REGULATION OF REACTIONS 301 



interaction between external and internal chemical con- 

 stituents, as, for example, a relation between acids and 

 alkalis, but to an effect on the general state of the organ- 

 ism as a whole. 



(10) Variability and modifiability in response to a given 

 external condition are striking characteristics in the be- 

 havior of all living beings. The optimum and sensitive- 

 ness in swarm-spores, diatoms, Euglena, Stentor, Volvox 

 and other similar organisms have been found to change in 

 accordance with the environment. If they are exposed to 

 strong illumination for some time the optimum intensity 

 increases and the sensitiveness decreases. If exposed to 

 low intensity the opposite change takes place. Not only 

 the light intensity, but also the time of exposure, is active 

 in these changes. Momentary exposure may produce 

 results just the opposite from those due to continued 

 exposure. Changes in response frequently occur with- 

 out any immediate changes in the environment. These 

 changes are regulated by internal factors, physiological 

 processes. Many of the responses to a given light con- 

 dition are extremely variable. This is due to the fact that 

 numerous factors, both internal and external, are involved 

 in these responses. Concerning the internal factors little 

 is as yet known. 



