TRANSFORMATION OF HELIOTROPIC ANIMALS 271 



tension of the muscles turning the animal to the right 

 and to the left is then the same. As soon, however, as 

 the rays of light SS 1 strike the right side, the tension of 

 the muscles which turn the animal toward the light side 

 either becomes (1) greater or (2) less; and this difference in 

 the tension of the symmetrically situated 

 muscles will in either case be greater at 

 the more irritable, oral end a of the animal 

 than at the less irritable, aboral end 6. In 

 the former case the animal will be forced 

 to assume the position 6a 1? and, further 

 more, under the same conditions, to bring 

 its median plane into the direction of the 

 rays of light; it is positively heliotropic. 

 In the latter case it will be forced to assume 

 the position ba. 2 ; it is negatively heliotropic. FIG - 65 



As soon as the plane of symmetry coincides with the direction 

 of the rays of light, symmetrically situated points on the 

 body of the animal are struck at the same .angle by equally 

 strong rays of light, and the animal can then no longer be 

 driven either to the right or to the left by the light, and 

 consequently continues to move in the direction of the rays 

 of light. As soon, however, as the animal is again disturbed 

 in its movements in this direction, through some other 

 external or internal stimulus, symmetrically situated points of 

 the animal are again stimulated unequally by the light. In 

 consequence there is a corresponding change in the tension 

 of the symmetrical muscles, and as a result of this the 

 animal is again brought into its proper orientation. 



I wish, however, particularly to emphasize the fact that 

 the progressive movement of heliotropic animals in the 

 direction of the rays of light is a fact which can be directly 

 observed and demonstrated, and is not a mere hypothesis. 



The question further arises whether facts are indeed at 



