264 Influence of environment on animals 



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 tiie 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 

 heliotropic, are attracted by the light, since it can be shown that 

 they go toAvards the source of light even if in so doing they move 

 fi'om 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 pro- 

 cesses in both eyes are identical. If, however, one eye is struck by 

 stronger light than the other, the symmetrical muscles will work 

 unequally ahd 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 exj)crimcnts of G. H. Parker and S. J. Holmes. The 

 former worked on a butterfly, Vanessa antiope, the latter on other 



