372 LIGHT AND THE BEHAVIOR OF ORGANISMS 



exists an endless number of organisms whose reactions to 

 light are useless. Indeed, according to this theory, there 

 must be more organisms in which the reactions to light 

 are or have been useless than there are in which they 

 are purposeful, for the author states, as quoted above, 

 that *' the number of animals in which the three groups 

 of conditions [purposeful mechanism] meet is much smaller 

 than where only two meet [useless reactions]." We have 

 demonstrated that, while there are isolated instances, 

 mostly under artificial conditions, in which orientation and 

 subsequent locomotion (heliotropism) lead to fatal results, 

 the orienting reactions are in general useful to the organism 

 in its life processes, and the same may be said regarding 

 all other reactions to light. Thus, it is evident that Loeb's 

 theory of the origin of purposeful reactions is not in har- 

 mony with the observed facts. 



Jennings' theory of behavior is founded upon the idea 

 that the reactions are fundamentally '* purposeful." He 

 admits that light and other external agents cause chemical 

 changes in the organism, and that all reactions are deter- 

 mined by chemical changes or states ; but that the chemical 

 change or state which causes a given reaction is not directly 

 and entirely the result of the external condition which 

 precedes the reaction; that what an organism does under 

 a given condition depends upon what it and its ancestors 

 have done and experienced in the past as well as upon the 

 present external conditions. The reactions are above all 

 things regulatory. External conditions are not the direct 

 cause of reactions. 



Reactions are defined as changes in the activity of or- 

 ganisms. Such changes may occur under constant exter- 

 nal conditions. They are therefore due primarily to 

 internal changes. External factors cause reactions not 

 directly, but indirectly, by altering internal processes 

 (physiological states). Variability in reaction to given 

 external conditions is due to changes in physiological 

 states. If an organism responds to light of a given inten- 



