272 LIGHT AND THE BEHAVIOR OF ORGANISMS 



after seventeen Ranatras, negative in a given light inten- 

 sity, had been exposed for an hour and forty minutes, all 

 became positive. 



The positive reactions in all of the animals just referred 

 to prove fatal under certain conditions, and Loeb (1905, pp. 

 42, 74) claims that the caterpillar of the willow borer and 

 the mud-inhabiting crustacean Cuma Rathkii are positive, 

 although in their natural environment they are never ex- 

 posed to light. Here, then, we have a number of reactions 

 which do not lead the organisms to their optimum, reactions 

 which under certain conditions are clearly not adaptive. 

 But, as already shown, these reactions are non-adaptive only 

 under artificial conditions. It was however reactions of this 

 character that led Loeb (1906, p. 159) to conclude "that 

 the tropisms could not have been acquired by the way of 

 natural selection," and to formulate an explanation of their 

 origin which we shall consider later, Chapter XX. 



b. Effect of change in temperature. — It is well known 

 that temperature affects the activity of organisms. If it is 

 increased above normal, organisms ordinarily become more 

 active and more sensitive to other stimuli until the optimum 

 is reached, when their activity and their response to other 

 stimuli decrease, as they usually do when the temperature 

 is decreased below normal. Changes in temperature may 

 however have quite a different effect on some organisms. 



Strasburger (1878, p. 606) found that haematococcus 

 swarm-spores, which were positive in a given light intensity 

 at 16° to 18° C, became negative when the temperature was 

 decreased to 4° C.,and more strongly positive when increased 

 to 35° C. He obtained similar though somewhat less striking 

 results with other swarm-spores. The degree of change of 

 temperature required to cause a reversal in reaction to light 

 was found to vary with the different organisms and with 

 the same organism under different conditions. Strasburger 

 says (p. 610), " SInd die photometrischen Schwarmer, mit 

 denen experimentirt werden soil, auf sehr hohe Lichtinten- 

 sitat gestimmt, so wird es, um sie auf den negativen Rand 



