REGULATION OF REACTIONS 271 



There are also many organisms which never become nega- 

 tive in their responses. This is true of the great majority 

 of higher plants and various animals. I exposed the ento- 

 mostracan Scapholeberis armata, and Caprella, Leptoplana 

 tremellaris and the early stages of Eudendrium, Arenicola, 

 Limulus, and many other forms in light of 15,000 ± ca. m. 

 intensity, and found that they remained positive although 

 many were soon injured by the intense light. Carpenter 

 (1908) was unable to make Drosophila negative to light, 

 although he exposed specimens in over 480,000 ca. m., an 

 intensity which produced convulsive reflexes and was un- 

 doubtedly injurious. Those organisms mentioned above 

 which do not become positive thrive in darkness. There 

 is, as stated in the preceding chapters, consequently no need 

 for a positive reaction to light. Those which do not become 

 negative thrive in strong light. Under natural environ- 

 mental conditions they rarely meet with intensities so high 

 as to be injurious. In these animals there is, then, no need 

 for negative reactions. 



Holmes says (1901, p. 233), " Talorchestia longicornis is 

 strongly and permanently positive both in weak and strong 

 light." These animals however come to rest in shaded 

 spots and are usually found under drifts of seaweeds. 

 Orchestia agilis, which is found in similar places, is negative 

 when first exposed, but it soon becomes positive, '* the more 

 quickly the stronger the light." After it is positive it 

 " remains so even in the strongest light, but it may be 

 rendered temporarily negative to exposure to light of lower 

 intensity." Similarly Holmes (1905) found that Ranatras 

 are negative when first taken from darkness and later posi- 

 tive, after which they remain positive as long as they are 

 in the light, no matter how intense the light may be or how 

 long they may be exposed. In general, exposure to light 

 tends to make them positive, whereas darkness tends to 

 quiet them and make them negative. It should be empha- 

 sized here that not only the intensity but also the time of 

 exposure has to do with these reactions. Holmes says that 



