PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 165 



end approach in their effects the conditions brought about by dark. 

 Graber ('83, '84) found that Triturus did not come to rest in the colors 

 toward the violet end of the spectrum when there was equal oppor- 

 tunity to remain in those nearer the opposite end. This was true of 

 blinded as well as normal animals. He also ('84) found that Rana 

 and Bufo reacted in much the same way. He states that his results 

 could not have been due to the effect of temperature, as he performed 

 experiments in which he used a heat screen for the blue and none for 

 the red light, and the results were the same. Kiihne ('78 a ) had pre- 

 viously observed that normal frogs went from green toward blue light, 

 while blinded individuals did not. Loeb ('90) states that the less re- 

 frangible rays do not affect light reactions to such an extent as those 

 of greater refrangibility, and in this connection he remarks that a frog 

 will jump towards a red cloth. (He found Rana to be negatively pho- 

 totropic.) Torelle (:03) in speaking of the frog recorded in a stronger 

 positive phototropism for blue light than for red, yellow, or green ; and 

 this was the same when the light was reflected, transmitted, or both. 

 The individuals she used were indifferent to red light. Reese (:06) 

 found blue to be most potent in causing reactions in Necturus and 

 Cryptobranchus. Yerkes (:03, p. 586) suggested that the frog might 

 be able to distinguish between red and white backgrounds, but, as he 

 says (:06, p. 548), there is nothing to show that these reactions might 

 not have been due to intensity differences. Holmes (:06, p. 350) in 

 speaking of frogs sums up the whole matter by stating that " in general 

 it may be said that where they are able to go toward one of two colors, 

 of equal intensity, they move to the color lying nearest the violet end 

 of the spectrum." 



The phototropic reactions of amphibia are apparently not due to the 

 direct stimulation of the central nervous system by light. Parker (:03 b ) 

 found that eyeless frogs responded positively when only the lower part of 

 the body was illuminated from the side in such a manner that the central 

 nervous organs were in shadow. The experiments of Dubois (:90) on 

 blinded Proteus, and Reese (:06) on Cryptobranchus and Necturus 

 offer additional evidence on this point. These animals reacted to a 

 beam of light thrown on the tail, and hence beyond the limits of the 

 central nervous organs. 



Various internal and external factors may influence the responses of 

 amphibians to light. It is probable that there are many factors which 

 exert such a modifying influence. Those which are enumerated in the 

 following paragraphs are known to alter the photic responses of certain 

 amphibians by producing changes in their physiological states. 



Breeding season. Jordan (:93, p. 271), in speaking of Diemyctylus, 



