52 LIGHT AND THE BEHAVIOR OF ORGANISMS 



(3) This period resulted in the origin of the iatromechani- 

 cal and iatrochemical schools. The object of these schools 

 was to explain all vital phenomena on purely physical and 

 chemical principles. 



(4) The failure to accomplish this purpose led to the 

 origin of the doctrine of vital force, during the first years 

 of the eighteenth century. This resulted in a period of 

 stagnation in research in this line which continued until 

 the appearance of Johannes Miiller, De CandoUe and many 

 others, early in the nineteenth century. 



(5) The establishment of the doctrine of evolution by 

 Darwin and the consequent interest in the origin of mental 

 phenomena in man led to special activity in the study of 

 behavior of animals from the psychological point of view, 

 and numerous anthropomorphic explanations of their 

 activity. 



(6) In plants activity was studied from the physico- 

 chemical point of view during this period. This study 

 resulted in the development of the idea that the actions 

 are definitely controlled by external agents, e.g., the direc- 

 tion of growth in roots and stems by gravity, moisture, 

 light, etc. The reactions thus definitely controlled were 

 called tropisms. At first the term tropism was used merely 

 to indicate the relation between the direction of bending 

 and the position of the source of stimulation (De Candolle, 

 1832). Tropisms were however in general regarded as 

 reactions unequivocally controlled by external agents. 



(7) The study of animal behavior from the physico- 

 chemical point of view was first taken up by Verworn and 

 Loeb in 1886 and 1887. The activity of the different 

 organs in animals had been studied from this point of view 

 for nearly three centuries, but not the reactions of the 

 animal as a whole. Loeb attempted to show that the 

 behavior In plants and animals is essentially the same, and 

 concluded that the behavior of animals is very largely un- 

 equivocally controlled by external agents. He and his 

 followers therefore described reactions in animals in terms 



