278 LIGHT AND THE BEHAVIOR OF ORGANISMS 



source of light. When the temperature was decreased 

 still more they became strongly positive. This experi- 

 ment was repeated several times with similar results. 

 Increase in temperature above normal ordinarily causes 

 these organisms to become more strongly positive until a 

 maximum is reached, when the movements become irregu- 

 lar and reaction to light ceases. In no instance was it 

 found that any of these organisms became negative owing 

 to changes in temperature. The interesting point in these 

 observations is the fact that they become quiet as the 

 temperature decreases and then active again when it 

 decreases still further, just as in case of Euglena, but the 

 former' become only more strongly positive, whereas the 

 latter change from positive to negative. 



Not all entomostraca can be made positive by decreasing 

 the temperature. On June i, Alona gracilis was found in 

 great abundance in a Paramecium culture jar. A few speci- 

 mens of Cypris were also found in the same jar. They were 

 strongly negative at room temperature (25°) in light of 

 250 ca. m. The temperature was lowered to freezing, but 

 the organisms were continuously negative whenever they 

 responded at all. 



It is thus evident that in some organisms a decrease in 

 temperature causes negative responses to light, whereas 

 in others it causes positive responses. How this is brought 

 about is very difficult to see from a physico-chemical point 

 of view, although there are chemical compounds in which 

 decrease in temperature facilitates reactions caused by 

 light, as shown in Part IV of this volume. The fact that 

 the organisms become quiet as the temperature decreases, 

 and then active again as it decreases still more, is particu- 

 larly puzzling. In some organisms the change in the sense 

 of reaction caused by change in temperature is clearly 

 adaptive, and from this point of view we get some light 

 on the causes of the change in reaction, but of course only 

 a superficial explanation for adaptation is itself a problem. 

 In Euglena and frogs, for instance, the negative reaction 



