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mely； to roots； and these roots grow indefinitely in length and attach themselves to solid bodies； while if the stem had remained in its normal position no further growth would have occurred in the lateral branches。 From the upper side of the horizontal stem new stems grow out； mostly directly from the original stem； occasionally also from the short lateral branches。 It is thus possible to force upon this hydroid an arrangement of organs which is altogether different from the hereditary arrangement。 The writer had called the change in the hereditary arrangement of organs or the transformation of organs by external forces HETEROMORPHOSIS。 We cannot now go any further into this subject； which should； however； prove of interest in relation to the problem of heredity。
If it is correct to apply inferences drawn from the observation on the frog's egg to the behaviour of Antennularia； one might conclude that the cells of Antennularia also contain non…miscible substances of different specific gravity； and that wherever the specifically lighter substance comes in contact with the sea…water （or gets near the surface of the cell） the growth of a stem is favoured； while contact with the sea…water of the specifically heavier of the substances； will favour the formation of roots。
VI。 THE EXPERIMENTAL CONTROL OF ANIMAL INSTINCTS。
（a） EXPERIMENTS ON THE MECHANISM OF HELIOTROPIC REACTIONS IN ANIMALS。
Since the instinctive reactions of animals are as hereditary as their morphological character； a discussion of experiments on the physico… chemical character of the instinctive reactions of animals should not be entirely omitted from this sketch。 It is obvious that such experiments must begin with the simplest type of instincts； if they are expected to lead to any results； and it is also obvious that only such animals must be selected for this purpose； the reactions of which are not complicated by associative memory； or； as it may preferably be termed； associative hysteresis。
The simplest type of instincts is represented by the purposeful motions of animals to or from a source of energy； e。g。 light； and it is with some of these that we intend to deal here。 When we expose winged aphides （after they have flown away from the plant）； or young caterpillars of Porthesia chrysorrhoea （when they are aroused from their winter sleep） or marine or freshwater copepods and many other animals； to diffused daylight falling in from a window； we notice a tendency among these animals to move towards the source of light。 If the animals are naturally sensitive； or if they are rendered sensitive through the agencies which we shall mention later； and if the light is strong enough； they move towards the source of light in as straight a line as the imperfections and peculiarities of their locomotor apparatus will permit。 It is also obvious that we are here dealing with a forced reaction in which the animals have no more choice in the direction of their motion than have the iron filings in their arrangement in a magnetic field。 This can 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 positi