The Mechanistic Conception of Life

1 Loeb, J., Der Heliotropismus der Tiere und seine Uebereinstimmung mit dem Heliotropismus der Pflanzen, Wurzburg, 1890. A preliminary note on these experiments appeared January, 1888. reflexes occur in plants possessing no nervous system, yet in animals where ganglion-cells are present the very existence of the ganglion-cells necessitates the presence in them of special reflex mechanisms. It was therefore necessary to find out if there w^ere not animals in which coordinated reflexes still continued to exist after the destruction of the central nervous system. Such a phenomenon could be expected only in forms in which a direct transmission of stimuli from the skin to the muscle or direct stimulation of the muscle is possible, in addition to the transmission through the reflex arc. This is the case, for instance, in worms and in ascidians. I succeeded^ in demonstrating in Ciona intestinalis that the complicated reflexes still continue after removal of the central nervous system.^

A study, then, of comparative physiology brings out the fact that irritability and conductibility are the only qualities essential to reflexes, and these are both common qualities of all protoplasm. The irritable structures at the surface of the body, and the arrangement of the muscles determine the character of the reflex act. The assumption that the central nervous system or the ganglion-cells are the specific bearers of reflex mechanisms cannot hold. But have we now to conclude that the nerves are superfluous and a waste ? Certainly not. Their value lies in the fact that they are quicker and more sensitive conductors than undifferentiated protoplasm. Because of these qualities of the nerves, an animal is better able to adapt itself to changing conditions than it possibly could if it had no nerves. Such power of adaptation is absolutely necessary for free animals.

3. While some authors explain ail reflexes on a psychical basis, the majority of investigators explain in this way only a 1 Loeb, J., Untersuchungen zur physiologischen Morphologic der Tiere, II, Wurzburg, 1892. 2 This animal closes the oral opening when we touch it. This is a reflex comparable to the closing of the eyelid if we touch the cornea. The central nervous sytem of the animal consists of one ganglion. When the latter is removed the oral opening still closes upon mechanical stimulation.

certain group of reflexes — the so-called instincts. Instincts are defined in various ways, but no matter how the definition is phrased the meaning seems to be that they are inherited reflexes so purposeful and so complicated in character that nothing short of intelligence and experience could have produced them. To this class of reflexes belongs the habit possessed by certain insects of laying their eggs on the material which the larvae will afterward require for food. When we consider that the female fly pays no attention to her eggs after laying them, we cannot cease to wonder at the seeming care which nature takes for the preservation of the species. How can the action of such an insect be determined if not by mysterious structures which can only be contained in the ganglion-cells ? How can we explain the inheritance of such instincts if we believe it to be a fact that the ganglion-cells are only the conductors of stimuli ? It was impossible either to develop a mechanics of instincts or to explain their inheritance in a simple way from the old standpoint, but our conception makes an explanation possible. Among the elements which compose these complicated instincts, the tropisms (heliotropism, chemotropism, geotropism, stereotropism) play an important part. These tropisms are identical for animals and plants. The explanation of them depends first upon the specific irritability of certain elements of the bodysurface, and, second, upon the relations of symmetry of the body. Symmetrical elements at the surface of the body have the same irritability; unsymmetrical elements have a different irritabiUty. Those nearer the oral pole possess an irritability greater than that of those near the aboral pole.

These circumstances force an animal to orient itself toward a source of stimulation in such a way that symmetrical points on the surface of the body are stimulated equally. In this way the animals are led without will of their own either toward the source of the stimulus or away from it. Thus there remains nothing for the ganglion-cell to do but to conduct the stimulus, and this may be accomplished by protoplasm in any form. For the inheritance of instincts it is only necessary that the egg contain certain substances — which will determine the different tropisms — and the conditions for producing bilateral symmetry of the embryo. The mystery with which the ganglion-cell has been surroimded led not only to no definite insight into these processes, but has proved rather a hindrance in the attempt to find the explanation of them.

It is evident that there is no sharp line of demarkation between reflexes and instincts. We find that authors prefer to speak of reflexes in cases where the reaction of single parts or organs of an animal to external stimuli is concerned; while they speak of instincts where the reaction of the animal as a whole is involved (as is the case in tropisms). 4. If the mechanics of a number of instincts is explained by means of the tropisms common to animals and plants, and if the significance of the ganglion-cells is confined, as in all reflex processes, to their power of conducting stimuli, we are forced to ask what circumstances determine the coordinated movements in reflexes, especially in the more complicated ones. The assumption of complicated but unkno\\Ti and perhaps unknowable structures in the ganglion-cells served formerly as a convenient terminus for all thought in this direction. In giving up this assumption, we are called upon to show what conditions are able to determine the coordinated character of reflex movements. Experiments on galvanotropism of animals suggest that a simple relation may exist between the orientation of certain motor elements in the central nervous system and the direction of the movements of the body which is called forth by the activity of these elements. This perhaps creates a rational basis for the further investigation of coordinated movements.^

1 Since this was written von UexkueU found a law which will go far in explaining the mechanism of coordination, namely, that a stretched muscle shows an increased irritability while the contracted muscle shows a decreased irritability. Since 5. We must also deprive the ganglion-cells of all specific significance in spontaneous movements, just as we have done in the case of simple reflexes and instincts. By spontaneous movements we mean movements which are apparently determined by internal conditions of the living system. Strictly speaking, no movements of animals are exclusively determined by internal conditions, for atmospheric oxygen and a certain range of temperature are always necessary in order to preserve the activity beyond a short period of time.

We must discriminate between simple and conscious spontaneity. In simple spontaneity we must consider two kinds of processes, namely, aperiodic spontaneous processes and rhythmically spontaneous or automatic processes. The rhythmical processes are of importance for our consideration. Respiration and the heart beat belong in this category. The respiratory movements seem to indicate that automatic activity can arise in the ganglion-cells, and from this the conclusion has been drawn that all automatic movements are due to specific structures of the ganglion-cells. Recent investigations, however, have transferred the problem of rhythmical spontaneous contractions from the field of morphology into that of physical chemistry. The pecuHar qualities of each tissue are partly due to the fact that it contains certain ions (Na, K, Ca, and others) in definite proportions. By changing these proportions, we can impart to a tissue properties which it does not ordinarily possess. If in the muscles of the skeleton the Na ions be increased and the Ca ions be reduced, the muscles are able to contract rhythmically Hke the heart. It is only the presence of Ca ions in the blood which prevents the muscles of our skeleton from beating rhythmically in our body. As the muscles contain no ganglion-cells, it is certain that the power of rhythmical spontaneous contractions is not due to the specific

the contraction of one group of muscles necessitates the stretching of their antagonists the coordinated character of locomotive action seems to become intelligible morphological character of the ganglion-cells, but to definite chemical conditions which are not necessarily confined to ganglion-cells.^ The coordinated character of automatic movements has often been explained by the assumption of a '^ center of coordination," which is supposed to keep a kind of police watch on the different elements and see that they move in the right order. Observations in lower animals, however, show that the coordination of automatic movements is caused by the fact that that element which beats most quickly forces the others to beat in its own rh^'thm. Aperiodic spontaneity is still less a specific function of the ganglion-cell than rhythmical spontaneity. The swarm spores of algae, which possess no ganglion-cells, show spontaneity equal to that of animals having ganglion-cells.

6. Thus far we have not touched upon the most important problem in physiology, namely, which mechanisms give rise to that complex of phenomena which are called psychic or conscious. Our method of procedure must be the same as in the case of instincts and reflexes. We must find out the elementary physiological processes which underlie the complicated phenomena of consciousness. Some physiologists and psychologists consider the purposefulness of the psychic action as the essential element. If an animal or an organ reacts as a rational human being would do under the same circumstances, these authors declare that we are dealing with a phenomenon of consciousness. In this way many reflexes, the instincts especially, are looked upon as psychic functions. Consciousness has been ascribed even to the spinal cord, because many of its functions are purposeful. We shall see in the follow^ing chapters that many of these reactions are merely tropisms which may occur in exactly the same form in plants. Plants must therefore have a psychic life, and, following the argument, we must ascribe it to machines also, for the tropisms depend

only on simple mechanical arrangements. In the last analysis, then, we would arrive at molecules and atoms endowed with mental qualities. We can dispose of this view by the mere fact that the phenomena of embryological development and of organization in general show a degree of purposefulness which may even surpass that of any reflex or instinctive or conscious act. And yet we do not consider the phenomena of development to be dependent upon consciousness.

On the other hand, physiologists who have appreciated the untenable character of such metaphysical speculations have held that the only alternative is to drop the search for the mechanisms underlying consciousness and study exclusively the results of operations on the brain. This would be throwing out the ^^heat with the chaff. The mistake made by metaphysicians is not that they devote themselves to fundamental problems, but that they employ the wrong methods of investigation and substitute a play on words for an explanation by means of facts. If brain physiology gives up its fundamental problem, namely, the discovery of those elementary processes which make consciousness possible, it abandons its best possibilities. But to obtain results, the errors of the metaphysician must be avoided and explanations must rest upon facts, not words. The method should be the same for animal psj'chology that it is for brain physiology. It should consist in the right understanding of the fundamental process which recurs in all psychic phenomena as the elemental component. This process, according to my opinion, is the activity of the associative rnemory, or of association. Consciousness is only a metaphysical term for phenomena which are determined by associative memory. By associative memory I mean that mechanism by which a stimulus brings about not only the effects which its nature and the specific structure of the irritable organ call for, but by which it brings about also the effects of other stimuli which formerly acted upon the organism almost or quite

simultaneously with the stimulus in question. ^ If an animal can be trained, if it can learn, it possesses associative memory. By means of this criterion it can be shown that Infusoria, Coelenterates, and worms do not possess a trace of associative memory. Among certain classes of insects (for instance, ants, bees, and wasps), the existence of associative memory can be proved. It is a comparatively easy task to find out which representatives of the various classes of animals possess, and which do not possess, associative memory. Our criterion therefore might be of great assistance in the development of comparative psychology.

7. Our criterion puts an end to the metaphysical ideas that all matter, and hence the whole animal world, possesses consciousness. We are brought to the theory that only certain species of animals possess associative memory and have consciousness, and that it appears in them only after they have reached a certain stage in their ontogenetic development. This is apparent from the fact that associative memory depends upon mechanical arrangements which are present only in certain animals, and present in these only after a certain development has been reached. The fact that certain vertebrates lose all power of associative memory after the destruction of the cerebral hemispheres, and the fact that vertebrates in which the associative memory either is not developed at all or only slightly developed (e.g., the shark or frog) do not differ, or differ but slightly, in their reactions after losing the cerebral hemispheres, support this view. The fact that only certain animals possess the necessary mechanical arrangements for associative memory, and therefore for consciousness, is not stranger than the fact that only certain animals possess the mechanical arrangements for uniting the rays from a luminous point in one point on the retina. The liquefaction of gases is an example of a sudden

1 Loeb, J., "Beitrage zur Gehirnphysiologie der Wiirmer," PflUgers Archiv, change of condition which may be produced when one variable is changed; it is not surprising that there should be sudden changes in the ontogenetic and phylogenetic development of organisms when there are so many variables subject to change, and when we consider that colloids easily change their state of matter. It becomes evident that the unraveling of the mechanism of associative memory is the great discovery to be made in the field of brain physiology and psychology. But at the same time it is evident that this mechanism cannot be unraveled by histological methods, or by operations on the brain, or by measuring reaction times. We have to remember that all life phenomena are ultimately due to motions or changes occurring in colloidal substances. The question is, Which peculiarities of the colloidal substances can make the phenomenon of associative memory possible? For the solution of this problem the experience of physical chemistry and of the physiology of the protoplasm must be combined. From the same sources we must expect the solution of the other fundamental problems of brain physiology, namely, the process of conduction of stimuli.

The mechanism of the action of the brain is entirely unknown to us. We are unable to look into the active brain and the objective results of brain action are in general so different in their nature from the external stimulus which leads to the action that we are prevented in most cases from drawing any conclusions concerning the nature of the processes occurring in the brain. From results obtained in experiments on dogs Munk stated years ago that there existed a projection of the retina on a part of the cortex which he had designated as the visual sphere and that the extirpation of definite parts of this sphere caused blindness in definite parts of the retina. I repeated these experiments but was not able to confirm his statements. Henschen has recently, however, furnished the proof, on the basis of excellent pathological observations on man, that such a projection after all exists, but that it is situated in another part of the cortex from where Munk had believed it to be, namely, in the area striata. Minkowski was able to confirm Henschen's conclusions through experi lents on dogs. These observations and experiments suggest the possibility that in vision an image is formed not only on the retina but also on the cortex.

The possibility that vision is based on the formation of an image in the brain is supported by a group of facts which to my knowledge have never received any consideration in this connection. 1 Reprinted from Physiologisches Centralblatt, XXV, No. 22, 1912. This note is given merely as a suggestion concerning the mechanism underlying certain brain processes. It has been known for some time that many animals, especially certain fishes, adapt their color and pattern to the ground upon which they happen to be. This fact has been extensively utilized for the theory of natural selection. It seems to me that the same facts furnish also the proof that an image of the objects is formed in the brain. Pouchet many years ago showed that the adaptation of fishes to the ground ceases as soon as their eyes are removed or as soon as the formation of retinal images is prevented through the turbidity of the refractive media of the eye. This fact (confirmed by many observers) proves that the harmony between color and pattern of the skin of fishes with their surroundings is transmitted through the retinal image; in other words, that the so-called adaptation of fishes to their surroundings is only the transmission of the retinal image to the skin.

It has, moreover, been shown that the destruction of the optic fibers and the optic ganglia in the brain acts like the extirpation of the eyes; and finally it has been proved that the cutting of the sympathetic fibers which go to the pigment cells of the skin also prevents the formation of a picture of the ground on the skin. Hence we know the path by which the retinal image is transferred to the skin of fishes. One station is the ending of the optic fibers in the brain. Since we are able to prove the existence of an image of the object on the retina of fishes; since it is proved that the image on the skin of the fish is a picture of the retinal image but not of the object (in this case the ground) itself; since, moreover, the transmission of the retinal image upon the skin takes place through the optic nerve, it follows that the image must pass the central stations of the optic nerve during the transmission to the skin.

An image consists of a number of points of different intensity of light, the mutual arrangement of which is definite and characteristic for the object. Sumner has shown that certain fishes are able to reproduce on their skin rather complicated patterns (e.g., a chess board), which form the bottom of the aquarium. This reproduction of the pattern is somewhat imperfect, but if we deduct the secondary disturbing factors the fact remains that the pattern on the skin is a tolerably true picture of the pattern of the ground. There exists, therefore, a definite arrangement of the images of the different luminous points of the ground on the retina and a similar arrangement of the images of the luminous points on the skin of the fishes. We may consider each point of the retinal image as a luminous or a stimulating point which produces a corresponding image point in the primary optic ganglia through the action of the nervefiber through which it is connected with the ganglia. Every image point in the primary optic ganglia may be considered again as a luminous or stimulating point which through the mediation of a special nerve-fiber influences an individual chromatophore or a small group of chromatophores of the skin. Considering the fact that the retina is a mosaic, we cannot well imagine the transmission of the retinal image upon the skin in any other way than by assuming that the relative arrangement of the individual points of the retinal image is preserved in the optic fibers and the end ganglia of the optic nerve. Under this assumption a relative distribution of the stimulating intensities must occur in the primary optic ganglion which corresponds to the distribution of the image points on the retina and which again can be called an image.

These observations in fish and the conclusions drawn in this note suggest the idea that vision is a kind of telephotography. The various organs of the higher animals have a definite arrangement; from the shoulders arms originate, from the hips legs, but we never see legs growing out from the shoulders or arms from the hips. In the lower animals the same definite arrangement of organs exists. Fig. 22 gives a diagram of a hydroid, Antennularia antennina, which is quite common in the Bay of Naples. From a bundle of roots or stolons a straight stem arises to a height of six inches or more. From this main stem originate, in regular succession, short and slender branches, which carry polyps on their upper sides.

In this animal we never find a root originating at the apex, or in place of a branch, or polyps originating on the under side of a branch. In observing these phenomena the question arose : What are the circumstances which determine that only one kind of organ shall originate at certain places in the body? It occurred to me that the answer to this question might be obtained by finding out first of all whether or not it were possible to make any desired organ of an animal grow at any desired place. In case this could be done, the question to be decided was whether the same circumstances by which the arrangement of organs can be changed experimentally also determine the arrangement of

1 Reprinted from Biological Lectvires delivered at the Marine Biological Laboratory of Woods Hole, 1893. by courtesy of Ginn & Co. 2 Untersuchungen zur physiologischen Morphologie der Tiere. I, Heteromorphosis, Wtirzburg, 1891. II. Organbildung und AVachsthum. Wurzburg, 1892. Translated in Studies in General Physiology. Fig. 23. — Diagram of normal regeneration if a piece a 6 of Antennularia is liung up vertically in the water. The piece forms roots W at the lower end b and a new stem »S at the upper end a. The old normal arrangement of organs is thus restored through the process of regeneration.

Fig. 24. — Diagram of heteromorphic regeneration in Antennularia. A piece a b cut out of the stem is hung up in an inverted position, i.e., the root end b upward and the stem end a downward. In this case the apical end a forms roots W, and the basal end b forms a new stem S which grows upward. organs in the natural development. The hydroid, Antennularia antennina, above mentioned, seemed to afford a suitable subject for experimentation in an attempt to solve this problem and the following simple experiments were performed.

A piece ab (Fig. 23) of an Antennularia was cut out and hung up vertically in the water of the aquarium, the apical end a above and the root end b below. It was found that after a few days the root end b had formed little roots, W, which Fig. 22. — A piece of the normal stem of Antennularia antennina,& hydroid of the Bay of Naples. Approximately natural size. S S, stem. W, stolons or roots. grew downward, and the apical end, a, had formed a new stem, S.

A similar piece was cut out from another specimen and was hung upside dowTi in the aquarium (Fig. 24). The root end b, which was now above, formed a new stem, *S, and the apical end a, which was below, formed roots, W. In the newly formed stem the arrangement of the organs was the same as in the normal animal, namely, the branches which were growing Fig. 25. — From nature. Regeneration of a piece a b cut out from the stem of Antennularia and put horizontally into the water. The branches on the lower side which had ceased to grow, grow downward as stolons and attach themselves to solid bodies. On the upper side a new stem c d grows vertically upward.

obliquely upward bore polyps on their upper side. From this we see that it was possible to substitute a root for a stem and an apex for a root. This phenomenon of the substitution of one organ for another I termed heteromorphosis. If the excised piece of an Antennularia was placed horizontally instead of vertically in the aquarium, something still more remarkable occured, namely, the branches on the lower side suddenly began to grow vertically downward, and these downward growing elements were no longer branches but roots (Fig. 25). This could be proved by their physiological reactions, for the roots attach themselves to the surface of solid bodies, e.g., the glass

of the aquarium, while the stems never show such a reaction. These new parts growing out from the branches of the under side of the stem attached themselves to the solid bodies with which they came in contact. Moreover, they were positively geotropic (that is, they grew toward the center of the earth), while the branches never showed any positive geotropism. The branches on the upper side were not transformed into roots. They either perished or gave rise to long, slender, perfectly straight stems, which grew vertically upward. These stems, as a rule, were too slender to bear branches, but at parts of the upper surface of the main stem there originated new stems (c d, Fig. 25), which grew vertically upward and produced the typical little branches bearing polyps.

If we brought the stem into an obhque position (Fig. 26), with the apex a upward, from every element of the main stem new stems and roots originated, but with this difference, that stems always originated from the upper side of an element and roots from its lower side. If the stem were placed in an oblique position, with the root end above, the branches on the mider side grew out as roots, and at the upper end a stem arose as usual. Fig. 26. — Diagrammatic regeneration in a piece o b of a stem of Antennularia put obliquely into the water. On the upper side of the stem a b new stems S, S„ S,„ grow vertically upward, while at the lower end of the piece a b opposite the new stems roots W, W„ W,„ grow out.

This influence of gravitation is foimd only in Antennularia antennina, not in other forms of Antennularia. What circumstances had all these experiments in common ? The stems always originated from the upper end or side of an element, and roots always from the lower side or end of the same element. These facts can be explained only on the assumption that in this case gravitation determines the place of origin of organs. Now we may ask whether the action of this force, gravitation, is also responsible for the natural arrangement of the organs in this form, namely, that roots appear only at the base of the stem and never at the apex or in the place of a branch. I believe that this is the case. By reason of its negative geotropism, the stem grows vertically upward. Gravitation does not permit roots to arise at any place except at the under side of the organs, and that is, under normal conditions, at the base of the stem. The same force determines that polyps can originate only on the upper side of branches, and thus the general arrangement of organs is brought about by gravitation. But how does gravitation determine that stems grow on the upper and roots at the under side? This is a question to which we shall return later.

Fig. 27 gives a drawing of an example of heteromorphosis in Margelis, a hydroid common at Woods Hole, upon which another set of experiments was carried on. If we cut off a stem, or a small piece of a stem of this hydroid, and place it in a dish containing sea-water, protecting it carefully from every motion, a curious change takes place in the organism. Almost all, and in some cases all, of the stems which touch the glass give rise to roots that spread out and very soon cover a large area of the glass. In this way the apical end of a stem may continue to grow as a totally different organ, namely, as a root. Every organ not in contact with some solid body gives rise to polyps. Even the main root, if not in contact with a solid body, no longer grows as a root, but gives rise to a great number of small polyps which appear at the end of long stems. Fig. 27, which Mr.