THEORY OF INSTINCTS 181 



It was possible to show that the heliotropism of 

 animals agreed in every point with that of plants. 

 If a moth be struck by the light on one side, those 

 muscles which turn the head toward the light become 

 more active than those of the opposite side, and 

 correspondingly the head of the animal is turned 

 toward the source of light. As soon as the head of 

 the animal has this orientation and the median-plane 

 (or plane of symmetry) comes into the direction of 

 the rays of light, the symmetrical points of the sur- 

 face of the body are struck by the rays of light at the 

 same angle. The intensity of light is the same on 

 both sides, and there is no more reason why the 

 animal should turn to the right or left, away from the 

 direction of the rays of light. Thus it is led to 

 the source of the light. Animals that move rapidly 

 (like the moth) get into the flame before the heat of 

 the flame has time to check them in their flight. 

 Animals that move slowly are affected by the increas- 

 ing heat as they approach the flame ; the high tem- 

 perature checks their progressive movement and they 

 walk or fly slowly about the flame. The more re- 

 fractive rays are the most effective in animals just as 

 in plants (i). 



Hence the " instinct" that drives animals into the 

 light is nothing more than the chemical and indirect- 

 ly the mechanical effect of light, an effect similar to 

 that which forces the stem of the plant at the window 

 to bend toward the source of light, or which forces 

 Palsemonetes to collect at the anode. The moth 



