even when the light strikes the crustacean (copepod) on one side only, the difference in the chemical changes on the two sides of the body remains still too small to call forth a difference in tension or action, in the muscles of the two sides of the body, sufficient to turn the animal toward the source of light. But if we add an acid this could act as a catalyzer, as, for instance, in the catalysis of esters. In the catalysis of esters, the acid acts, according to Stieglitz, only to the extent of increasing the active mass of the substance which undergoes a chemical change. In order to provisionally fix our ideas, we will assume that the acid makes the animal more strongly positively heliotropic by increasing the active mass of the photosensitive substance. By this means it becomes possible for the same intensity of light which before produced no heliotropic reaction now to cause a very pronounced positively heliotropic reaction; because if now the animal is struck on one side only by the light, the difference in the reaction product in both retinæ becomes rapidly great enough to cause automatically a difference in the action of the muscles of both sides of the body and a turning of the head towards the source of light.

A second consideration must also be mentioned here. In certain forms, for instance, in daphnia and in certain marine copepods, a decrease in temperature also increases the tendency to positive heliotropism. If the mere addition of acid is not sufficient to make the daphnia positively heliotropic, this may often be accomplished by simultaneously reducing the temperature. From the physico-chemical standpoint we must assume that likewise in the dark, at the ordinary temperature, the photosensitive substance is destroyed so rapidly that its active mass is generally rather too low to cause a heliotropic reaction. By reducing the temperature the rate of decomposition of the photosensitive substance is decreased more than the rate of its formations.

This illustration may suffice, under the limitations of the space allowed us, to indicate how the facts in this field might be correlated when viewed from the standpoint of physical chemistry.


The animals which are strongly positive heliotropic and those animals which do not react at all to light offer the observer no difficulties. Nevertheless, some zoologists, apparently not very familiar with the laws of physical chemistry, seem to have found difficulty in explaining the behavior of those animals which come between the two extremes. For instance, one writer has asserted that with greater intensity of light the laws of heliotropic orientation hold good, while with a lessened light-intensity the animals react to light by the method of "trial and error." From a chemical standpoint the behavior of animals at low intensity is easily to be understood. If a positively heliotropic animal is illuminated from one side a compulsory turning of the head toward the source of light occurs only when the difference in the rate of certain photochemical reactions in the two eyes reaches a certain value. If the intensity of the light is sufficient and the active mass of photochemical substance in the animal great enough, it is only a short time, for instance, the fraction of a second, before the difference in the mass of the reaction products formed on the two sides of the animal reaches the value necessary for the compulsory turning of the head toward the source of light. In this case the animal is a slave of the light; in other words, it has hardly time to deviate from the direction of the light rays; for if it turns the head even for the fraction of a second from the direction of the light rays, the difference in the photochemical reaction-products in the two retinas becomes so great that the head is at once automatically turned back toward the source of light. But if the intensity of the light is lessened (or the photosensitiveness of the animal lessened) the animal may deviate for a longer period from the direction of the light rays. Such animals do eventually reach the lighted side of the vessel, but they no longer go straight toward it, but move instead in zig-zag lines or very irregularly. It is, therefore not a case of a qualitative, but of a quantitative, difference in the behavior of heliotropic animals under greater or lesser illumination, and it is therefore erroneous to assert that heliotropism determines the movement of animals toward the source of light only under strong illumination, but that under weaker illumination an essentially different condition exists.

Still another point is to be considered. We have seen that acid increases the sensitiveness of certain animals to light and probably, as we assume, by increasing the active mass of the photochemical substance. Now every animal is continually producing acids in its cells, especially carbonic acid and lactic acid. It probably produces also substances which could have the opposite effect and which decrease the heliotropic sensitiveness of the animals. Fluctuations in the rate of production of these substances will also produce fluctuations in the heliotropic sensitiveness of the animal. Now if, for instance, the active mass of the photosensitive substance in a copepod is relatively small, a temporary increase in the production of carbonic acid can increase the photosensitiveness of the animal sufficiently for it to move for the period of a few seconds directly toward the source of light. Later the production of carbonic acid decreases and the animal again becomes indifferent to light and can move in any other direction. Then the production of carbonic acid increases again and the animal goes again, for a short time, toward the light. Such animals finally gather at the lighted side of the vessel because the algebraic sum of the movements in the other directions becomes zero according to the law of chance. But it is plain that such animals do not reach the source of light by a straight path. A writer who is not trained to interpret the variations in the behavior of such an animal chemically and physiologically, can naturally give no explanation of their significance. If he is forced to find an explanation he will wind up at the method of "trial and error" which is no more chemical nor scientific than the explanations of metaphysicians in general.

Some authors have, it seems, worked only with animals which were not pronouncedly heliotropic and the photo-sensitiveness of which wavered about the threshold of stimulation in the manner described above. A writer trained in physical chemistry would have understood that such animals are unsuitable for experiments in heliotropism and that it is necessary to first increase their photo-sensitiveness if the laws of the action of light upon them are to be investigated.

I also believe that observations upon animals which are not sufficiently photo-sensitive have caused many writers to assert that heliotropic animals do not place themselves directly in the line of the rays of light,[9] but that they first have to learn the right orientation. But a very striking experiment contradicts this assertion. The larvæ of Balanus perforatuS develop entirely in the dark. If the ovary filled with mature larvæ is, in the dark, placed in a watch crystal filled with sea water, the larvæ emerge at once and, if they are brought into the light, they move at once to the side of the watch crystal nearest to the window. They were, therefore, pronouncedly positively heliotropic before they came under the influence of the light.

In experiments with winged aphids I often found that after having gone through the heliotropic reactions a few times they react much more quickly to light than at the beginning. This might be interpreted as a case of "learning." In so far as it is not a case of a lessening of the stickiness of the feet or the removal of some other purely mechanical factor which retards the rate of movement, it may be brought about by the carbonic or lactic acids produced through the muscular activity.[10]


As far back as twenty years ago I pointed out that the photo-sensitiveness of an animal is different in different physiological conditions and that, therefore, under natural conditions, heliotropism is found often only in certain developmental stages, or in certain physiological states of an animal. I have already mentioned that in the aphids distinct heliotropic reactions may only be expected when the animals have developed wings and left the plant. The influence of the chemical changes which take place in animals upon heliotropism is much more distinct in the larvæ of Porthesia chrysorrhæa. The larvæ hatch from the eggs in the fall and, as young larvæ, hibernate in a nest. The rising temperature in the spring drives them out of the nest and they can be driven out of the nest in winter also by an increase in temperature. When they are driven out of the nest in this condition they are strongly positively heliotropic and I have never found in natural surroundings any animals whose heliotropic sensitiveness was more pronounced than it is in the young larvæ of Chrysorrhea under these conditions. But as soon as the animals have once eaten the positive heliotropism disappears and does not return if they are again allowed to become hungry.[11] In this case it is clear that the chemical changes connected with nutrition directly or indirectly lead to a permanent diminution or disappearance of the photochemical reaction. In ants and bees the influence of substances from the sexual organs seems to be the determining factor in the production of positive heliotropism. The ant workers show no heliotropic reactions while in the males and females, at the time of sexual maturity, a distinct positive heliotropism develops, the intensity of which continues to increase.

According to Kellogg the case of the bees is similar. It is a well known fact that during sexual maturity special substances are formed which influence various organs. For instance, Leo Loeb has found that the substances which are set free by the bursting of an egg follicle cause a special sensitiveness in the non-pregnant uterus, so that every mechanical stimulus causes the latter to form a decidua. In this way he could cause the formation of any number of deciduæ in non-pregnant uteri, while without the follicle substance the uterus did not react in this manner.

It is a common phenomenon that animals in certain larval stages are positively heliotropic, while in others they are not sensitive to light or are even negatively heliotropic. In order to save time I will not now discuss further these facts which are easily comprehensible in the light of what has been said and I refer the readers to my earlier papers.

This change in the heliotropic sensitiveness, produced by certain metabolic products in the animal body is of great biological significance. I have already shown that it even serves to save the lives of the above-mentioned young larvæ of Chrysorrhœa. When the young larvæ are awakened from their winter sleep by the spring sunshine they are actively positively heliotropic. The positive heliotropism leaves them no freedom of movement, but forces them to creep (eindeutig) straight upward to the top of a tree or branch. Here they find the first buds. In this way the heliotropism guides them to their food. Should they now remain positively heliotropic they would be held fast on the ends of the twigs and would starve to death. But we have already mentioned that after they have eaten they lose the positive heliotropism once more. They can now creep downwards, and the restlessness which is characteristic of so many animals[12] forces them to creep downwards until they reach a new leaf, the odor or tactile stimulus of which stops the progressive movement of the machine and sets in motion further eating activity.

The fact that ants and bees become positively heliotropic at the time of sexual maturity plays an important rôle in the vital economy of these creatures. As is well known, the mating of these insects takes place during flights, the so-called nuptial flight. Now I have watched and found that among the male and female ants of a single nest the heliotropic sensitiveness increases steadily up to the time of the nuptial flights and that the direction of their nuptial flights follows the direction of the rays of the sun in the afternoon. I gained the impression that this nuptial flight is merely the consequence of a very highly developed heliotropic sensitiveness. The case must be similar among the bees according to the following experiment described by Kellogg. The bees were ready to swarm out of the opening of the box used for the experiment when he suddenly removed the dark covering of the box so that the light now entered it from above. The heliotropic sensitiveness of the animals was so great that they crept upward within the box, following the direction of the light rays and were not able to make the nuptial flight. Thus, according to these observations the bees at the time of the nuptial flight are positively heliotropic machines.

These observations may serve as examples of the way in which analyses of the vital phenomena of certain animals show tropisms to be elements of these phenomena. Many observations of a similar nature are found in the papers of George Bohn, Parker, Rádl[13] and myself. What appear to us upon incomplete analysis as acts of will or instinct prove upon more careful analysis, in a series of cases, to be tropisms, the theories of which we have explained in the foregoing pages.


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. But now the exact doctrine of heredity, founded by Mendel and advanced to the position of a systematic science in 1900, reduces this false 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. The opposite view, however, namely, that every reaction or every tropism which an animal possesses is for its interest, or of great benefit to it, is just as incorrect as the view that every structural characteristic of a species must be useful to it.

Galvanotropism illustrates this in a striking manner. If a galvanic current is passed through a trough filled with water, and if 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 animals and that the organisms move in the direction either of the positive or of the negative current. In this case we speak of galvanotropism. In galvanotropism the current lines or the current curves play the same role as the light rays in heliotropism. The explanation is that at those points where the current curves enter the cells[14] a collection of ions takes place which influences the chemical reactions. The number of organisms which show typical galvanotropic reactions is not so large as the number of those which show typical heliotropism. According to 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 of the animal. Thus, in general, 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 simple structure, as, for instance, in one-celled 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 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 influence.: 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 one-celled 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 which possess both forms of sensitiveness. And this expectation is fulfilled. The colonial algæ 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.

Claparède raises the objection that the galvanotropic reactions are purely compulsory, while the heliotropic reactions are governed by the "interest of the animal."[15] Such a view, however, is not supported by the facts. The reason that heliotropism 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 larvæ of the goat moth, which live under the bark of trees, may show positive heliotropism. I found, moreover, that the crab, Cuma Rathkii, which lives in the mud of the harbor of Kiel, when brought into the light and removed from the mud shows positive heliotropism. It is, therefore, just as incorrect to assert that the heliotropic reactions are governed by the biological 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 case of a suggestion forced upon the investigators by the extreme application of the natural selection theory. I have made experiments upon a large number of animals, but, with a clear 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 shown that positively heliotropic animals are positive with any intensity of light above the threshold. Thus winged plant lice or wingless larvæ of Chrysorrhœa or copepods, which have been made heliotropic by acids, go toward the light regardless of 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 intensity of light required for the reaction is passed. 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.[16] A "selection" of a suitable light intensity I have never observed.

What probably lies behind these interpretations of the "selection of suitable light intensity" 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 larvæ 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 larvæ soon become negatively heliotropic. For these experiments the larvæ 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 larvæ of Polygordius. But it would be wrong in this case to speak of an adaptation of the animal to a certain light intensity. 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 even 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 semi-darkness 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 sudden change of light intensity are shown most clearly in marine tube worms, whose gills are exposed to light. If the light intensity in the aquarium is suddenly diminished the worms withdraw quickly into their tubes. A sudden increase of light intensity has no such effect. With other forms, for instance, with planarians, a sudden decrease of the intensity of the light causes a decrease in movement. Such animals gather chiefly in parts of the space where the light intensity is relatively small. I have designated such reactions as the expression of sensitiveness to change in intensity of a stimulus (Unterschiedsempfindlichkeit), in order to distinguish them from tropisms.[17]

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 sense of difference is very pronounced in response to sudden touch or sudden alteration of the chemical medium, and like the tube-worms 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 to demonstrate the laws of heliotropism, especially since they possess, moreover, only a slight photochemical sensitiveness. But Jennings has with special preference used observations on such organisms to argue against the theory of tropisms, and he has with these arguments caused much confusion in the minds of zoologists. One writer has, if I am not mistaken, asserted that the significance of tropisms is limited by the demonstration of the sense of difference. This writer overlooks the fact that it is a question of tracing psychical phenomena, and not merely tropisms, back to physico-chemical processes. Just as in muscles and nerves the action of a constant current is different from that of an intermittent current, so we find in the action of light an analogous case. 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 sense of difference, 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 can not discuss that further here. Instead the reader is referred to my aforementioned book, as well as the newer work of Bohn, "La naissance de l'intelligence."[18] Let us bear in mind that "ideas" also can act, much as acids do for the heliotropism 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.[19]


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 contact 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, and thence the geotropic bending.[20] In the frog's egg, we can actually directly demonstrate 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. 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. I cannot, however, agree with this, 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 internal ear by a mild current of seawater the orientation of the animal does not suffer so easily.

In the latter case, the doubt is present as to whether all the otolith powder has been removed from the ear. The matter was decided by experiments on flounders, which have only a single large otolith which 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 by this means the nerve endings in the ears were injured. We have, therefore, no right to say 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 given above. Since then I have looked through the literature 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 to simpler reactions like those of plants and finally to physicochemical laws, the opposite tendency has lately been gaining influence. Some botanists, namely, Haberlandt, Nĕmec 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 (and other physico-chemical laws), they try to show that "sense organs" exist in the cells of plants and France even attributes to the latter a "soul" and an "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, namely, to try to explain the simpler by means of the more complex. 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 of this assertion.

These writers represent 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 which in this case introduces a demonstrable error of animal physiology into plant physiology.


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 under this law is 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 deviations (from these laws), however, it has not always been possible to recognize at once the causes of the same. 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 rôle 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 make possible for us a rationalistic conception of the psychological reactions of animals on the basis of chemical mass action, 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 writers, 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. Jennings draws from this fact the following conclusions: "The symmetrical position is an incident of the reaction, not its essence."

In other words, he uses these observations of Holmes to 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 different animals, and that, secondly, there is a special peculiarity in regard to the function of the two retinas whereby they do not act like symmetrical surface elements. I believe that a new discovery may be made here.[21]


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 source 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 physical chemistry than to the specialists in psychology or zoology, for it is in general hardly 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 about by means of a secretion of the reproductive glands; then we have given, I believe, a group of facts, within which the analogies necessary for psychiatry can be experimentally called forth and investigated.

These experiments may also attain a similar value for ethics. The highest manifestation of ethics, namely, the condition that human beings could be willing to sacrifice their lives for an idea is comprehensible neither from the utilitarian standpoint nor from that of the categorical imperative. In this case also it might possibly be that under the influence of certain ideas chemical changes, for instance, internal secretions within the body, might be 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. To-day, 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 to us so strange that what the philosopher terms an "idea" is a process which can cause chemical activity in the body.

  1. Lecture given at the Sixth International Psychological Congress at Geneva, 1909. Published by Johann Ambrosius Barth, Leipzig, 1909. (Translated by Grace B. Watkinson, New York, February, 1911.)
  2. Loeb, Sitsungsber. der Würzburger Physik.-Med. Gesellsch., 1888.
  3. Loeb, "Der Heliotropismus der Tiere und seine Ubereinstimmung mit dem Heliotropismus der Pflanzen," Würzburg, 1890 (Erschienen, 1889).
  4. Luther, "Die Aufgaben der Photochemie," Leipzig, 1905. C. Neuberg, Biochem. Zeitschr., Bd. 13, S. 305, 1908. Loeb, "Vorlesungen über die Dynamik der Lebenserscheinungen," Leipzig, 1906. In addition, the work of Ciamician, as also Wolfgang Ostwald (Biochem. Zeitschr., 1907).
  5. Loeb, "Comparative Physiology of the Brain and Comparative Psychology," New York and London, 1900.
  6. If two equally powerful sources of light are present at equal distances from the animal, the animal will move in a line at right angles to a line connecting the two sources of light, because in this case both eyes are similarly influenced by the light. Herein, as Bohn has rightly said, the machine-like heliotropic reaction of animals differs from the movement of a human being toward one of two sources of light, the movement in the latter case not being determined by heliotropism.
  7. Heliotropism is unusually common, namely, among the larvæ of marine animals and insects, but also not lacking in sexually mature individuals.
  8. Loeb, Pflügers Archiv, Bd. 115, S. 564, 1906.
  9. Provided that only a single source of light is present.
  10. The phenomenon of "steps" ("Treppe") upon stimulation of a muscle is ascribed, probably rightly, also to the formation of acid. The phenomenon of "steps," that is, the increase of the amount of contraction with every new stimulus is, however, comparable to or identical with the increase in the rate of reactions in the experiments described here.
  11. Loeb, l. c., p. 24. (This latter fact has been overlooked by several writers.)
  12. The physico-chemical cause of this "restlessness" which is noticeable in many insects and crustaceans is at present unknown.
  13. Rádl, "Der Phototropismus der Tiere," Leipzig, 1903.
  14. Or where the movement of the ions within the cell is retarded.
  15. Claparède, "Les tropismes devant la Psychologie," Journ. f. Psychologie und Neurologie, Bd. 13, S. 150, 1908.
  16. 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 "Vorlesungen über die Dynamik der Lebenserscheinungen."
  17. Loeb, "Uber die Umwandlung positiv heliotropischer Tiere usw," Pflügers Archiv, 1893. See also the recent investigations of Georg Bohn, "La naissanee de l'intelligence," Paris, 1909; "Les essais et les erreurs chez les étioles de mer," Bull. Inst. gén. psychol., 1907; "Intervention des réactions oscillatoires dans les tropismes," Ass. franc, d. Sciences, 1907.
  18. "Comparative Physiology of the Brain and Comparative Psychology," New York and London, 1900.
  19. Paris, "Bibliothèque de Philosophie scientifique," 1909.
  20. Chapter Tropismen in "Vorlesimgen über die Dynamik der Lebenserscbeinungen."
  21. 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 pleuronectids, which I can not, however, discuss further here. However, probably no one will maintain that the existence of the pleuronectids invalidates all laws in regard to the symmetrical body structure.