HEL1OTROP1SM 135 



animals collected at the top, another at the bottom of the tube. When 

 these two groups were separated in two different vessels and exposed 

 to the light, it was found that those animals that had collected at the 

 bottom of the tube in the dark room were invariably negatively helio- 

 tropic, while the others were positively heliotropic. The reverse was 

 also true; namely, that if positively and negatively heliotropic larvae 

 of Polygordius were put into vertical tubes in the dark, the positively 

 heliotropic specimens invariably gathered at the top, the others at the 

 bottom of the tube. In Limulus larvae I noticed that when positively 

 heliotropic they swam at the surface of the dish, while in the negatively 

 heliotropic state they crept at the bottom. It is, however, questionable 

 how far this observation can be generalized. In the Nauplii of Bala- 

 nus I have noticed that negatively heliotropic larvee swim with the 

 same velocity toward the room side as positively heliotropic animals 

 move in the opposite direction. 



Heliotropism, and especially positive heliotropism, is extremely 

 common among animals, particularly pelagic animals. I have found 

 pelagic larvae of fish which reacted in just as machinelike a manner 

 to light as caterpillars or Crustaceans ; but in adult fish, and particularly 

 in higher vertebrates, typical heliotropic reactions can no longer be 

 demonstrated. It rarely happens that animals endowed with the 

 mechanisms of associative memory react in such a machinelike manner 

 to the elementary forces of nature as the heliotropic animals which 

 we have discussed. 



Heliotropism plays a wide role in determining the behavior of 

 animals, and there are animals whose life becomes at certain periods 

 of their existence, at least, a function of light. Since I have treated 

 the bearing of heliotropism upon the theory of animal instincts in 

 another place * it need not be repeated here. 



4. THE REACTION OF ANIMALS TO SUDDEN CHANGES IN THE INTEN- 

 SITY OF LIGHT 



One source of endless misunderstandings and waste of time among 

 scientists results from the indiscriminate application of one principle 

 to all those cases which by accident have one feature in common with 

 the cases covered by the principle, but differ in almost every other re- 

 gard. We have already mentioned the absurdity of the idea that every 

 kind of turning to the light should be a case of heliotropism. Helio- 

 tropism covers only those cases where the turning to the light is com- 



* Loeb, Comparative Physiology of the Brain and Comparative Psychology, G. P. Put- 

 nam's Sons. 



