The Science and Philosophy of the Organism

The second complication in the theory of tropisms appears whenever the general conditions of life are altered. In this case a change say of the general temperature of the medium changes the " sense " of say heliotropism ; a fact that has been named " heterogeneous induction " by Noll. This change of the sense of a tropism very often plays a true morphogenetic, or, rather, restitutive role : if a pine is decapitated, one of the side branches assumes the negative geotropism of the lost main axis, and a similar phenomenon holds for roots. The general organisatory state of the organism is the "general condition" that was altered in

this case. Whenever parts of a plant change the sense of a tropism, according to their age or state of fertility, we find something very similar. Here already the concept of the " whole " with regard to functioning in its relation to out side factors presents itself, though perhaps not in a manner sufficient to refute the " machine theory " of life.1 The last step of complication is reached if two or more stimuli are in competition with one another. This case is best shown by the behaviour of roots in the ground ; gravity, moisture, heat, chemicals are the principal stimuli concerned here. The effect is not a simple sum or resultant, but a sort of unity of a very peculiar kind : each single component may change the organism's sense of irritability, or " Stimmung," towards any other component. A certain sort of innate direction relative to the axis may be among the components that influence the behaviour of a certain organ (" autotropism "). It would at least be difficult to apply the machine theory of life in these cases.

Are the directive movements in freely moving Protista or animals, called " taxis," explainable in the same way as tropisms ? It is clear that the direction and the movement are two different things. It is the direction only that is considered here, and so we may better say : " taxis " signifies the specific orientation of a specific axis of the organism with regard to the direction of any directed agent of the medium. 1 A very strange case belonging here is discussed by France (Zeitschr. f. d. Ausbau d. Entwickelungslehre, i. 4, 1907).

If the taxis is combined with or followed by movement, there will, of course, be a specific direction in this move ment also. The word " taxis " thus applies only to the correspondence of directions. It does not say the least thing about the means of movement, by which the orientation of the organ ism goes on ; it does not even seek to point out that the process of orientation is quite a simple process. In fact, a very easy consideration shows that the process of " taxis " is by no means simple in many cases.

Imagine an organism, — say a protozoon or a crayfish, in order to show from the beginning that the particular motor organs in question are of no consequence — and imagine it placed with its long axis at a certain angle towards the direction say of the rays of light proceeding from a radiant point. Then " taxis," in this case " phototaxis " or " heliotaxis," would be said to occur, if the organism carries out some sort of turning movement so long as there is any deviation between the direction of its axis and the rays of light ; the movement being performed equally well by the cilia of the protozoon or by the legs of the crayfish. Certainly the " taxis " here is neither immediate nor simple ; it is a combination of very many single motor acts, leading to taxis as a result, though this result must be said to have been reached in an unbroken line. We have to assume that the motor organs of one side of our organism are stimulated by the rays of the light as long as there is no symmetrical arrangement of both of its sides with regard to the direction of the light ; of course, the result of stimulation of this kind would be finally a symmetry of orientation.

Nothing of course would be explained by calling any process of movement of this sort " taxis " : but " taxis " certainly would be a good name for embracing a rather simple class of co-ordinated movements, which have a very apparent common feature in the fixed relation of the directions between the stimulus or cause and the final effect, reached without any interruption in an unbroken line. It is true, the phenomena of this so-called taxis were known not to be so simple as described here ; there were all the kinds of complications known from the phenomena of tropisms. Taxis was called " positive " in the case when the anterior end of the organism was finally placed towards the stimulating source, and it was called " negative " in the opposite case. Now it was found that the same organism, which had proved to be positively phototactic or chemotactic, could react negatively when the intensity of the stimulus increased, and conversely. But the point of this change was by no means fixed for a given individual ; the organism could become adapted or acclimatised to a stimulus which at first had caused the avoiding or negative reaction, and could thus become positive without any change of the medium. But other conditions of the medium, such as its salinity or temperature, were also found to have an influence upon the " sense " of taxis, say with regard to the rays of the sun (J. Loeb).

That was the general state of the apparently well established theory of taxis about six years ago. Was it possible to explain all these facts as being simple and machine -like in the same way as simple reflexes ? The difficulties, as with tropisms, lay in the variability of the point of changing the tactical sense and in the phenomena of simultaneous irritation by different stimuli.1 But these difficulties might perhaps not be regarded as sufficient to force us to accept vitalism, though, of course, to deny the logical necessity of a mtalistic conception of biological facts does not imply the impossibility of mtalistic agents being actually at work in them.

So much about the aspect of the theory of " taxis " a few years ago. Now it is very important for our present purposes to observe that " taxis," in the sense we have analysed, seems to occur to a rather limited extent only. There is a true and real " galvanotaxis " amongst Infusoria, and there are a few " tactical " phenomena in animals, as for instance when Hydra or a flatworm turns its head towards a strong light or towards a mechanical stimulus. But very much of what had been called phototaxis or chemotaxis or thermotaxis, among Protozoa as well as among higher animals, has actually been shown to be not taxis at all, that is, not a final correspondence of direction reached in an unbroken line comparable to the tropisms in plants, but something very different. It therefore must be regarded as possible at least, that in the future still more cases of " taxis " will prove to be illusory, though, as must be mentioned, J. Loeb and certain other writers only

1 Compare the suggestive article, "Die Lichtsinnesorgane der Algen," by R. H. France, Stuttgart, 1908. France's conception of " Reizverwertung " — originally created by Kohnstamm in a purely psychological sense— is very well descriptive of what happens. concede a very limited validity to the views recently brought into the field, and maintain the old " taxis "- theory. The new doctrine of "taxis," and at the same time quite a new theory of the elements of animal movements in general, is due to Herbert Jennings.1 Jennings made his important discoveries by studying not only the final result of any directed agent acting upon the organism, but also the moving individual itself in the very act of moving. This very act of moving, especially in the case of Protozoa, was proved to be anything but a single and unbroken act of turning. " Taxis " thus became a mere resultant of the most various single motor acts, and, with the sole excep tion of galvanotaxis, ceased to be a proper name for the process.

I shall be only following the historical line of events, if I now try first to give a short sketch of Jennings' solution of the problem of taxis, and then begin the real systematics of animal motions. The infusorium Paramecium is " positively chemotactic " to a weak solution of acetic acid, that is to say, a number of these Protista living in a dish that contains a drop of such a solution in any part of the water after a certain time will be found to be all in a certain region around this drop, which, of course, is slowly diffusing into the surrounding water. The old theory would say in this case, that the

1 Compare his work, Behaviour of Lower Organisms (New York, 1906), where the full literature is to be found. lines of diffusion of the acetic acid orient the Paramecia positively according to their direction, and that thus the Paramecia reach the solution by simply swimming forward after the orientation is completed. But that would be far from the truth. Jennings found, on^the^cpju^rary, by observing the single individuals, that all the Infusoria swim at random and enter the solution at random also, but that then they are kept within the limits of a certain concentra tion of the diffusing acid by a very strange feature : as soon as they reach those limits the passing of which would bring them out of the region of the acid, they give a certain very typical motor reaction, which makes them remain in the region where they were. The reaction consists in a swim ming backward, combined with a revolution round the long axis and a turning to the aboral side.

And quite the same holds for " negative chemo taxis," as happening, for instance, in the presence of a solution of ordinary salt. All of the animals which by their ordinary forward motion would reach the region of a certain con centration of the diffusing chloride of sodium, perform the reaction just named in the very moment of entering this region. Thus they never really penetrate to this region, for the reaction may be repeated as often as necessary; but the few organisms which were in the region of the salt at the beginning of the experiment may freely leave it. In the end, of course, all the animals are out of range of the solution, just as in " positive chemotaxis " all the animals were in range.

It must be granted that Loeb, in establishing what he called " Unterschiedsempfindlichkeit," i.e. the reactions of animals to differences of intensity say of light, came very near to the views sketched here, though he was (and is) far from admitting the resolution of all kinds of "taxis" in this way.1 Chemotaxis thus is proved by Jennings to be a mere resulting effect of many different single performances, and is not a simple and immediate process of orientation at all.2

And what holds with regard to chemicals is also true with regard to heat, light, contact, and any other stimulus except the galvanic current, and applies not only to Infusoria, but also to Flagellata, and Bacteria, and Rotatoria, and all other sorts of invertebrate animals ; as far at least as experiments in the style of Jennings have been carried out. Therefore, though we cannot say at present that no case whatever of " taxis " exists (except galvanotaxis), we shall ' not, I believe, be very far wrong in saying that probably the range of " taxis " will prove finally to be at least very restricted.

It now might seem that the typical motor reaction shown by Paramecium, either in leaving or in entering the solution applied in the experiment, is of the type of a true reflex of the most simple kind, and that, therefore, in spite of the resolution of the concept of " taxis," as maintained by Jennings, the simple reflex would be the actual basis of 1 I cannot agree with Walter (Journ. exp. Zool. 5, 1907. — Here full literature on the subject), when, in his studies on the reactions of Planaria to light, he applies the term " Phototaxis " to reactions of this worm towards .differences of the intensity of illumination. The word "taxis" strictly depends on the theory that refers to the direction of a stimulus exclusively.

2 I should not believe that the resolution of "taxis," according to the analysis of Jennings, would apply to the phenomena of the wandering of embryonic cells to specific localities in the case of " directive stimuli " (see vol. i. p. 104). The old theory might also hold perhaps in cases of "in flammation " and the protective migrations of cells in general (Metschnikoff ; see also vol. i. p. 206). all movement whatever. So indeed Jennings thought in the first period of his work, but a more thorough study taught him very differently.

This now is the right point to begin the systematic study of the types of animal movements ; let us consider, in the first place, what may be called single motor acts. The " simple reflex " is one of these acts, but it is far from being the most original or the most widely distributed of them ; it seems to be restricted to certain specific typesof motion among the higher classes of animals ; even what is performed by our Paramecium is not a simple reflex.

The most original motor act, that is to say, the most elemental one both ontogenetically and systematically (" phylogenetically "), is " motion at random" i.e. an in definitely variable motor effect following some sort of a stimulus and having no specific relation to the locality of the latter, whether the locality of possible stimulation be a limited and fixed one, as for instance in many Infusoria, or not, as in many higher animals and in all Amoebae.

There are two classes of original movements at random requiring to be distinguished. The first consists of such single motor acts as show an absolute contingency, the second of those which show a relative one. All Amoebae are a good instance of the first type : any stimulation may be followed by every possible movement in every geometrically possible direction out of a strictly indefinite number of possibilities ; the same holds for many worms. But in Infusoria, as in all animals that are more specifically

organised with regard to their locomotory organs, the number of motor possibilities is more restricted : Paramecium for instance always swims backward, revolves round the axis, and turns to the aboral side. That might seem to be a typical reflex, but in fact is far from being so. One of the components of the motor reaction allows an indefinite variety of motions at random even here — the revolving round the long axis. This act may be performed to any possible amount, and, of course, the slightest variety in performing it would bring the animal to quite a different part of the dish in the course of its subsequent movements. Jennings has introduced the appropriate name of " action system " to signify the typical restriction of possible move ments, indefinite1 in spite of it, which are founded upon the typical locomotory organisation : it is clear that all higher animals possess such a system, and that man for instance is restricted by it from flying.

Thus then all single motor acts that could be actually observed were found to be of the type of "movement at random," occurring either on a definite action system or on an absolutely indefinite one. There was scarcely any reflex of the true kind, in the sense of an absolutely fixed correspondence of locomotory cause and eft'ect. The concept of the contingency of single motor acts embraces the fact of their modifiability. But as our mind is forced to conceive all that happens as being univocally

1 We might speak here of an indefiniteness of different orders, as mathematics does. determined, the problem at once arises, by what factors or conditions the actual performance of a particular movement in a particular case is actually determined as such. Let us first remark that motion in itself by no means requires a separate external cause for each of its single phases. On the contrary, not only can periodic movements like those in medusae or in the heart of animals be said to be due to innate causes or stimuli, and to be, so to say, the normal permanent state of the animal or the organ, but changes of the specific type of random-movements may also occur from within. In Hydra such an innate change of different contingent motions may be studied with the greatest advantage.

This possibility of a change of single random-motions from within now gives us the key to an understanding of their change as occurring in response to an external stimulus. It is always the interior general state of the organism that determines which particular motor performance is to go on, whether the state of rest is to be changed into a state of some possible movement, or whether permanent motion is to change its type. Yet we may speak of motions occurring " at random " although we know that they are determined, provided that we know nothing specific about the general state of the organism in question. In fact, the movements of an animal which otherwise would not move at all, or the changes of motion in a permanently moving organism, may properly be called " random," if they do not follow any specific law with regard to their sequence, if they go on until the stimulus from without, that has caused them, is escaped quite accidentally during and by the moving.

Jennings has spoken of the method of " trial and error " in these cases as well as in others to be studied hereafter. I should like to avoid this term, for, besides its psychological aspect, which seems to be out of place here, the word " trial " seems to me to imply some sort of so-called " experience." But here in the simple fact of movement at random there is nothing of that sort as far as we know ; it only might be, that the true random-motions might offer the material for " experience," as will be seen on a later occasion.

Contingency thus is the leading characteristic of the performance of all these most elemental single motor acts, as well as of their being stopped. But there are cases where something more definite may be said about the factors that determine the type of each single motion. Typical interior states — not only quite generally conceived ones — may change the type of reaction as well as stop motion altogether in spite of the external stimulus being still present. Thus it is well known, especially from the studies of Coelenterata, that a hungry animal reacts otherwise or not at all, if compared with a fed one, with regard to the same stimulus, and there are also differences of reaction corresponding to the different embryonic stage or the age of an organism.

And moreover we find that a competition among various external stimuli may determine the type of reaction. The effect of a second external stimulus may be either that there is no longer any reaction to the original stimulus, or that a sort of resultant reaction goes on, or that the type of the original reaction is other wise changed. Here we must recall attention to the so-called reversal of the <c sense " of the reaction, as asserted by the theory of " taxis " to occur if the intensity of the original stimulus was increased, or if other stimuli came into play. The facts were quite true, but their real explanation now proves to be of a much more general kind. In fact, there may also be "acclimatisation," say to chemical stimuli ; then the avoiding reaction shown at first will not be shown any longer after a certain time : " nega tive chemotaxis " will cease to exist. And other kinds of stimuli, coming into competition with the original one, may result in the same effect.1

But now we come to two classes of modifications of single motor acts, which possess a great importance for all that is to follow. There may be a typical series of consecutive different single motor reactions, whenever the first or any following one of these reactions has not avoided the external stimulus or has not reached the condition " desired," and this typical series may go on until the " desired " state is actually reached. Such typical lines of different single reactions have been well studied by Jennings and his followers in many cases, the most typical ones occurring in the infusorium Stentor and in Actinians. If a Stentor is disturbed, say by some sort of light powder falling upon it, it first bends to one side

1 A very remarkable fact of this class has recently been discovered by Minkiewicz (Arch. ZooL exp. et gen. 4 ser. 7, notes, 1907) : the crab Maia may change the qiuility — not the "sense" — of its " chromotropism," which is independent of its reaction to light in general, according to the colour of the ground it lives upon, and another crab, Hippolyte, changes its colour and its chromotropism correspondingly. In this case the whole phenomenon falls most markedly under the concept of what we have called "physiological adaptation " in the first volume of this book. Indeed, the question may arise, whether all modifications of primitive motor irritability may not be considered under this heading in further analytical studies. Of course, what Minkiewicz calls chromo-" tropism " ought rather to be styled chromo-" taxis," and, most probably, is no real "taxis."

several times, but, if it is not freed from the stimulus, a second type of reaction sets in : the direction of the ciliary movement is reversed. Again without success ; even the third type of reaction, contraction into the tube, is un successful, and it is only the last kind of motion, swimming away, that definitively frees our animal from the " disliked " condition. Here quite decidedly the fact that one type of movement has occurred determines the type of the next reaction : the word " trial," though not quite correct even here, seems at least to have a better meaning than if applied to mere movement at random.

It also might seem to be a typical sequence of reaction types, if to a very weak stimulus our Stentor first answers in its usual original manner, and after that does not react any more : but it seems to me that here we have nothing but the well-known fact of acclimatisation. To the last typical class of modifiability of simple motor acts only a few words may be devoted in this connexion. If Stentor, after going through the whole series of possible reactions, is stimulated in exactly the same way once more, it answers with the ultimate reaction at once, supposing the intermediate time has not been very long. And similar features in simple motor actions have been observed in other Protozoa, in Actinians, and some worms. Did these animals acquire any " experience," even of the most simple kind ? And what does " experience " mean in natural science ? A later chapter will have to deal with this most fundamental question.