THE ANALYSIS OF BEHAVIOUR IN ANIMALS 159 



contributions concern the behaviour of animals under the 

 influence of light. That in many animals response to light 

 predominates over other modes of response is proverbial. 

 The behaviour of moths, and many other nocturnal insects is 

 very widely known. When the organism moves towards the 

 light it is said to be positively phototropic. The opposite 

 type of reaction, negative phototropism, is well shown by 

 blowfly maggots. Loeb was first attracted by the bending 

 of sedentary organisms like Tubularia or tubicolous worms 

 towards the source of illumination, a phenomenon which 

 superficially resembles the effect of light on the growth of 

 plants. He observed that in doing so the animal tends to 

 take up such a position that its photo-sensitive surfaces are 

 symmetrically illuminated ; and advanced the hypothesis 

 that orientation depends upon reflex muscular tone maintained 

 through the photo-receptors. When the animal bends towards 

 the light it does so because the tone of the muscles on the side 

 exposed to light is increased by stimulation of the photosensitive 

 surface on which the incident rays fall. The consequent 

 flexion of the body eventually brings it into a position when the 

 photo-sensitive surfaces are equally illuminated, so that the 

 muscular tension on either side of the body is balanced. 

 Anthropomorphic bias ascribes the movement of the moth 

 towards the candle to the preference of the animal for the 

 light. This view does not permit us to make verifiable 

 predictions which can be inferred from a more objective 

 attitude to the problem. It follows that if the insect's move- 

 ments are mainly concerned with the direction of the rays, 

 it will move from a strongly to a weakly illuminated situation, 

 when conditions are so arranged that by doing so it continues 

 to move along the path of the incident beam with both eyes 

 equally illuminated. These conditions are easy to arrange 

 by projecting on to a tube containing some positively photo- 

 tropic organism such as caterpillars of Porthesia a slanting 

 beam of parallel rays, the intensity of one half of the beam being 

 artificially reduced, and in this way the animal is induced to 

 move from the light into the shade, which is contrary to what 

 the anthropomorphic view would lead us to anticipate. If 



