6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



philosophical point of view at further analysis of causa- 

 tion. Not until the work of Galen (131-200 ± A.D.), four 

 hundred years later, was there anything approaching ex- 

 perimental analysis. 



2. First Attempts at Mechanical Explanation of Life 



Phenomena 



Galen studied the structure of animals by direct obser- 

 vation, even practicing vivisection on pigs and monkeys, 

 and thus he sought to learn the functions of the various 

 organs. But others did not continue the experimental 

 work begun by him, and nearly thirteen centuries passed 

 without any progress. It was not until early in the six- 

 teenth century that interest in vital phenomena was again 

 aroused, and it was a century later before Harvey made his 

 important discovery on the circulatory system and pre- 

 sented mechanical explanations for many factors involved 

 in the process of circulation, all of them based on experi- 

 mental evidence. 



A few years later, building on Descartes' idea " that the 

 bodies of animals and men act wholly like machines and 

 move in accordance with purely mechanical laws," Borelli 

 undertook to reduce the movements of the organic motor 

 apparatus to purely physical principles. The work of 

 Borelli formed the foundation of the iatromechanical 

 school, the members of which sought to explain all vital 

 phenomena in animals by the application of physical prin- 

 ciples. Other investigators of this period recognized the 

 importance of chemical reactions in animal activity, and, 

 under the leadership of Sylvius, founded the iatrochemical 

 school, a school which admitted the importance of physical 

 principles in explaining animal activity, but which strongly 

 emphasized the influence of chemical phenomena in vital 

 processes. The seventeenth century, and part of the 

 eighteenth, formed a period in which mechanical explana- 



