Influence of Environment on Animals 219 



filings in their arrangomont in n mnp:notir fi^ld. 'Pliis can be 

 proved very nicely in the case of starving caterpillars of I'or- 

 thesia. The writer put such caterpiUars into a ghiss tube the 

 axis of which was at right angles to the ])lane of the window: 

 the caterpillars went to the window side of the tubi- and 

 remained there, even if leaves of their food plant wcn^ 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 toward the source of 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 tlie following theory of these 

 instinctive reactions. Animals of the type of those mentioned 

 are automatically oriented b}^ the light in such n 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 photosensitiveness 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 i)articipate e(iually 

 in locomotion, the symmetrical muscles work with vq\u\\ energy 

 as long as the photochemical processes in both eyes are identi- 

 cal. If, however, one eye is struck by stronger light than the 

 other, the symmetrical muscles will work une(iually and in 

 positively hehotropic animals those muscles will work with 

 greater energy which brings the plan(^ of symmetry back into 

 the direction of the rays of light and the head toward the source 

 of light. As soon as both eyes are struck by the rays of light 



