8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



SO, their importance cannot readily be overestimated, for 

 they formed the foundation of later work which led to 

 most fruitful results. 



3. Period of Vitalism 



It was fully realized before the close of the eighteenth 

 century that the mechanical explanations thus far pre- 

 sented were inadequate to account for many fundamental 

 phenomena at which they were directed. Especially was 

 this true with reference to movements of various kinds, in 

 both plants and animals. It led to the postulation of a 

 controlling principle in living beings, foreign to chemistry 

 and physics, a hypermechanical principle known as vital 

 force. Those who believed in this principle were called 

 vitalists. Some vitalists considered the postulated force 

 inscrutable, and consequently abandoned all hope of gain- 

 ing an insight into vital processes through experimental 

 means. Others, however, among the foremost of whom were 

 the botanist, De Candolle, and the famous physiologist, 

 Johannes Miiller, held the opinion that this force was 

 subject to further experimental analysis. 



The prevalence of the former view was however un- 

 doubtedly the chief cause of stagnation in general physiology 

 in its broadest sense, during this period, for there was no 

 corresponding unproductive period in the development of 

 physical sciences. As a matter of fact many who had been 

 prominent investigators in both biological and physical 

 sciences, now abandoned the former, and devoted their 

 entire energies to the latter. 



4. Return to Mechanical Explanations 



Miiller realized the weakness of the iatromechanical 

 school as well as the inadequacy of pure philosophical 

 speculation. On the one hand he recognized the importance 



