CHAPTER X
ANIMAL INSTINCTS AND TROPISMS[217]
1. The idea that the organism as a whole cannot be explained from a physicochemical viewpoint rests most strongly on the existence of animal instincts and will. Many of the instinctive actions are “purposeful,” i. e., assisting to preserve the individual and the race. This again suggests “design” and a designing “force,” which we do not find in the realm of physics. We must remember, however, that there was a time when the same “purposefulness” was believed to exist in the cosmos where everything seemed to turn literally and metaphorically around the earth, the abode of man. In the latter case, the anthropo- or geocentric view came to an end when it was shown that the motions of the planets were regulated by Newton’s law and that there was no room left for the activities of a guiding power. Likewise, in the realm of instincts when it can be shown that these instincts may be reduced to elementary physicochemical laws the assumption of design becomes superfluous.
If we look at the animal instincts purely as observers we might well get the impression that they cannot be explained in mechanistic terms. We need only consider what mysticism apparently surrounds all those instincts by which the two sexes are brought together and by which the entrance of the spermatozoön into the egg is secured; or the remarkable instincts which result in providing food and shelter for the young generation.
We have already had occasion to record some cases of instincts which suggest the possibility of physicochemical explanation; for example the curious experiment of Steinach on the reversal of the sexual instincts of the male whose testes had been exchanged for ovaries. There is little doubt that in this case the sexual activities of each sex are determined by specific substances formed in the interstitial tissue of the ovary and testes. The chemical isolation of the active substances and an investigation of their action upon the various parts of the body would seem to promise further progress along this line.
Marchal’s observations on the laying of eggs by the naturally sterile worker wasps are a similar case. The fact that such workers lay eggs when the queen is removed or when they are taken away from the larvæ may be considered as a manifestation of one of those wonderful instincts which form the delight of readers of Maeterlinck’s romances from insect life. Imagine the social foresight of the sterile workers who when the occasion demands it “raise” eggs to preserve the stock from extinction! And yet what really happens is that these workers, when there are no larvæ, can consume the food which would otherwise have been devoured by the larvæ; and some substance contained in this food induces the development of eggs in the otherwise dormant ovaries. What appeared at first sight as a mysterious social instinct is revealed as an effect comparable to that of thyroid substance upon the growth of the legs of tadpoles in Gudernatsch’s experiment (Chapter VII).
2. If we wish to show in an unmistakable way the mechanistic character of instincts we must be able to reduce them to laws which are also valid in physics. That instinct, or rather that group of instincts, for which this has been accomplished are the reactions of organisms to light. The reader is familiar with the tendency of many insects to fly into the flame. It can be shown that many species of animals, from the lowest forms up to the fishes, are at certain stages—very often the larval stage—of their existence, slaves of the light. When such animals, e. g., the larvæ of the barnacle or certain winged plant lice or the caterpillars of certain butterflies, are put into a trough or test-tube illuminated from one side only, they will rush to the side from which the light comes and will continue to do this whenever the orientation of the trough or test-tube to the light is changed; while they will be held at the window side of the vessel if the light or the position of the vessel remains unchanged. This instinct to get to the source of light is so strong that, e. g., the caterpillars of Porthesia chrysorrhœa die of starvation on the window side of the vessel, with plenty of food close behind. This powerful “instinct” is, as we intend to show, in the last analysis, the expression of the Bunsen-Roscoe law of photochemical reactions. A large number of chemical reactions are induced or accelerated by light, and the Bunsen-Roscoe law shows that the chemical effect is in these cases, within certain limits, equal to the product of the intensity into the duration of illumination.
The “attraction” or “repulsion” of animals by the light had been explained by the biologists in an anthropomorphic way by ascribing to the animals a “fondness” for light or for darkness. Thus Graber, who had made the most extensive experiments, gave as a result the statement that animals which are fond of light are also fond of blue while they hate the red, and those which are fond of the “dark” are fond of red and hate the blue.[218] In 1888 the writer published a paper in which he pointed out that the so-called fondness of animals for light and blue and for dark and red was simply a case of an automatic orientation of animals by the light comparable to the turning of the tips of a plant towards the window of the room in which the plant is raised.[219]
The phenomenon of a plant bending or growing to the source of light is called positive heliotropism (while we speak of negative heliotropism in all cases in which the plant turns away from the light, as is observed in many roots). The writer pointed out that animals which go to the light are positively heliotropic (or phototropic) and do so because they are compelled automatically by the light to move in this direction, while he called those animals which move away from the light negatively heliotropic; they are automatically compelled by the light to move away from it. What the light does is to direct the motions of the animals and to explain this the following theory was proposed. Animals possess photosensitive elements on the surface of their bodies, in the eyes, or occasionally also in epithelial cells of their skin. These photosensitive elements are arranged symmetrically in the body and through nerves are connected with symmetrical groups of muscles. The light causes chemical changes in the eyes (or the photosensitive elements of the skin). The mass of photochemical reaction products formed in the retina (or its homologues) influences the central nervous system and through this the tension or energy production of the muscles. If the rate of photochemical reaction is equal in both eyes this effect on the symmetrical muscles is equal, and the muscles of both sides of the body work with equal energy; as a consequence the animal will not be deviated from the direction in which it was moving. This happens when the axis or plane of symmetry of the animal goes through the source of light, provided only one source of light be present. If, however, the light falls sidewise upon the animal, the rate of photochemical reaction will be unequal in both eyes and the rate at which the symmetrical muscles of both sides of the body work will no longer be equal; as a consequence the direction in which the animal moves will change. This change will take place in one of two ways, according as the animal is either positively or negatively heliotropic; in the positively heliotropic animal the resulting motion will be toward, in the negatively heliotropic from, the light. Where we have no central nervous system, as in plants or lower animals, the tension of the contractile or turgid organs is influenced in a different way, which we need not discuss here.
The reader will perceive that according to the writer’s theory two agencies are to be considered in these reactions: first, the symmetrical arrangement of the photosensitive and the contractile organs, and second, the relative masses of the photochemical reaction products produced in both retinæ or photosensitive organs at the same time. If a positively heliotropic animal is struck by light from one side, the effect on tension or energy production of muscles connected with this eye will be such that an automatic turning of the head and the whole animal towards the source of light takes place; as soon as both eyes are illuminated equally the photochemical reaction velocity will be the same in both eyes, the symmetrical muscles of the body will work equally, and the animal will continue to move in this direction. In the case of the negatively heliotropic animal the picture is the same except that if only one eye is illuminated the muscles connected with this eye will work less energetically. The theory can be nicely tested for negatively heliotropic animals in the larvæ of the blowfly when they are fully grown, and for positively heliotropic animals on the larvæ of Balanus, and many other organisms.
| Fig. 43 |
| Fig. 44 |
One of the difficulties in identifying the motions of animals to or from the light with the positive and negative heliotropism of plants consisted in the fact that plants are mostly sessile (and respond to a one-sided illumination with heliotropic curvatures to or from the light), while most animals are free moving and respond to the one-sided illumination by being turned and compelled to move to or from the light. This difficulty was overcome by the observation that sessile animals like the hydroid Eudendrium (Fig. 43) or the tube worm Spirographis (Fig. 44) react to a one-sided illumination also with heliotropic curvatures like sessile plants.[220] On the other hand, it had been found before by Strassburger that free-swimming plant organisms like the swarmspores of algæ move to or from the source of light as do free-swimming animals.
3. The writer suggested in 1897[221] that the light acts chemically in the heliotropic reactions and in 1912 that the heliotropic reactions probably follow the law of Bunsen and Roscoe,[222] and it was possible to confirm this idea by direct experiments.[223] This law states that the photochemical effect of light equals i t where i is the intensity of the light and t the duration of illumination. The experiments were carried out on young regenerating polyps of Eudendrium by measuring the time required to cause fifty per cent. of the polyps to bend to the source of light. The intensity of light was varied by altering the distance of the source of light from the polyps. Table VI gives the result.
TABLE VI
| Distance between Polyps and Source of Light | Time Required to Cause Fifty Per Cent. of the Polyps to Bend towards the Source of Light | |
|---|---|---|
| Observed | Calculated from Bunsen-Roscoe Law | |
| Metres | Minutes | Minutes |
| 0.25 | 110 | |
| 0.50 | between 35 and 40 | 140 |
| 1.00 | 150 | 160 |
| 1.50 | between 360 and 420 | 360 |
We must therefore conclude that the heliotropic curvature of the polyps is determined by a photochemical action of the light. The light brings about or accelerates a chemical reaction which follows the Bunsen-Roscoe law. As soon as the product of this reaction on one side of the polyp exceeds that on the other by a certain quantity, the bending occurs. When the product i t is the same for symmetrical spots of the organism no bending can result. This is what our theory suggested.
It is very difficult to prove directly the applicability of the Bunsen-Roscoe law for free-moving animals, but it can be shown that intermittent light is as effective as constant light of the same intensity, provided that the total duration of the illumination by the intermittent light is equal to that of the constant light, and the duration of the intermission is sufficiently small (Talbot’s law). Talbot’s law is in reality only a modification of the Bunsen-Roscoe law. Ewald has proved in a very elegant way the applicability of Talbot’s law to the orientation of the eyestalk of Daphnia.[224] This makes it probable that the law of Bunsen-Roscoe underlies generally the heliotropic reaction of animals.
It is of importance for the theory of the identity of the heliotropism of animals and plants that in the latter organisms the law of Bunsen and Roscoe is also applicable. This had been shown previously by Fröschel[225] and by Blaauw.[226] In the following table are given the results of Blaauw’s experiments on the applicability of the Bunsen-Roscoe law for the heliotropic curvature of the seedlings of oats (Avena sativa). The time required to cause heliotropic curvatures for intensities of light varying from 0.00017 to 26520 metre-candles was measured. The product i t, namely metre-candles-seconds, varies very little (between 16 and 26).
TABLE VII
| I Duration of Illumination | II Metre-Candles | III Metre-Candles-Seconds | I Duration of Illumination | II Metre-Candles | III Metre-Candles-Seconds | |||
|---|---|---|---|---|---|---|---|---|
| 43 | hours | 0.00017 | 26.3 | 25 | seconds | 1 | .0998 | 27.5 |
| 13 | " | 0.000439 | 20.6 | 8 | " | 3 | .02813 | 24.2 |
| 10 | " | 0.000609 | 21.9 | 4 | " | 5 | .456 | 21.8 |
| 6 | " | 0.000855 | 18.6 | 2 | " | 8 | .453 | 16.9 |
| 3 | " | 0.001769 | 19.1 | 1 | " | 18 | .94 | 18.9 |
| 100 | minutes | 0.002706 | 16.2 | 2⁄5 | " | 45 | .05 | 18.0 |
| 60 | " | 0.004773 | 17.2 | 2⁄25 | " | 308 | .7 | 24.7 |
| 30 | " | 0.01018 | 18.3 | 1⁄25 | " | 511 | .4 | 20.5 |
| 20 | " | 0.01640 | 19.7 | 1⁄55 | " | 1255 | 22.8 | |
| 15 | " | 0.0249 | 22.4 | 1⁄100 | " | 1902 | 19.0 | |
| 8 | " | 0.0498 | 23.9 | 1⁄400 | " | 7905 | 19.8 | |
| 4 | " | 0.0898 | 21.6 | 1⁄800 | " | 13094 | 16.4 | |
| 40 | seconds | 0.6156 | 24.8 | 1⁄1000 | " | 26520 | 26.5 | |
It is, therefore, obvious that the blind instinct which forces animals to go to the light, e. g., in the case of the moth, is identical with the instinct which makes a plant bend to the light and is a special case of the same law of Bunsen and Roscoe which also explains the photochemical effects in inanimate nature; or in other words, the will or tendency of an animal to move towards the light can be expressed in terms of the Bunsen-Roscoe law of photochemical reactions.
The writer had shown in his early publications on light effects that aside from the heliotropic reaction of animals, which as we now know depends upon the product of the intensity and duration of illumination, there is a second reaction which depends upon the sudden changes in the intensity of illumination. These latter therefore obey a law of the form: Effect = f (di/dt).[227] Jennings has maintained that the heliotropic reactions of unicellular organisms are all of this kind, but investigations by Torrey and by Bancroft[228] on Euglena have shown that Jennings’s statements were based on incomplete observations.
4. In these experiments only one source of light was applied. “When two sources of light of equal intensity and distance act simultaneously upon a heliotropic animal, the latter puts its median plane at right angles to the line connecting the two sources of light.”[229] This fact has been amply verified by Bohn, by Parker and his pupils, and especially by Bradley Patten, who used it to compare the relative efficiency of two different lights.
The behaviour of the animals under the influence of two lights is a confirmation of our theory of heliotropism inasmuch as the animal moves in such a direction that the symmetrical elements of the surface of the body are struck by light of the same intensity at the same angle, so that as a consequence equal masses of photosensitive substances are produced in symmetrical elements of their eyes or skin in equal times. The effect on the symmetrical muscles will be identical. As soon as one of the lights is a little stronger the animal will deviate towards this light, in case it is positively heliotropic and towards the weaker light if it is negatively heliotropic. This deviation again is not the product of chance but follows a definite law as Patten[230] has recently shown. He used the negatively heliotropic larvæ of the blowfly. These larvæ were made to record their trail while moving under the influence of the two lights. The results of the measurements of 2500 trails showing the progressive increase in angular deviation of the larvæ (from the perpendicular upon the line connecting the two lights), with increasing differences between the lights, are given in the following table. Since the deviation or angular deflection of the larvæ is towards the weaker of the two lights it is marked negative.
TABLE VIII
| Percentage Difference in the Intensity of the Two Lights | Average Angular Deflection of the Two Paths of the Larvæ towards the Weaker Light | |
|---|---|---|
| Per Cent. | Degrees | |
| 0 | 0-0.09 | |
| 8 | 1⁄3 | 0-2.77 |
| 16 | 2⁄3 | 0-5.75 |
| 25 | 0-8.86 | |
| 33 | 1⁄3 | -11.92 |
| 50 | -20.28 | |
| 66 | 2⁄3 | -30.90 |
| 83 | 1⁄3 | -46.81 |
| 100 | -77.56 | |
Let us assume that the negatively heliotropic animal is at an equal distance from the two unequal lights and placed so that at the beginning of the experiment its median plane is at right angles to the line connecting the two lights, but with its head turned away from them. In that case the velocity of reaction in the symmetrical photosensitive elements of the eyeless larvæ is greater on the side of the stronger light. Since the animal is negatively heliotropic this will result in a greater relaxation or a diminution of the energy production of the muscles turning the head of the animal towards the side of the stronger light. Hence the animal will automatically deviate from the straight line towards the side of the weaker light. By the alteration of the position of its body the photosensitive elements exposed to the stronger of the two lights will be put at a less efficient angle and hence the rate of photochemical reaction on this side will be diminished. The deviation from the perpendicular in which the animal will ultimately move will be such that as a consequence, the rate of photochemical reaction in symmetrical elements is again equal. The ultimate direction of motion will, according to our theory always be such that the mass of chemical products formed under the influence of light in symmetrical photosensitive elements during the same time is equal.
Patten also investigated the question whether the same difference of percentage between two lights would give the same deviation, regardless of the absolute intensities of the lights used. The absolute intensity was varied by using in turn from one to five glowers. The relative intensity between the two lights varied in succession by 0, 81⁄3, 162⁄3, 25, 331⁄3, and 50 per cent. Yet the angular deflections were within the limits of error identical for each relative difference of intensity of the two lights no matter whether, 1, 2, 3, 4, or 5 glowers were used. The following table shows the result.
TABLE IX
A Table Based on the Measurements of 2700 Trails Showing the Angular Deflections at Five Different Absolute Intensities
| Number of Glowers | Difference of Intensity between the Two Lights | |||||
|---|---|---|---|---|---|---|
| 0 per cent. | 81⁄3 per cent. | 162⁄3 per cent. | 25 per cent. | 331⁄3 per cent. | 50 per cent. | |
| Deflection in Degrees | ||||||
| 1 | -0.550 | -2.32 | -5.270 | -9.04 | -11.86 | -19.46 |
| 2 | -0.100 | -3.05 | -6.120 | -8.55 | -11.92 | -22.28 |
| 3 | +0.450 | -2.60 | -5.650 | -8.73 | -13.15 | -20.52 |
| 4 | -0.025 | -2.98 | -6.600 | -9.66 | -11.76 | -19.88 |
| 5 | -0.225 | -2.92 | -5.125 | -8.30 | -10.92 | -19.28 |
| Average | -0.090 | -2.77 | -5.750 | -8.86 | -11.92 | -20.28 |
Such constancy of quantitative results is only possible where we are dealing with purely physicochemical phenomena or where life phenomena are unequivocally determined by purely physicochemical conditions.
5. It seems difficult for some biologists, even with the validity of the Bunsen-Roscoe law proven, to imagine that the movements of the animals under the influence of light are not voluntary (or not dictated by the mysterious “trial and error” method of Jennings).[231] But one wonders how it is possible on such an assumption to account for the fact that the angle of deflection of the larva of the fly when under the influence of two lights of different intensities should be always the same for a given difference in intensity; or why the time for curvature in Eudendrium should vary inversely with the intensity of illumination. It is, however, possible to complete the case for the purely physicochemical analysis of these instincts. John Hays Hammond, Jr., has succeeded in constructing heliotropic machines which in the dark follow a lantern very much in the manner of a positively heliotropic animal. The eyes of this heliotropic machine consist of two lenses in whose focus is situated the “retina” consisting of selenium wire. The two eyes are separated from each other by a projecting piece of wood which represents the nose and allows one eye to receive light while the other is shaded. The galvanic resistance of selenium is altered by light; and when one selenium wire is shaded while the other is illuminated, the electric energy (supplied by batteries inside the machine) which makes the wheels turn (these take the place of the legs of the normal animal) no longer flows symmetrically to the steering wheel, and the machine turns towards the light. In this way the machine follows a lantern in a dark room in a way similar to that of a positively heliotropic animal. Here we have a model of the heliotropic animal whose purely mechanistic character is beyond suspicion, and we may be sure that it is not “fondness” for light or for brightness nor will-power nor a method of “trial and error” which makes the machine follow the light.
Fig. 45
6. It may also be of interest to know that in heliotropism the motions of the legs are automatically controlled by the chemical changes taking place in symmetrical elements of the retina. In order to prove this point we will turn to the phenomenon of galvanotropism. The galvanic current forces certain animals to move in the direction of one of the two electrodes just as the light forces the heliotropic animals to move towards (or from) the source of light. The change in the concentration of the ions at the boundary of the various organs, especially the nerves, determines the galvanotropic reactions. When the shrimp Palæmonetes is put into a trough with dilute salt solution through which a current of a certain intensity flows, the animal is compelled to move towards the anode.[232] It can walk forwards, backwards, or sidewise. Here we can observe directly that the effect of the current consists in altering the tension of the muscles of the legs in such a way as to make it easy for the animal to move toward the anode and difficult to move toward the cathode. Thus if the current be sent sidewise through the animal, say from left to right (Fig. 45), the legs of the left side assume the flexor position, those of the right the extensor position. With this position of its legs the animal can easily move to the left, i. e., the anode, and only with difficulty to the right, i. e., the cathode. This change in the position of the legs occurs when the animal is not moving at all, thus showing that the galvanotropic movements take place not because the animal intends to go to the anode, but that the animal goes to the anode because its legs are practically prevented by the galvanic current from working in any other way. This is exactly what happens in the heliotropic motions of animals.[233]
Fig. 46
To understand what happens when the current goes lengthwise through the body it should be stated that Palæmonetes uses the third, fourth, and fifth pairs of legs for its locomotion. The third pair pulls in the forward movement, and the fifth pair pushes. The fourth pair generally acts like the fifth, and requires no further attention. If a current be sent through the animal longitudinally, from tail to head, and the strength be increased gradually, a change soon takes place in the position of the legs (Fig. 46). In the third pair the tension of the flexors predominates, in the fifth the tension of the extensors. The animal can thus move easily with the pulling of the third and the pushing of the fifth pairs of legs, that is to say, the current changes the tension of the muscles in such a way that the forward motion is rendered easy, the backward motion is difficult. Hence it can easily move toward the anode, but only with difficulty toward the cathode. If a current be sent through the animal in the opposite direction, namely, from head to tail, the third pair of legs is extended, the fifth pair bent; that is, the third pair can push, and the fifth pair pull. The animal will thus move backward easily and forward with difficulty, and it is thus driven to the anode again.
The explanation which Loeb and Maxwell proposed for this influence of the current on the legs assumes that there are three groups of ganglion cells in the central nervous system of these animals which are oriented according to the three main axes of the body; (1) right-left and left-right, (2) backward, and (3) forward. It depends upon whether the ganglion cells or the nerve elements are in anelectrotonus, which muscles are bent and which relaxed. It would lead us too far to recapitulate the theory in this place, and the reader who is interested in it is referred to Loeb and Maxwell’s paper.[234] The importance of the observations lies in the fact that they show that any element of will or choice on the part of the animal in these motions is eliminated, that the animal moves where its legs carry it, and not that the legs carry the animal where the latter “wishes” to go.
7. This may be the place to dispel an error which has sometimes crept into the discussion of the tropistic reactions of animals. It has been stated occasionally that it is the energy gradient and not the automatic orientation of the animal by the light which makes the positively heliotropic animal move towards the source of light and the negatively heliotropic away from it. Thus the positively heliotropic animal would be compelled to move towards the source of light as a consequence of the fact that the intensity of the light increases the more the nearer the animal approaches the source of light. If the source of light be the reflected sky-light the difference of intensity at both ends of a microscopic organism is so slight that it is beneath the limit capable of influencing the motions.
Fig. 47
A simple experiment published by the writer in 1889 suffices to dispel the idea that the energy gradient determines the direction of the motion of an animal in tropistic reactions. Let direct sunlight (S, Fig. 47) fall through the upper half of a window (w w) upon a table, and diffused daylight (D) through the lower half of the window on the same table. A test-tube a c is placed on the table in such a way that its long axis is at right angles to the plane of the window; and one half a b is in the direct sunlight, the other half in the shade. If at the beginning of the experiment the positively heliotropic animals are in the direct sunlight at a, they promptly move toward the window, gathering at the window end c of the tube, although by so doing they go from the sunshine into the shade.[235] This experiment is in harmony with our idea that the effect of light consists in turning the head of the animal and subsequently its whole body toward the source of light. By going from the strong light into the shade the reaction velocity in both eyes is diminished equally and hence there is no reason for the animal to change its orientation, though its progressive motion may be stopped for an instant by the change. But at the boundary between sunlight and daylight a sudden change from strong to weak light occurs. If the energy gradient determined the direction of the positively heliotropic animal, the motion should stop at the boundary from strong to weak light, which may happen for an instant but which will not interfere with the progressive motion of the animal.
8. Graber had found that when animals are put into a trough covered half with blue and half with red glass, those that are “fond” of light go under the blue, those that are “fond” of darkness go under the red glass. The writer pointed out that this result should be expected on the basis of his theory of heliotropism, if the assumption be correct that the red light is considerably less efficient than light which goes through blue glass (such glass also allows green rays to go through). The botanists had already shown that red glass is impermeable for the rays which cause heliotropic reactions of plants, and the writer was able to show the same for the heliotropic reactions of animals. Red glass acts, therefore, almost like an opaque body for these animals.
A closer examination of the most efficient rays for the heliotropic reactions of different organisms has revealed the fact that for some organisms a region in the blue λ = 460–490 µµ, for others a region in the yellowish-green, near about λ = 520–530 µµ is the most efficient.[236] For many plants and for some animals, like Eudendrium and the larvæ of the worm Arenicola, a region in the blue is most efficient; for certain, if not most, animals a region in the yellow-green is most efficient. Among unicellular green algæ, Chlamydomonas, has its maximal efficiency in the yellowish-green and Euglena in the blue. According to observations by Mast, some green unicellular organisms like Pandorina, Eudorina, and Spondylomorum seem to behave more like Chlamydomonas, while certain others behave more like Euglena.[237] Wasteneys and the writer suggested that there are two groups of heliotropic substances, one with a maximum of photosensitiveness in the blue, the other in the yellowish-green; and that the latter group may or may not be related or identical with the visual purple which is most rapidly bleached by light of a wave length near λ = 520–530 µµ.
The ophthalmologist Hess[238] has utilized the heliotropic reactions of animals in an attempt to prove that all animals from the lowest invertebrates up to the fishes inclusive suffer from total colour-blindness. This statement was based on the observation that for most positively heliotropic animals the region in the yellowish-green near λ = 520 µµ seems the most efficient. Since this region of the spectrum appears also as the brightest to a totally colour-blind man, he concluded that all these animals are totally colour-blind. There is no reason why heliotropic reactions should be used as an indicator for colour sensations; if totally colour-blind human beings were possessed of an irresistible impulse to run into a flame Hess’s assumption might be considered, but no such phenomenon exists in colour-blind man. Moreover, v. Frisch[239] has shown by experiments on the influence of the background on the colouration of fish as well as by experiments on bees and on Daphnia that the reactions of these animals to light of different wave-lengths indicate different effects besides those of mere intensity. Thus v. Frisch could train bees to go to a blue piece of cardboard distributed among many cardboards of different shades of grey. Bees thus trained would alight on any blue object even if it contained no food. It would be impossible to do this with totally colour-blind organisms.
9. Heliotropic reactions play a great rôle in the preservation of individuals as well as of species. In order to understand this rôle it must be stated that the photosensitive substances appear often only under certain conditions and that their effect is inhibited under other conditions. Thus among ants the winged males and females alone show positive heliotropism,[240] while the wingless workers are free from this reaction. This positive heliotropism becomes violent at the time of the nuptial flight and this phenomenon itself seems to be a heliotropic phenomenon since it takes place in the direction of the light. When the queen founds her nest she loses her wings and becomes negatively heliotropic again. Kellogg[241] has shown that the nuptial flight of the bees is also a purely heliotropic phenomenon. When a part of the hive remote from the entrance is illuminated the bees rush to the light and can thus be prevented from swarming. These phenomena suggest that the presence of some substance secreted by the sex glands may cause the intensification of the heliotropism which leads to the nuptial flight.
In certain species of Daphnia, fresh-water copepods, and of Volvox, a trace of CO2 suffices to make negatively heliotropic or indifferent specimens violently positively heliotropic.[242] Certain forms of marine copepods and the larvæ of Polygordius can be made positively heliotropic by lowering the temperature[243] and the larvæ of the barnacle can be made negatively heliotropic by strong light.[244] It is quite possible that a change in the sense of heliotropism by temperature and light is to some extent at least responsible for the periodic depth migrations of heliotropic animals. Many if not all positively heliotropic animals can be made negatively heliotropic by exposure to ultraviolet light.[245]
A most interesting example of the rôle of heliotropism in the preservation of a species is shown in the caterpillars of Porthesia chrysorrhœa. The butterfly lays its eggs upon a shrub. The larvæ hatch late in the fall and hibernate in a nest on the shrub, as a rule not far from the ground. As soon as the temperature reaches a certain height, they leave the nest; under natural conditions, this happens in the spring when the first leaves have begun to form on the shrub. (The larvæ can, however, be induced to leave the nest at any time in the winter provided the temperature is raised sufficiently.) After leaving the nest, they crawl directly upward on the shrub where they find the leaves on which they feed. Should the caterpillars move down the shrub, they would starve, but this they never do, always crawling upward to where they find their food. What gives the caterpillar this never-failing certainty which saves its life, and for which a human being might envy the little larva? Is it a dim recollection of experiences of former generations? It can be shown that it is the light reflected from the sky which guides the animal upward. When we put these animals into a horizontal test-tube in a room, they all crawl toward the window, or toward a lamp; the animal is positively heliotropic. It is this positive heliotropism which makes them move upward where they find their food, when the mild air of the spring calls them forth from their nest. At the top of the branch, they come in contact with a leaf, and chemical or tactile influences set the mandibles of the young caterpillar into activity. If we put these larvæ into closed test-tubes which lie with their longitudinal axes at right angles to the window, they will all migrate to the window end, where they stay and starve, even if their favourite leaves are close behind them. They are slaves of the light.
The few young leaves on top of a twig are quickly eaten by the caterpillar. The light, which saved its life by making it creep upward where it finds food, would cause it to starve could it not free itself from the bondage of positive heliotropism. The animal, after having eaten, is no longer a slave of the light, but can and does creep downward. It can be shown that a caterpillar, after having been fed, loses its positive heliotropism almost completely and permanently. If we submit unfed and fed caterpillars of the same nest contained in two different test-tubes to the same artificial or natural source of light, the unfed will creep to the light and stay there until they die, while those that have eaten will pay little or no attention to the light. Their sensitiveness to light has disappeared; after having eaten they become independent of light and can creep in any direction. The restlessness which accompanies the condition of hunger makes the animal creep downward—which is the only direction open to it—where it finds new young leaves on which it can feed. The wonderful hereditary instinct, upon which the life of the animal depends, is its positive heliotropism in the unfed condition and its loss of this heliotropism after having eaten. The latter phenomenon is in harmony with the experiments which show that the heliotropism of certain species of Daphnia disappears when the water becomes neutral.
And finally it may be pointed out that the majority of green plants could not exist if their stems were not positively, their roots negatively, heliotropic. It is the positive heliotropism which makes the top grow toward the light, which enables the leaves to get the light necessary for assimilation, and the roots to grow into the soil where they find the water and nutritive salts.
10. While we do not wish to deal here with the different tropisms it should be stated that aside from heliotropism, chemotropism as well as stereotropism play the most essential rôle in the so-called instinctive actions of animals. It is a problem of orientation by the diffusion of molecules from a centre when a male butterfly is deviated from its flight and alights on the wooden box in which is enclosed a female of the same species. We have already alluded to certain phenomena of chemotropism in Chapter IV. Certain organisms have a tendency to bring their bodies as much as possible on all sides in contact with solid bodies; thus the butterfly Amphipyra, which is a fast runner, will come to rest under a glass plate when the plate is put high enough above the ground so that it touches the back of the butterfly. The animals which live under stones or underground or in caves are as a rule both negatively heliotropic and positively stereotropic. Their tropisms predestine or force them into the life they lead.
The sensitive area which forms the basis of tropisms is as a rule developed not in the whole organism but only in certain segments of the body. Thus the eyes are located in the head. But when the action of one segment becomes overpowering the whole organism follows the segment. It has been customary among physiologists to speak of reflexes in such cases. Thus, e. g., the arms of the male frog develop a powerful positive stereotropism on their ventral surface during the spawning season. It would avoid confusion to realize that there is nothing gained in applying to this tropism the meaningless term “reflex”; it is better to call them tropisms since the organism as a whole is involved. If necessary we might speak of segmental tropisms. The act of seeking the female as well as that of cohabitation are in many cases combinations of chemotropism and stereotropism. The development of these tropisms depends upon the presence of certain specific substances in the body, a fact emphasized already in the case of heliotropism. In case of the development of the segmental stereotropism of the male frog at the time of spawning it has been shown that it depends on an internal secretion from the testes.
It has been suggested by some authors that the tropistic reactions are determined by some feeling or emotion on the part of the organism. We have no means of judging the emotions of lower animals (except by “intuition”). The writer suggested in 1899 in his book on brain physiology that emotions may be determined by specific substances which also determine the tropistic reaction (as well as phenomena of organ formation, although this latter phenomenon has nothing to do with the subject of instincts); and the excellent work of Cannon[246] has shown the rôle of adrenalin in the expression of fear. It is, therefore, both unwarranted and unnecessary to state that hypothetical emotions determine the tropistic reactions.