ii2 TEE POPULAR SCIENCE MONTHLY 



daphnia positively heliotropic, this may often be accomplished by 

 simultaneously reducing the temperature. From the physico-chemical 

 standpoint we must assume that likewise in the dark, at the ordinary 

 temperature, the photosensitive substance is destroyed so rapidly that 

 its active mass is generally rather too low to cause a heliotropic reaction. 

 By reducing the temperature the rate of decomposition of the photo- 

 sensitive substance is decreased more than the rate of its formations. 



This illustration may suffice, under the limitations of the space 

 allowed us, to indicate how the facts in this field might be correlated 

 when viewed from the standpoint of physical chemistry. 



IV 



The animals which are strongly positive heliotropic and those ani- 

 mals which do not react at all to light offer the observer no difficulties. 

 Nevertheless, some zoologists, apparently not very familiar with the laws 

 of physical chemistry, 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 " trial and error/' 

 From a chemical standpoint the behavior of animals at low intensity is 

 easily to be understood. If a positively heliotropic animal is illuminated 

 from one side a compulsory turning of the head toward the source of 

 light occurs only when the difference in the rate of certain photo- 

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

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

 stance in the animal great enough, it is only a short time, for instance, 

 the fraction of a second, before the difference in the mass of the re- 

 action 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 direc- 

 tion of the light rays, the difference in the photochemical reaction- 

 products in the two retinas becomes so great that the head is at once 

 automatically turned back toward the source of light. But if the in- 

 tensity of the light is lessened (or the photosensitiveness of the animal 

 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, but move instead in 

 zig-zag lines or very irregularly. It is, therefore not a case of a qualita- 

 tive, but of a quantitative, difference in the behavior of heliotropic 

 animals under greater or lesser illumination, and it is therefore errone- 

 ous to assert that heliotropism determines the movement of animals 



