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be proved very nicely in the case of starving caterpillars of Porthesia。 The writer put such caterpillars into a glass tube the axis of which was at right angles to the plane of the window： the caterpillars went to the window side of the tube and remained there； even if leaves of their food…plant were put into the tube directly behind them。 Under such conditions the animals actually died from starvation； the light preventing them from turning to the food； which they eagerly ate when the light allowed them to do so。 One cannot say that these animals； which we call positively helioptropic； are attracted by the light； since it can be shown that they go towards the source of the light even if in so doing they move from places of a higher to places of a lower degree of illumination。
The writer has advanced the following theory of these instinctive reactions。 Animals of the type of those mentioned are automatically orientated by the light in such a way that symmetrical elements of their retina （or skin） are struck by the rays of light at the same angle。 In this case the intensity of light is the same for both retinae or symmetrical parts of the skin。
This automatic orientation is determined by two factors； first a peculiar photo…sensitiveness of the retina （or skin）； and second a peculiar nervous connection between the retina and the muscular apparatus。 In symmetrically built heliotropic animals in which the symmetrical muscles participate equally in locomotion； the symmetrical muscles work with equal energy as long as the photo…chemical processes in both eyes are identical。 If； however； one eye is struck by stronger light than the other； the symmetrical muscles will work unequally and in positively heliotropic animals those muscles will work with greater energy which bring the plane of symmetry back into the direction of the rays of light and the head towards the source of light。 As soon as both eyes are struck by the rays of light at the same angle； there is no more reason for the animal to deviate from this direction and it will move in a straight line。 All this holds good on the supposition that the animals are exposed to only one source of light and are very sensitive to light。
Additional proof for the correctness of this theory was furnished through the experiments of G。H。 Parker and S。J。 Holmes。 The former worked on a butterfly； Vanessa antiope； the latter on other arthropods。 All the animals were in a marked degree positively heliotropic。 These authors found that if one cornea is blackened in such an animal； it moves continually in a circle when it is exposed to a source of light； and in these motions the eye which is not covered with paint is directed towards the centre of the circle。 The animal behaves； therefore； as if the darkened eye were in the shade。
（b） THE PRODUCTION OF POSITIVE HELIOTROPISM BY ACIDS AND OTHER MEANS AND THE PERIODIC DEPTH…MIGRATIONS OF PELAGIC ANIMALS。
When we observe a dense mass of copepods collected from a freshwater pond； we notice that some have a tendency to go to the light while others go in the opposite direction and many； if not the majority； are indifferent to light。 It is an easy matter to make the negatively heliotropic or the indifferent copepods almost instantly positively heliotropic by adding a small but definite amount of carbon…dioxide in the form of carbonated water to the water in which the animals are contained。 If the animals are contained in 50 cubic centimetres of water it suffices to add from three to six cubic centimetres of carbonated water to make all the copepods energetically positively heliotropic。 This heliotropism lasts about half an hour （probably until all the carbon…dioxide has again diffused into the air。） Similar results may be obtained with any other acid。
The same experiments may be made with another freshwater crustacean； namely Daphnia； with this difference； however； that it is as a rule necessary to lower the temperature of the water also。 If the water containing the Daphniae is cooled and at the same time carbon…dioxide added； the animals which were before indifferent to light now become most strikingly positively heliotropic。 Marine copepods can be made positively heliotropic by the lowering of the temperature alone； or by a sudden increase in the concentration of the sea…water。
These data have a bearing upon the depth…migrations of pelagic animals； as was pointed out years ago by Theo。 T。 Groom and the writer。 It is well known that many animals living near the surface of the ocean or freshwater lakes； have a tendency to migrate upwards towards evening and downwards in the morning and during the day。 These periodic motions are determined to a large extent； if not exclusively； by the heliotropism of these animals。 Since the consumption of carbon…dioxide by the green plants ceases towards evening； the tension of this gas in the water must rise and this must have the effect of inducing positive heliotropism or increasing its intensity。 At the same time the temperature of the water near the surface is lowered and this also increases the positive heliotropism in the organisms。
The faint light from the sky is sufficient to cause animals which are in a high degree positively heliotropic to move vertically upwards towards the light； as experiments with such pelagic animals； e。g。 copepods； have shown。 When； in the morning； the absorption of carbon…dioxide by the green algae begins again and the temperature of the water rises； the animals lose their positive heliotropism； and slowly sink down or become negatively heliotropic and migrate actively downwards。
These experiments have also a bearing upon the problem of the inheritance of instincts。 The character which is transmitted in this case is not the tendency to migrate periodically upwards and downwards； but the positive heliotropism。 The tendency to migrate is the outcome of the fact that periodically varying external conditions induce a periodic change in the sense and intensity of the heliotropism of these animals。 It is of course immaterial for the result； whether the carbon…dioxide or any other acid diffuse into the animal from the outside or whether they are produced inside in the tissue cells of the animals。 Davenport and Cannon found that Daphniae； which at the beginning of the experiment； react sluggishly to light react much more quickly after they have been made to go to the light a few times。 The writer is inclined to attribute this result to the effect of acids； e。g。 carbon…dioxide； produced in the animals themselves in consequence of their motion。 A similar effect of the acids was shown by A。D。 Waller in the case of the response of nerve to stimuli。
The writer observed many years ago that winged male and female ants are positively helioptropic and that their heliotropic sensitiveness increases and reaches its maximum towards the period of nuptial flight。 Since the workers show no heliotropism it looks as if an internal secretion from the sexual glands were the cause of their heliotropic sensitiveness。 V。 Kellogg has observed that bees also become intensely positively heliotropic at the period of their wedding flight； in fact so much so that by letting light fall into the observation hive from above； the bees are prevented from leaving the hive through the exit at the lower end。
We notice also the reverse phenomenon； namely； that chemical changes produced in the animal destroy its heliotropism。 The caterpillars of Porthesia chrysorrhoea are very strongly positively heliotropic when they are first aroused from their winter sleep。 This heliotropic sensitiveness lasts only as long as they are not fed。 If they are kept permanently without food they remain permanently positively heliotropic until they die from starvation。 It is to be inferred that as soon as these animals take up food； a substance or substances are formed in their bodies which diminish or annihilate their heliotropic sensitiveness。
The heliotropism of animals is identical with the heliotropism of plants。 The writer has shown that the experiments on the effect of acids on the heliotropism of copepods can be repeated with the same result in Volvox。 It is therefore erroneous to try to explain these heliotropic reactions of animals on the basis of peculiarities （e。g。 vision） which are not found in plants。
We may briefly discuss the question of the transmission through the sex cells of such instincts as are based upon heliotropism。 This problem reduces itself simply to that of the method whereby the gametes transmit heliotropism to the larvae or to the adult。 The writer has expressed the idea that all that is necessary for this transmission is the presence in the eyes （or in the skin） of the animal of a photo…sensitive substance。 For the transmission of this the gametes need not contain anything more than a catalyser or ferment for the synthesis of the photo…sensitive substance in the body of the animal。 What has been said in regard to animal heliotropism might； if space permitted； be extended； mutatis mutandis； to geotropism and stereotropism。
（c） THE TROPIC REACTIONS OF CERTAIN TISSUE…CELLS AND THE MORPHOGENETIC EFFECTS OF THESE REACTIONS。
Since plant…cells show hel