44 The Mechanistic Conception of Life 



this may often be accomplished by simultaneously reducing the 

 temperature. 



IV 



The animals which are strongly positively heliotropic and 

 those animals which do not react at all to light offer no diffi- 

 culties to the observer. Nevertheless, some zoologists seem to 

 have found difficulty in explaining the behavior of those animals 

 which come between the two extremes. For instance, one writer 

 has asserted that with greater intensity of light the laws of 

 heliotropic orientation hold good, while with a lessened light 

 intensity the animals react to light by the method of 'Hrial 

 and error." From a chemical standpoint the behavior of 

 animals at low intensity is easily to be understood. If a posi- 

 tively heliotropic animal is illuminated from one side, a com- 

 pulsory turning of the head toward the source of light occurs 

 only when the difference in the rate of certain photochemical 

 reactions in the two eyes reaches a certain value. If the inten- 

 sity of the light is sufficient and the active mass of photochemical 

 substance in the animal great enough, it requires only a short 

 time, for instance, the fraction of a second, before the difference 

 in the mass of the reaction products formed on the two sides 

 of the animal reaches the value necessary for the compulsory 

 turning of the head toward the source of light. In this case the 

 animal is a slave of the light; in other words, it has hardly time 

 to deviate from the direction of the light rays; for if it turns 

 the head even for the fraction of a second from the direction 

 of the light rays, the difference in the photochemical reaction 

 products in the two retinae becomes so great that the head is 

 at once turned back automatically toward the source of light. 

 But if the intensity of the light or the photosensitiveness of 

 the animal is lessened the animal may deviate for a longer 

 period from the direction of the light rays. Such animals 

 do eventually reach the lighted side of the vessel, but they no 

 longer go straight toward it, moving instead in zig-zag lines 



