through the source of light, provided only one source of light be present. If, however, the light falls sideways on the animal the rate of photochemical reaction will be unequal in both eyes and the rate at which the symmetrical muscles on 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.'

(5) If this is true, it follows that the animal will obey the BunsenRoscoe Law. This is rather troublesome to prove for free-moving animals. The following table shows the applicability of the law to regenerating polyps of Eudendrium. The intensity of the light was altered by varying the distance between the source of light and the polyps.

The calculated values of t tend to be somewhat larger than the observed results. Schwarzschild observed that when development followed exposure to light the formula should be modified to

it constant.

For silver bromide gelatine plates, the value of the exponent p varies between 0-8 and 1 according to the brand of the plate.

Talbot's Law is the Bunsen-Roscoe Law modified to make it applicable to intermittent light. Intermittent light is as effective as constant light of the same intensity provided that the total duration of the intermittent light is equal to that of the constant light.

(6) What is going to be the result when the organism is subjected to two sources of light? One might predict that, if Loeb's hypothesis is correct, the organism will be oriented so that it comes to rest in a position where it is symmetrically stimulated. (a) If the two sources of light are of equal intensity and duration and

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are set at an equal distance from the organism it should be oriented with its plane of symmetry at right angles to the line joining the sources of light. (b) If the lights are of unequal intensity, the animal should move so that its photosensitive elements are in a position to absorb equal amounts of light energy. Further, the absolute intensities of light should have no effect on the deviation. of the path of the organism from the perpendicular path outlined at first. The relative intensities should be the governing factor. These three predictions have been amply proved experimentally. The following results (Table LXVI.) from Patten's investigations illustrate the nature of the findings.

This table shows clearly that (col. 1) when the intensity of the two lights was equal, the animal varied on the average only 0.09 degree from the perpendicular. It also demonstrates that when the one light was reduced to three-quarters (col. 2) the intensity of the other, the angle of deviation was about 8-86, and that when a further reduction to a half was made, the angle of deviation was more than doubled. Finally, the figures show that the angle of deviation depends on the relative differences of light intensity and is independent of absolute intensity (provided sufficient light is present to overcome inertia (cf. stimulation, p. 193).

(7) A model with a heliotropic mechanism has been constructed by Hays Hammond, the inventor of the dirigible torpedo. The principle on which the machine depends is the alteration in the electrical resistance of metallic selenium when exposed to light. The " eyes are lenses separated from each other by a projecting nose " which permits the shading of one eye while the other is illuminated. The lenses are cach focused on separate selenium

cells. The heliotropic machine consists of a rectangular box about 3 ft. long, 1 ft. wide and 1 ft. high mounted on three wheels, two of which are geared to a driving motor. The third wheel, mounted at the rear-end, controls the steering. It may be turned right or left by the differential action of two solenoid electromagnets. The selenium cells are in circuit both with the driving motors and with the steering magnets. In the former case, the selenium cells control a series of very sensitive relays (cf. nervous system) in such a way that the amount of energy sent through the driving motors and hence their speed is determined by the intensity of the light falling on the lenses. The steering magnets are opposed, i.e. if both selenium cells are illuminated equally, both magnets will receive the same current and the steering wheel will lie parallel to the driving wheels. If more light falls on one selenium" retina" than on the other, the former has its power to conduct electricity increased in proportion to the relative increase in intensity; consequently the magnets controlling the position of the rear wheel are activated asymmetrically. The wheel is pulled over to make an angle with the previous line of traction of the "dog." The mechanism is so arranged that this steering movement turns the machine towards the light. It will continue to turn till both lenses are equally illuminated. As soon and as long as both eyes are equally illuminated in sufficient intensity, the machine moves in a straight line towards the source of the light. The apparatus is fitted with a reversing switch which will convert it from a positively to a negatively heliotropic machine.

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If, say, a portable electric light be turned on in front of the machine it will immediately start to follow (or run from) the light at a speed which may attain to 100 yds. per min. On reducing the intensity of the light, the "dog" will slow down, and on switching off, it will stop. In this way, the machine follows a lantern in a dark room just like a positively heliotropic animal. By reversing the direction of the current one may make the machine negatively heliotropic.

II. Stereotropism is the term applied to the tendency of certain organisms to bring their bodies as much as possible on all sides in contact with solid bodies. "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." Man orients himself partly by appreciation of the tactile influences on the soles of the feet. When these are weakened as in locomotor ataxia, and when the orienting

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influence of the eyes is removed, the patient finds difficulty in standing and in walking (Romberg's sign).

III. Chemotropism plays an important part in the life of the lower organisms. By it, the animal is drawn towards or draws away from certain chemical substances. The organ stimulated asymmetrically is oriented so that the stimulating impacts on it are symmetrical.

IV. Orientation in space is determined mainly by three factors, light, tactile sense and gravitation. Normal equilibrium or normal geotropic orientation is defined as that position in which the plane of symmetry of the animal passes through the centre of the earth. Any deviation from that position causes unilateral stimulation and corrective movements are instituted. The tight-rope walker perceives that his centre of gravity is tending towards unstable equilibrium and voluntary (though generally sub-consciously) corrects his balance. In the labyrinths, we have a delicate mechanism for detecting alterations in our orientation in space.

Sufficient has been said to show the nature and indicate the mechanism of those actions termed tropisms. In principle they depend on unilateral stimulation of a symmetrical animal. How far they can be accepted as explanations of all the instinctive actions of the lower organisms or of any of the actions of the higher animals remains an open and debatable question.

CHAPTER XXXIII

ADAPTATION

"The free use of final causes to explain what seems obscure was temptingly easy ... Hence the finalist was often the man who made a liberal use of the ignava ratio, or lazy argument: when you failed to explain a thing by the ordinary process of causality, you could explain' it by reference to some purpose of nature or of its Creator."

PRINCIPAL GALLOWAY quoted by D'ARCY THOMPSON.

IF the environment exerts such an all-powerful effect on the organism, can the organism alter itself according to the principle of Le Chatelier so that it may live with the least possible expenditure of energy? That is, has the animal the power of adaptation? There is no doubt whatever as to the adaptation of growing bone or growing tissue of any sort to the stresses and strains incident upon it. Various organs are known to adapt themselves to meet alterations in the conditions under which they work.

When one comes to consider the organism as a whole, the evidence for adaptation is not so conclusive. The arctic fox and the polar bear are not white because they have adapted themselves to a white background, but because their coloured relatives, not having their invisibility, have paid the penalty. It has been said that trypanosomes may be obtained which are almost unaffected by treatment with arsenic. The process for producing them is to give their host a high but non-lethal dose of arsenic, infect another host with the survivors and so on. This is clearly a case of the survival and propagation of the most resistant strains.

Animals which live in dark or semi-dark places have generally defective eyesight. Is this due to atrophy from want of use or might one not argue that the environment of the cave was the fittest for the blind or semi-blind animal? Not only would they be at a manifest disadvantage in the struggle for existence outside, but they have a distinct advantage in the cave over any seeing animal that may stray in.