Forced Movements, Tropisms, and Animal Conduct

each other in the middle of the dish. The light A is fixed while the I light B is movable on an optical bench. The experiment is made in I a dark room and the lights A and B are enclosed in a box. At the beginning of the experiments the pipette is filled with a dense suspension of larvae in sea water and then put with its point touching the bottom of the dish. The animals flow out in a fine stream which is narrow at the opening of the pipette and widens slightly, owing probably to the negative ©tereotropism of the animals.

A glass plate (Fig. 32.) hihl, which has a strong red line na and a fine parallel line pq (cut with a diamond), is then put on the dish and so adjusted that pq is in the middle of the stream fg of the ani¬ mals. Then the angle a which pq makes with the perpendicular from A on ah is measured. This perpendicular is marked in the form of a red line on the black base on which the glass vessel ahcd stands. The angle a is measured with a goniometer. When the lights are equal in intensity « should be 45°; if the two lights have different intensities and if A be the stronger light a should become smaller with increas¬ ing difference in intensity. The individual measurements vary com¬ paratively little, as long as the difference in the intensity of the two lights is not too great; for this reason our observations do not go beyond a wider ratio of the two lights than 10:1, though 4:1 is perhaps the limit for good results. Table VII gives the results. A is always the stronger light. Each table is the average of from 40 to 60 individual observations, each being the average of the path of many thousands of animals.

In the next series of experiments an opaque rotating disk with one sector cut out was placed before light B. In one set of experiments the sector cut out was 90®. The rate of rotation (by an electric motor) was 1,500 to 2,500 revolutions per minute. The other light was constant and its distance was chosen on the assumption of the validity of the Bunsen-Roseoe law for these cases. Thus when the two lights without sector were equal at a given distance of A, by putting 90° sector before

Bj it was assumed that the ratio of effects would be the same as if, with constant light, B had been placed at the double distance and the ratio of intensities of the two lights had been 4:1. Going on such a calculation we should expect the same values for a as in Table VII. As one sees from Table VIII, the observed values are slightly smaller but practically identical with the values obtained when the two lights are constant. The deviation is probably due to the well established fact that the photochemical efficiency of an intermittent light is a trifle less than that calculated on the basis of the Bunsen-Koscoe law.

Value of a when one light is inter¬ mittent (90° sector) and the other constant, and the efficiency of the two lights is calculated on the basis of the validity of the Bunsen-Eoscoe photochemical law We carried out some experiments with a sector of 144°. When the efficiency of both lights was equal on the assumption of the validity of the Bunsen-Boscoe law a was found to be 44.9° (instead of 45°), and for the ratio 2:10: was found to be 38.8°. The values are, within the limits of error, identical with the values in Tables VII and VIII.’^®^

Bradley M. Patten also showed that for the heliotropic reactions of the negatively heliotropic larvae of the fly the law of Bunsen and Eoscoe holds. Photochemical processes have a very small tempera¬ ture coefficient and it agrees with this that lowering of temperature within the limits compatible with the motility of animals does not affect the heliotropic response; on the contrary, we shall see that in certain crustaceans {e.g., Daphnia) lowering of the temperature may enhance posi¬ tive heliotropism.^^®

We must, therefore, conclude that the light produces in an eye or an element of the photosensitive skin a chemi- cal reaction whicli results in the formation of a certain mass of a reaction product. This mass acts on the periph¬ eral nerve endings and brings about an as yet unknown change in the brain elements with which these nerve end¬ ings are connected. This change in turn affects the tone or tension of the muscles with which the brain elements are connected. When the rate of photochemical reaction is the same in both eyes or in the photosensitive elements on both sides of the body, the change of tone in the sym¬ metrical muscles of both sides of the body is the same and no change in the position or direction of motion of the organism should occur. If the rate of illumination is dif¬ ferent in both eyes, differences in the relative tension of the symmetrical muscles occur, which make the motion to the source of light easy and in the opposite direction more difficult when the animal is positively heliotropic. ■For the negatively heliotropic animal the opposite effect will be brought about.

These experiments, therefore, show that the tropism theory not only allows us to predict the nature of the animal reactions but allows us to predict them quantita¬ tively. Thus far the tropism theory is the only one which satisfies this demand of exact science. The degree of directness with which a heliotropic ani¬ mal goes to or from a source of light depends, aside from the degree of perfection of its locomotor apparatus, upon the intensity of the light and the relative sensitiveness of the animal. Animals which in strong light will move in approximately straight lines to or from the source of light may in weak light reach their goal in a more or less irregular zigzag line. This is easily understood. When

aix animal by chance gets its median plane too far out of the direction of the rays of light (we assume them to be parallel), the rate of photochemical reaction will be¬ come different in both eyes. As soon as the difference between the photochemical reaction products in both eyes tixcoeds a certain limit the animal wiU automatically put its plane of symmetry again into the direction of the rays of light. The weaker the light and the less sensitive the animal, the longer it will take until this happens, and the greater the freedom of the animal to deviate from the straight line.

It may prove necessary to make a similar assumption for the elfect of a constant illumination as was made by Nernst for the theory of the action of galvanic currents, namely that there are two antagonistic processes going on, one being the photochemical effect of light a-nrl the second either a process of diffusion of the substances formed or a chemical reaction of the opposite character as that caused by the action of the light. Many animala which are oriented by constant illumination react by a quick, jerky movement when the intensity of light is either rapidly increased or diminished. In this case the effect is determined by the rapidity of the change in the intensity, |, and not by the product of intensity into dura¬ tion of illumination, These twitchiug or jerking

effects caused by a rapidly changing intensity of light are comparable to the twitching brought about in a muscle by a rapid increase or decrease in the intensity of a cur¬ rent. The writer described such reactions first for tube worms like Serpula, which withdraws suddenly into its tube when a shadow passes over it or when the intensity of light is suddenly diminished in some other way. The anthropomorphists, of course, declare this reaction to be induced by the instinctive fear of an enemy, oblivious of the fact that if they were consistent they would have to give the same explanation for the twitching of a muscle upon rapid changes in the intensity of a current. The

problem to be solved is ia both cases a purely physico¬ chemical one. It was also found that the motions of certain animals stop when they come suddenly from strong light into weak light. This was observed in planarians which as a consequence collect in greater density in spots Fia. 33.—Difference in place of gathering be¬ tween heliotropic animals and animals which come to rest when reaching a relative minimum in the intensity of light. In a circular vessel a cb d and W W representing the window, positively helio- tropic animals will collect at a, negatively heliotropic animals at b, and animals which come to rest where the intensity of light is a relative minimum at c and d.

tropic animals; they may lead, however, to collections of animals. Jennings has maintained that all reactions of unicel¬ lular organisms are due to “fright” or “avoiding reae- of the space where the intensity of light is a relative minimum.^®^ The difference in the conduct of helio¬ tropic organisms like Daphnia which go to or from the light and animals like planarians which come to rest where the intensity of light is a rela¬ tive minimum can be demonstrated by putting them into a circular vessel (Fig. 33). The positively heliotropic animals collect at a, the negatively heliotropic at h, while the planarians collect at c and d where the intensity of light is a minimum. Eeaetions de¬ termined by the value do not lead to phenomena of orientation, though such (improperly called) “fright reactions”®- occur in many helio-

a The reader should notice the difference in the treatment of animal conduct from the point of view of the physicist and of the introspective psy¬ chologist. What the physicist expresses correctly by the term the an¬ thropomorphic biologist explains in terms of human analogy as avoiding reaction ” or “ fright reaction,” a term which not only assumes the existence of sensations without any adequate proof, but removes the problem from the field of quantitative experimentation.

tions,” and it seems as if at one time lie even intended to deny the existence of tropisms and to maintain that all animals were influenced only by rapidly changing intensi¬ ties of light. It is needless to discuss such an idea (which he probably no longer holds) in view of the contents of the preceding chapters. He seems, however, to cling to it as far as asymmetrical unicellular organisms are concerned. When moving about Faramcecia often reverse the direc¬ tion of their progressive motion for a moment, but then do not return in the old direction, moving sidewise, on account of the asymmetry in the arrangement of their cilia. Jennings is probably right in assuming that this factor can lead to collections of such infusorians, since it may prevent their leaving a drop and going into the sur¬ rounding medium. When, e.g., at the boundary of the two media such a reversal of the action of the cilia occurs, the organisms are prevented from crossing from one medium into the other.

But Jennings goes too far' in this attempt, when he tries to explain the heliotropic reactions of certain uni¬ cellular organisms, e.g., Euglena, in this way. He main¬ tains that unicellular organisms like Euglena go to the light on account of shock movements produced by the shading of the photosensitive region of the animal. Euglena moves with a constant rotation around its longi¬ tudinal axis and Jennings assumes that in a certain phase of the rotation a photosensitive element (the eye spot) of the organism is shaded. This he thinks causes a shock movement, whereby the animal is swerved to the light again during the next half of the spiral revolution, and so on. Similarly in negatively heliotropic Euglena the swerving away from the light is, according to Jennings, the shock movement caused by the increased illumination

of the photosensitive end of the animal produced by swerv¬ ing toward the light during the previous half of the spiral revolution. Bancroftshowed tiiat Jennings’s theory was based upon incomplete facts. Aceordiiii^ to JeioiiogH’H pcwitivi* holitdropiwm is ooiuiitioiied by and should be aeetutipanircl by slioek moveinente produred by sudden shading (= shading reaction), and negative hidiotropisin should alwayit be accompanied liy shock movcmniiits produced by sudden illumination.

It lias been found, Innvever, that tliin uhiiiiI nsHofliiticui of slioek movcnictnts with iropism i» not a neca^iiry one, but that it can be dffltroyed if the proper means be taken. Ctonsequently the view that the heliotropic swerving is a shock movcufieiii must fall When Euglenm from (hdtiire B were placed in the rays of the are light, at a distance of four or iv# feet from the light, they were stitnigly positively heliotropic and gave lliii shading reaction. When, liowover, they were gradiially lirought iicmrer to light a point was reaclitui at which the hiiliotropism ciiiappeared hut tlie shading reaction pi*T- «sted. When moved still cdoser to the light they became ne^tividy heliotropie but still without any clianpi of the slifidiiig renetion. When moved still closer to the light, there was a iliort lime when no slicM^k movements could b# obtained, but mnt the illuifiiiiation reaction ap¬ peared. At the same tiniii the ncgalivc hcfliolmpbiti breaiiicf tiioro prompt and precise* Finally, when tlici light was Mill further increiiMed aii4 allowed to act for a eonsidemlile tiiiit, iivtti the illuirniiatlon reacticitin frequently dkappeared complitdy, and a iiioit proiioiiiieed and cotniiell- Ing nogative hdiotropkia held full sway. * . •

It is very evident, then, that the* ifivnriiljin cwiTciathm of positivii haliotropam with the ihading ritcliwi, wliich m impirtd by theory, does not Both of heiliitropiiiii may lie fijisficlal«il with cither the fthadiiig or the llltitiiiiiatkiti rifaiilioii. Aecordliigly, it must bo concluded that tiiii litliotropk mecliaiiiiiii clo« not dcpciid itpciri thi mechaakai for th© ihoek ttiovmiifite, but that tlio two riiochariwiii^ are ini^indent.^* the orientation of flagolliiteH ilofipink upon riipicl changes

made or broken, but not while the constant current lasts. Hence a rapidly alternating current throws the muscle into tetanus, while the constant current has no effect. If it is the rapid change in the intensity of light which causes the swimming of a positively heliotropic Euglena to the light, an intermittent light, of a sufficient number of alternations per second, should be much more efficient than a constant light; while in case the positive helio- tropism is determined by constant illumination, this should not be the case and the Bunsen-Eoscoe law should hold.

Mast has recently published experiments on the relative efficiency of the various parts of the spectrum by a method based on the assumption of the validity of the Bunsen-Eoscoe law for the heliotropic orientation of these organisms. If his assumption is correct, it con¬ tradicts the theory which Jennings and Mast have de¬ fended now for more than fifteen years; if his assumption is wrong, his experiments on the relative efficiency of various parts of the spectrum cannot be correct. Since, however. Mast’s results with this method coincide with those by Loeb and Wasteneys obtained by a direct method, it is very probable that the law of Bunsen and Eoscoe holds for the heliotropie reactions of Euglena and unicellular flagellates in general, and, if this is true, the heliotropie reactions of unicellular algae {Euglena in¬ cluded) are determined by light of constant intensity.

b He does not seem to have noticed that his method was based on this assumption. ]. The valhlity of the Jiunscii-KoHeoe law for the lielio- tropic reactioiiH of animaLs and plants hnivi's no doubt that these reactions arc dct(HTniiKHl by the rate; of plioto- chemieal iirocesses. Ileliotropic reactions d(>pend, how¬ ever, not only ujjon tlie intensity hut also npon the wave length of light. Photochemistry shows tliat tin* most efficient wave length varies witli the nature of the photo¬ chemical substance and that comparatively slight changes in the constitution of a molecule may bring about con¬ siderable changes in the relativi* f‘fliei<»ncy of difTerent wave lengths. The sean-h for ditTei’eneea in the helio¬ tropic effect of dilfercuit wave hmgths can be of serviee in detecting tb<^ nature! of the photochemical substances responsible for heliotropie reactions.

The investigations on the relative heliotrofjic efficiency of different wave lengths have generally been undertakfin for a different purpose, namely, to get information con¬ cerning the color sensations of animals. Oraber gave it as the result of his ol)8ervntion8 that all animals which were fond of light were also fond of blue, and animals which were fond of dark were also fond of redd'"* ITe put animals into a box half of whicli was covered with trans¬ parent glass and half with an opaque object, and then counted the relative numbers of organisms in both halves of the box. ITe then replace<I these screens by colorefi glasses and obtained the above-mentionotl result. The

writer showed that the animals are neither fond of bine nor of red but are oriented by the light in the same way as are plants, and the statement that aniTnalp which were “fond” of light also were “fond” of blue and those which were “fond” of “dark” were “fond” of red the writer explained in a simpler way, namely that the light filtered through red glass had a smaller orienting effect than the light filtered through blue glassHence red glass acted like an opaque, blue glass like a trans¬ parent screen. This had already been known to be true for the heliotropic reactions of plants for which Sachs had shown that they occur behind a blue glass in the same way as behind common window glass, while behind red glass heliotropic reactions do not occur at all or occur very slowly as if the fight were weak. The writer was able to show that the same is true for animals.^®’ When positively heliotropic animals are put into a box covered with blue glass they go as rapidly to the window side as when the box is uncovered; while when it is covered with red glass the animals will go to the window but more slowly and irregularly. Behind a red screen they behave therefore as if they were exposed to weak light.

Blue glass is permeable not only for blue but also for rays which produce the sensation of green. Paul Bert had already made experiments with positively heliotropic Daphnia in a solar spectrum and found that the an i mals “accouraient beaucoup plus rapidement au jaune ou au vert qu’ a toute autre couleur.”a. Bert concluded from this that to the eye of a Daphnia those parts of the spec¬ trum appear brightest which also appear brightest to the human eye. Bert’s aim was to find out whether the sensa-

a This method of ascertaining the most efficient part of the spectrum is not reliable and has be^ replaced by other methods. tions caused by light in lower ajaimals are the same as those caused in a human being. But even if the relative efficiency of the various parts of the spectrum were the same for the sensations of brightness in human beings and for the hehotropic reactions iu lower animals, it would not prove that the latter also have sensations of brightness. For we have no guarantee that the heliotropic reactions of lower animals are due to or accompanied by sensations of brightness. If the yellow-green rays are the most efficient in causing heliotropic reactions in an organism, it suggests only that in such an organism the photosensi¬ tive substance responsible for the heliotropic response is most easily decomposed by the yellow-green part of the spectrum.

A similar error of reasoning as that by Bert has recently been made by Hess. Hess corroborated what the writer had already pointed out, that the red rays of the visible solar spectrum are the least efficient, and he found, moreover, as Bert had found for Daphnia, that for the heliotropic reactions of a number of animals, from the fishes down, the yellow-green region of the solar spectrum is the most efficient. Now it happens that to a totally color blind human being, to whom the different parts of the solar spectrum appear only as shades of gray, the region X — 540 w in the yellow-green appears to be the brightest; while the red part of the spectrum gives a very faint sensation of brightness. From this similarity or apparent identity between the relative effects of dif¬ ferent wave lengths upon the heliotropic effects of certain Iqwer animals and upon the sensations of brightness of a totally color blind human being, Hess draws the con¬ clusion that these animals are totally color blind. In our opinion the only conclusion which Hess has a right to

draw is that the photosensitive substance which causes sensations of brightness in the eye of the color blind human being is either identical with or is affected in a similar way by light waves as is the substance giving rise to heliotropic reactions in certain animals. This assumption is entirely adequate and harmonizes better with the facts than the assumption made by Hess. The substance responsible for the sensations of brightness in the eyes of the totally color blind human being is visual purple which is bleached most rapidly by light of A. = 540 nfn.. That our objection is justified is proved by the experiments of v. Frisch on bees.

V. Frisch^®® has shown by very ingenious and care¬ ful experiments that bees can be trained to discriminate between blue and yellow but not between different shades of gray. On a table were put square cardboards of dif¬ ferent. shades of gray and among them one blue piece of cardboard. On each gray square was put a watch crystal containing water, while the watch crystal on the blue con¬ tained sugar water. The bees of an observation hive visiting the sugar crystal were marked with a fine paint brush.* After a sufficient period of training it was found that the marked bees always went directly to those crys¬ tals which were on a blue piece of cardboard, whether they contained sugar water or pure water; and when there was no sugar water on the blue cardboard they alighted on any blue object, e.g., a blue pencil. The crystals and cardboard pieces were always renewed in different tests, to avoid any influence of odor. It was never possible to train the bees to select a piece of cardboard with a definite shade of gray among cardboards of different shades of gray.

bees the yellowish-green part of the spectrum is most efficient, and he concluded that bees are totally color blind. To a totally color blind person the blue cardboard appears only like a shade of gray, and such a person is unable to learn to discriminate between a blue and gray piece of cardboard, v. Frisch’s experiments support the con¬ clusion that it is unjustifiable to use experiments on heliotropisan to draw conclusions concerning light or color sensations, v. Frisch and Kupelwieser have also demonstrated selective effects of different light waves for DapJinia which differ from those found for the eye of a totally color blind person, and their observations have been confirmed by Ewald.^^’^