480 H. S. Jennings. 



organism from the favorable conditions; in other words, to pro- 

 duce interference with the life processes. Unfavorable stimuli, 

 in these organisms, cause a change in behavior; favorable stimuli 

 cause none. It is perhaps a general rule in organisms, high or 

 low, that continued completely favorable conditions do not lead 

 to definite reactions. Of course while the external conditions 

 remain the same, the internal processes may change in such a 

 way that these conditions are no longer favorable, and now the 

 behavior may change. This frequently happens. 



When the organism is not completely enveloped by favorable 

 conditions, but is on the boundary, if we may so express it, be- 

 tween favorable and unfavorable ones, there is often a definite 

 change in the behavior leading toward the favorable conditions — 

 a positive reaction. To understand such reactions, we may start 

 from the fact, already mentioned, that unfavorable internal con- 

 ditions (as well as external ones) cause a change of behavior. 

 It is a general fact, for example, that the animal whose metabolic 

 processes suffer interference from lack of material — the hungry 

 animal — sets in operation trains of activity diff^ering from the 

 usual ones. Interference with respiration, or an increase in 

 temperature above that favorable for the physiological processes, 

 has similar efi^ects. This is indeed a general rule for all internal 

 changes interfering with the usual physiological processes. 



But the activities thus induced are in themselves undirected, 

 save by structural conditions. There is nothing in the cause 

 producing them to direct them with reference to external things. 

 Let us suppose, however, that certain of these movements lead 

 to a condition which relieves the interference with the internal 

 processes. The cause for a change of behavior is now removed, 

 hence the organism continues its present movement. But perhaps 

 later — sometimes at the very next instant — this same movement 

 may tend to remove the organism from the favorable condition — 

 as when a Paramcecium in a heated preparation passes across a 

 small area of water cooled to the optimum, and reaches the oppo- 

 site side, or when a hungry organism comes in contact with food, 

 which will be lost if there is further movement. Thereupon the 

 cause for a change — interference with the life processes — is again 

 set in operation, and the present movement is changed. Thus the 

 animal changes all behavior that leads away from the favorable 

 condition, and continues that which tends to retain it, so that we 



