The Mechanistic Conception of Life

A mechanistic conception of life is not complete miless it includes a physico-chemical explanation of psychic phenomena. Some authors hold that even if a complete physico-chemical analysis of these phenomena were possible today it would leave the 'Hruly psychical" imexplained. We do not need to enter into a discussion of such an objection since we are still too far from the goal. We are at least able to show for a limited group of animal reactions that they can be explained unequivocally on a purely physico-chemical basis, namely, phenomena which the metaphysician would classify under the term of animal 'Svill."

Through the writings of Schopenhauer and E. von Hartmann I became interested in the problem of vrill. When as a student I read Munk's investigations on the cerebral cortex I believed that they might serve as a starting-point for an experimental analysis of will. Munk stated that he had succeeded in proving that every memory image in a dog's brain is locahzed in a particular cell or group of cells and that any one of these memory images can be extirpated at desire. Five years of experiments 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 may give data concerning the path of nerves in the central nervous system but that it teaches little about the dynamics of brain processes.

A better opportunity seemed to offer itself in the study of the comparative psychology of the lower animals in which 1 Lectiire delivered at the Sixth International Psychological Congress at Geneva, 1909. (After a translation in Popular Science Monthly by Miss Grace B. Watkinson.) Reprinted by courtesy of Professor James McKeen Cattell. the mechanism for memory is developed but sHghtly or not at all. It seemed to me that some day it must become possible to discover the physico-chemical laws underlying the apparently random movements of such animals; and that the word ''animal will" was only the expression of our ignorance of the forces which prescribe to 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 movements of the planets and should begin to think about them, he would probably come to the conclusion that a ''will action" guides the movements of the planets just as a chance observer is today inclined to assume that "will" causes animals to move in a given direction.

The scientific solution of the problem of will seemed then to consist in finding the forces which 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 agencies, any number of individuals of a given kind of animals to move in a definite direction by means of their locomotor apparatus. Only if we succeed in this have we the right to assume that we know the force which under certain conditions seems to a layman to be the ^\ill of the animal. But if one part only of the animals moves in this definite direction and the other does not, we have not succeeded in finding the force which unequivocally determines the direction of their movement.

One other point should be observed. If a sparrow flies down to a seed lying on the ground, we speak of an act of will, but if a dead sparrow falls upon the seed this does not appear to us as such. In the latter case purely phj^sical 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 the methods of prescribing to animals the direction of their progressive movements. In order to obtain the material, potted rose bushes or Cinerarias 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 (plant lice), 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 easily be collected by holding a test-tube underneath and touching one animal at a time from above with a pen or scalpel, which causes the animals to drop into the test-tube. In this manner a sufficiently large number, perhaps twenty-five or fifty suitable subjects for the experiment, may be 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 so arranged 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 toward it in as straight a line as the imperfection of their locomotor apparatus allows, approaching as near to the source of light as their prison (the test-tube) permits. 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 180"^ the animals again go straight toward the source of light until the interference of the glass stops their further progressive movements.^ It can be demonstrated in these animals that the

1 Loeb, Der Heliotropismus der Tiere und seine Uebereinstimmung mit dem Heliotropismus der Pfiamen, Wurzburg, 1889. Translated in Studies in General Physiology, 1906. 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 today that a great many chemical reactions of organic bodies are accelerated by light. Especially is this true of oxidations.^ The mass of facts is already so great that we are justified in assuming 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, e.g., oxidation, increases.

The second factor is the symmetrical structure of the animal. As expressed in the gross anatomy of the animal, 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, by which I mean that s^inmetrical 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 retinae, 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

1 Luther, Die Aufgaben der Photochemie, Leipzig, 1905; C. Neuberg, Biochem. Zeitschr., Xlll, 305, 1908; Loeb, The Dynamics of Living Matter, New York, 1906. In addition, see the work of Ciamician, as also of Wolfgang Ostwald (Biochem. Zeitschr., 1907). 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 sjnmmetry of the two retinae are chemically alike, the non-symmetrical points chemically unlike.

Now if an unequal amoimt of light falls upon the two retinae the photochemical reactions in the one which receives more light will also be more accelerated than those in the other. The same naturally holds true for every other pair of symmetrical photosensitive surface elements. For it should be mentioned that photochemical substances are not found in the eyes only, but also in other places on the surface of many animals. In planarians, as my experiments and those of Parker have shown, not only the eyes, but also parts of the skin, are photosensitive. But if more light falls upon one retina than upon the other, the chemical reactions will also be more accelerated in the 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 CD. Snyder have demonstrated, independently of each other, that the rate of the nerve impulse has a temperature coefficient of the order of magnitude which is characteristic for chemical reactions. From this we must conclude that when two retinae (or other points of symmetry) are illuminated with unequal intensity, chemical processes, also of unequal intensity, take place in the two optic nerves (or in the sensory nerves of the two illuminated points) . This inequality of chemical processes passes from the sensory to the motor nerves and eventually to the muscles connected with them. We conclude from this that with equal illumination of both retinae 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 the action of symmetrical muscles of the two sides of the body is a change in the direction of movement on the part of the animal. The change in the direction of movement can result either in a turning of the head and, in consequence, of the whole animal toward the source of light, or in a turning of the head and the animal in the opposite direction. The structure of the central nervous system is segmental and the head segments generally determine^ the behavior of the other segments with their accessory parts.

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 toward the source of light will be increased.- 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 retinae become illuminated equally. There is therefore no longer any 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 light is the ''will" of the animal which determines the direction of its movement, just as it is gravity in the case of a falling stone or the movement of a planet. The action of gravity upon the 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 is caused only by means of an acceleration of photochemical reactions to move in a definite direction.

1 Loeb, Comparative Physiology of the Brain and Comparative Psychology, New York and London, 1900. ' If two sources of light of equal intensity are at an equal distance from the animal, it will move in a direction at right angles to a line connecting the two sources of light, because in this base 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.

We will now designate as positively heliotropic those animals which are forced to turn their head or move toward the source of light, and as negatively heliotropic those animals which are oriented or compelled to move 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. We cannot, of course, give here an account of all these cases. The reader who is interested in them must look into the voluminous literature upon this subject. Heliotropism is unusually common among the larvae of marine animals and insects, but also not lacking in sexually mature individuals.

Heliotropic animals are therefore in reality photometric machines. According to photometric laws the intensity of light varies with the sine of the angle at which the light strikes a surface element of the animal (or with the cosine of the angle of incidence). The animal is oriented by the light in such a way that symmetrical elements of its photosensitive surface are struck at about the same angle. In the presence of only one source of light this condition is fulfilled if the axis of symmetry of the animal moves in the direction of the rays of light. In this case the velocity of photochemical reactions on both sides of the animal is the same and there is no reason why it should deviate from this direction in its progressive motions.

Experiments on the heliotropism of plants as well as on the perception of light by our retina have shown that the effect of light equals the product of the intensity into the duration of illumination. This law is identical with the general law of Bunsen and Roscoe which states that the chemical effect of light is within wide limits equal to this product. We do not yet know whether or not Bunsen's law holds good for the heliotropic animals. If it does, we shall have to substitute 5\(^^

1 Whether an animal is positively or negatively heliotropic depends upon the fact whether the light causes an increase or a decrease in the tension of the muscles . Why light should have these opposite effects is as yet imknown. this law for what the metaphysician calls the will of these animals. The winged aphids serve as an example, because they fulfil the above-mentioned requirement, namely, that all individuals, without exception, move toward the light. For mechanistic science it is a methodological postulate that the same law acts without exception, or that the exception must be satisfactorily explained. It was soon found, as was to 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 in a degree as pronounced as do the winged aphids. The problem therefore presented itself of producing heliotropism artificially in animals which, imder natural conditions, show no positive heliotropism. If small crustaceans of a fresh-water pond or lake are taken with a plankton net at noontime or in the afternoon and placed in an aquarium which is illuminated from one side only, it is often found that these animals move about in the vessel pretty much at random and distribute themselves irregularly. Some seem to go more toward the source of light, 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 of the animal. To every 50 c.c. of the fresh water a few cubic centimeters of water charged with carbon dioxide are slowly added. If the correct amount is added all the individuals become actively positively heliotropic and move in as straight a line as the imperfection of their swimming movements permits, toward the source of light, and remain there closely crowded together on the illuminated side of the vessel. If the vessel is turned 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 weakly and much more slowly. Animals which were previously indifferent to light become, under carbonic acid treatment, complete slaves of the light.i 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 fix our ideas provisionally we will assume that the acid makes the animal more strongly positively heliotropic by increasing the active mass of the photosensitive substance. In this way the same intensity of light which before produced no heliotropic reaction now may 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 products in both retinae becomes rapidly large enough to cause automatically a difference in the action of the muscles of both sides of the body and a turning of the head toward the source of light.

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 Daphniae positively heliotropic, this may often be accomplished by simultaneously reducing the temperature. The animals which are strongly positively helio tropic and those animals which do not react at all to light offer no difficulties to the observer. Nevertheless, some zoologists 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 ^^ith 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 requires 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 raj's, the difference in the photochemical reaction products in the two retinae becomes so great that the head is at once turned back automatically toward the source of light. But if the intensit}^ of the light or the photosensitiveness of the animal is 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, moving 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, possibly by increasing the active mass of the photochemical substance. Every animal is continually producing acids in its cells, especially carbonic acid and lactic acid; and such acids increase the tendency in certain animals to react hehotropically. 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 the production of these substances will also produce fluctuations in the heliotropic sensitiveness of the animal. 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 to cause 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 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 \^Titer 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 with the suggestion of ''trial and error" which is no more chemical or scientific than the explanations of metaphysicians in general. Some authors have, it seems, worked only with animals which were not pronouncedly heliotropic and whose photosensitiveness wavered about the threshold of stimulation in the manner described above. Such animals are not suitable for experiments in heliotropism and it is necessary to first increase their photosensitiveness if the laws of the action of light upon them are to be investigated.

I also believe that observations upon animals which are not sufl&ciently photosensitive have caused many writers to assert that heliotropic animals do not place themselves directly in the line of the rays of light,^ but that they first have to learn the right orientation. A very striking experiment contradicts this assertion. The larvae of Balanus perforatus develop entirely in the dark. If the ovary filled with mature larvae is placed in a watch crystal filled with sea-water in the dark, the larvae 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 .^

2 The so-called " staircase " phenomenon of stimulation of a muscle is ascribed, probably rightly, also to the formation of acid. This phenomenon, 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. As far back as 1889 I pointed out that the photosensitiveness 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 have left the plant. The influence of the chemical changes which take place in animals upon heliotropism is much more distinct in the larvae of Porthesia chrysorrhoea. The larvae hatch from the eggs in the fall and, as young larvae, hibernate in a nest. The rising temperature in the spring drives them out of the nest, and they can also be driven out of the nest in winter by an increase in temperature. When driven out of the nest in this condition they are strongly positively hehotropic and I have never found in natural surroundings any animals whose heliotropic sensitiveness was more pronounced than it is in the young larvae of Chrysorrhoea. But as soon as the animals have once eaten, the positive heliotropism disappears and does not return even if they are again allowed to become himgry.^ In this case it is clear that the chemical changes directly or indirectly connected with nutrition 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.

It is a well-known fact that during sexual maturity special substances are formed which influence various organs. For 1 Loeb, op. cit., p, 24. (This latter fact has been overlooked by several writers.) instance, Leo Loeb has found that the substances which are set free by the bursting of an egg foUicle 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 deciduae in nonpregnant uteri, while without the circulation of follicle substance in the blood 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. I will not discuss these facts further in this place, but refer my 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 pointed out in former papers that it serves to save the lives of the above-mentioned yoimg larvae of Chrysorrhoea. When the young larvae are awakened from their winter sleep by the sunshine of the spring they are positively heliotropic. Their positive heliotropism leaves them no freedom of movement, but forces them to creep straight upward to the top of a tree or branch. Here they find the first buds. In this way their 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 having eaten they once more lose their positive heliotropism. They can now creep downward, and the restlessness which is characteristic of so many animals^ forces them to creep downward until they reach a new leaf, the odor or tactile stimulus of which stops the progressive movement of the machine and sets their eating activity again in motion.

The fact that ants and bees become positively heliotropic at the time of sexual maturity plays an important role in the 1 The physico-chemical cause of this "restlessness" which is noticeable in many insects and crustaceans is at present unknown. vital economy of these creatures. As is well kno^\Ti, the mating of these insects takes place during the so-called nuptial flight. I found that among the male and female ants of a nest the heliotropic sensitiveness increases steadily up to the time of the nuptial flight and that the direction of their flight follows the direction of the rays of the sun. I gained the impression that this nuptial flight is merely the consequence of a very highly developed heliotropic sensitiveness. The case seems to 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.