Animal Instincts and Tropisms 265 



used it to compare the relative efficiency of two different 

 lights. 



The behaviour of the animals under the influence of 

 two lights is a confirmation of our theory of heliotro- 

 pism inasmuch as the animal moves in such a direction 

 that the symmetrical elements of the surface of the 

 body are struck by light of the same intensity at the 

 same angle, so that as a consequence equal masses of 

 photosensitive substances are produced in symmetrical 

 elements of their eyes or skin in equal times. The 

 effect on the symmetrical muscles will be identical. 

 As soon as one of the lights is a little stronger the 

 animal will deviate towards this light, in case it is 

 positively heliotropic and towards the weaker light if it 

 is negatively heliotropic. This deviation again is not 

 the product of chance but follows a definite law as 

 Patten 1 has recently shown. He used the negatively 

 heliotropic larva? of the blowfly. These larvae were 

 made to record their trail while moving under the 

 influence of the two lights. The results of the measure- 

 ments of 2500 trails showing the progressive increase 

 in angular deviation of the larva? (from the perpendicular 

 upon the line connecting the two lights), with increasing 

 differences between the lights, are given in the follow- 

 ing table. Since the deviation or angular deflection of 

 the larvae is towards the weaker of the two lights it is 

 marked negative. 



1 Patten, Bradley M., Am. Jour. Physiol., 1915, xxxviii., 313. 



