Significance of Tropisms for Psychology 45 



or very irregularly. It is therefore not a case of a qualitative, 

 but of a quantitative, difference in the behavior of heliotropic 

 animals under greater or lesser illumination, and it is there- 

 fore erroneous to assert that heliotropism determines the 

 movement of animals toward the source of light only under 

 strong illumination, but that under weaker illumination an 

 essentially different condition exists. 



Still another point is to be considered. We have seen 

 that acid increases the sensitiveness of certain animals to light, 

 possibly by increasing the active mass of the photochemical 

 substance. Every animal is continually producing acids in its 

 cells, especially carbonic acid and lactic acid; and such acids 

 increase the tendency in certain animals to react hehotropically. 

 It probably produces also substances which could have the 

 opposite effect and which decrease the heliotropic sensitiveness 

 of the animals. Fluctuations in the rate of the production of 

 these substances will also produce fluctuations in the helio- 

 tropic sensitiveness of the animal. If, for instance, the active 

 mass of the photosensitive substance in a copepod is relatively 

 small, a temporary increase in the production of carbonic acid 

 can increase the photosensitiveness of the animal sufficiently 

 to cause it to move for the period of a few seconds directly 

 toward the source of light. Later the production of carbonic 

 acid decreases and the animal again becomes indifferent to light 

 and can move in any direction. Then the production of car- 

 bonic acid increases again and the animal goes again, for a 

 short time, toward the light. Such animals finally gather at 

 the lighted side of the vessel because the algebraic sum of the 

 movements in the other directions becomes zero according to 

 the law of chance. But it is plain that such animals do not 

 reach the source of light by a straight path. A \^Titer who is 

 not trained to interpret the variations in the behavior of such 

 an animal chemically and physiologically, can naturally give 

 no explanation of their significance. If he is forced to find an 



