§4] PHOTOTAXIS AND PHOTOPATHY 203 



p. 127 ; '84, p. 137) showed that Daphnia and some ants are 

 very sensitive to the violet rays, and Graber ('83, p. 214) 

 found that the photophob earthworm withdraws from ultra- 

 violet rays. This result is unusual, however, for most experi- 

 menters have agreed with this much of Bert's ('78, p. 989) 

 conclusions, that " the animals see . . . only those rays which 

 we ourselves see," or, better, that the range of irritability of 

 the protoplasm of our retina is as great as that of any other 

 protoplasm. 



Below the blue, some authors have believed the yellow rays, 

 the brightest of the spectrum, to be prevailingly photopathic. 

 Thus both Bert ('68, p. 381) and Lubbock ('83, p. 214) find 

 that Daphnia accumulates especially in the yellow and green 

 parts of the spectrum. Regarding these results I have only 

 the comment that they need further confirmation. 



0. Phototaxis vs. Photopathy. — We have hitherto assumed 

 the existence of two dissimilar sorts of locomotor response to 

 light — phototaxis and photopathy. Phototaxis we defined as 

 migration in the direction of the light rays, and photopathy as 

 migration towards a region of greater or less intensity of light. 

 Are we justified in making this distinction ? 



The chief ground for this distinction is the existence of two 

 sorts of phenomena which, not having been generally recog- 

 nized as different, have led to extensive discussion. The best- 

 established of these phenomena is phototaxis, which was proved 

 to exist by certain crucial experiments of Strasbtjrgbr on 

 Protista, and of Loeb on Metazoa. Mr. W. B. Cannon and 

 I have used Strasburgbr's methods on Daphnia, and con- 

 firmed his results. Figure 59 gives a view of our apparatus, 

 which was essentially the same as Strasbtjrger's. It con- 

 sisted of a hollow prism P, containing a dark solution and 

 placed over the trough T, with its organishis. Strasbtjrgbr 

 ('78, p. 585) put swarm-spores of Botrydium and Bryopsis into 

 the trough, and reflected the light perpendicularly through the 

 prism upon the trough. There was now a perfect gradation in 

 intensity from the thick end to the thin edge of the prism. 

 Yet the organisms showed no tendency to aggregate at the 

 clearer end. The light was now permitted to enter the trough 

 obliquely, the thicker end of the prism being next the source 



