310 BEHAVIOR OF THE LOWER ORGANISMS 



by structural conditions. There is nothing in the cause that produces 

 them, taken by itself, to specifically direct them with reference to exter- 

 nal 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 — continues in the direc- 

 tion, we will say, that has led to the favorable conditions. But perhaps 

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

 tend to remove the organism from the favorable conditions — as when 

 a heated Paramecium passes across a small area of cool water, or a hun- 

 gry organism comes against food. Thereupon the cause for a change — 

 interference with the life processes — is again set in operation, and this 

 movement changes to another. Thus the animal changes all behavior 

 that leads away from the favorable condition, and continues that which 

 tends to retain it, so that we get what we call a positive reaction. The 

 change of behavior is due primarily in each case to the unfavorable 

 condition, internal or external — perhaps in last analysis always in- 

 ternal. 



Behavior of this character is seen with diagrammatic clearness in 

 the free-swimming infusoria. These animals continue their movements 

 so long as they lead to favorable conditions, changing at once such move- 

 ments as lead away. They thus retain favorable conditions by avoiding 

 unfavorable ones; the positive reaction is seen to be a secondary result 

 of negative ones. 



In the infusoria we have then the most elementary condition of the 

 positive reaction. Let us now examine a more pronounced type of 

 positive reaction, — movement directly toward the favorable condition. 

 Amoeba flows toward and follows a food body with which it comes in 

 contact, as illustrated in Fig. 19, p. 14. Take, for example, its action 

 at 3 in this figure. It moves forward with broad front, part of the 

 movement taking it toward the food, part away. On coming in contact 

 with the food, all movement is changed which takes it away, only that 

 being retained which keeps the animal in contact with the food. We 

 have here then, as in infusoria, a case of selection from varied movements, 

 the central point being the changing of all motion that leads to less favor- 

 able conditions. 



This is, perhaps, the fundamental condition of affairs, from which 

 all positive reactions are derived. The animal moves (partly or entirely 

 from internal impulse, as we have seen), but changes all movements that 

 lead to less favorable conditions. It therefore moves toward the favor- 

 able conditions. In many higher animals, even, this behavior is seen 

 in the random movements by which food is sought, by the aid of the 



