The Mechanistic Conception of Life 27 



animals cannot resist. It appears as if this blind instinct which 

 these animals must follow, although it may cost them their life, 

 might be explained by the same law of Bunsen and Roscoe, 

 which explains the photochemical effects in inanimate nature. 

 This law states that within wide limits the photochemical effect 

 equals the product of the intensity of light into the duration 

 of illumination. It is not possible to enter here into all the 

 details of the reactions of these animals to light; we only wish 

 to point out in which way the light instinct of the animals 

 may possibly be connected with the Bunsen-Roscoe law. 



The positively hehotropic animals — i.e., the animals which 

 go instinctively to a source of light — have in their eyes (and 

 occasionally also in their skin) photosensitive substances which 

 undergo chemical alterations by light. The products formed 

 in this process influence the contraction of the muscles — mostly 

 indirectly, through the central nervous system. If the animal 

 is illuminated on one side only, the mass of photochemical 

 reaction products formed on that side in the unit of time is 

 greater than on the opposite side. Consequently the develop- 

 ment of energy in the symmetrical muscles on both sides of the 

 body becomes unequal. As soon as the difference in the masses 

 of the photochemical reaction products on both sides of the 

 animal reaches a certain value, the animal, as soon as it moves, 

 is automatically forced to turn toward one side. As soon as 

 it has turned so far that its plane of symmetry is in the direction 

 of the rays, the symmetrical spots of its surface are struck by 

 the light at the same angle and in this case the intensity of light 

 and consequently the velocity of reaction of the photochemical 

 processes on both sides of the animal become equal. There 

 is no more reason for the animal to deviate from the motion in 

 a straight line and the positively heliotropic animal will move 

 in this line to the source of light. (It was assumed that in 

 these experiments the animal is under the influence of only 

 one source of light and positively heliotropic.) 



