COMPARISON OF PROTOZOA AND METAZOA 263 



leading from the optimum produces a negative reaction, while a change 

 leading toward the optimum produces no reaction, or a positive one. 

 The optimum from this standpoint usually corresponds, in a broad 

 way, to the optimum for the general interests of the organism. These 

 relations hold equally for Protozoa and Metazoa. 



12. In both the Protozoa and the Metazoa that we have studied, the 

 behavior is based to a considerable degree on the selection of certain 

 conditions through the production under stimulation of varied move- 

 ments (see Chapter XII). This shows itself in two characteristic types. 

 In the one case the organism when subjected to a change leading away 

 from the optimum responds by a movement that subjects it successively 

 to many different conditions, finally remaining in that one which is 

 nearest the optimum. This form of reaction is strongly developed in 

 Paramecium. In the second type, which may be considered a devel- 

 opment of the first, the organism first responds by one reaction, then 

 by another, continuing at intervals to change its response until one of 

 the reactions frees it from the stimulation. This way of behaving is 

 well seen in Stentor. Both methods of reaction may be expressed as 

 follows : When the organism is subjected to an irritating condition, it 

 tries many different conditions or many different ways of ridding itself 

 of this condition, till one is found which is successful. 



All together, there is no evidence of the existence of differences of fun- 

 damental character between the behavior of the Protozoa and that of 

 the lower Metazoa. The study of behavior lends no support to the 

 view that the life activities are of an essentially different character in the 

 Protozoa and the Metazoa. The behavior of the Protozoa appears to 

 b>e no more and no less machinelike than that of the Metazoa ; similar 

 principles govern both. 



Further, the possession of a nervous system brings with it no observ- 

 able essential changes in the nature of behavior. We have found no 

 important additional features in the behavior when tl^e nervous system 

 is added. In the lower Metazoa, experiment has shown the nervous 

 system to have two chief functions, — the maintenance of tonus, and the 

 bringing of the parts of the body into relation with each other by serving 

 for conduction. But both these functions are performed in the Protozoa 

 without a nervous system. The body of Paramecium maintains marked 

 tonus, and the different parts of the body work together. A comparison 

 of the behavior of the Protozoa with that of the lower Metazoa lends 

 powerful support to that view of the functions of the nervous system 

 which is so ably maintained by Loeb in his brilliant work on "The Com- 

 parative Physiology of the Brain and Comparative Psychology. " Accord- 

 ing to this view we do not find in the nervous system specific qualities 



