206 C. F. CURTIS RILEY 



away from the source of illumination. After orientation is com- 

 pleted the animals retain the position assumed with reference 

 to the light, the medium longitudinal axis of the body being 

 kept practically parallel with the incoming rays. So far as the 

 writer's observations are concerned, there is no definite evidence 

 that young toads orient to light according to the method of 

 "selection of random movements," as advocated by Holmes 

 (1905) (though the present writer believes that it may function 

 in some modified form), or by that of "trial " so ably propounded 

 by Jennings (1904) and (1906), also discussed by Mast (I.e.,), and 

 many other writers. Usually the young toads orient promptly 

 and definitely. If the head is pointing toward the light, they 

 make a turn of 180 so that the anterior end of the body is 

 pointed directly away from the source of illumination. Should 

 the toad be in such a position that the median longitudinal 

 axis of the body lies at right angles to the rays and facing the 

 light, the animal makes a turn of 90 °, thus bringing the head 

 into such a position that it points directly away from the source 

 of illumination. There are no preliminary movements, either 

 "trial" or "random" ones, during or immediately preceding the 

 orienting response, so far as the writer observed. But as Mast 

 (I.e., p. 214) has suggested, these facts do not preclude the possi- 

 bility of preliminary movements when other forms of stimuli 

 impinge upon the toads. After orientation is completed the 

 young toads jump away from the source of illumination along 

 a comparatively straight path, the medium longitudinal axis of 

 the body being parallel with the rays of light. During such 

 motor responses the direction of the rays in the field may be a 

 guiding factor. It is not impossible that they may be both a 

 guiding and a correcting factor, if we should apply the theory 

 advocated by Holmes (I.e., pp. 108-109). While there is little 

 evidence of such responses on the part of young toads, neverthe- 

 less the applicability of Holmes's modified "trial and error 

 method" with reference to the responses of animals which 

 orient themselves "according to the usual scheme" is worthy 

 of careful consideration. The present writer believes that it is 

 absolutely futile to attempt to explain the responses of all 

 animals by any one theory, or from any one point of view. 



Such a response on the part of young toads as has been 

 described by the writer is a tropic response in so far as it fits 



