EXPERIMENTAL ZOOLOGY I9I 



effected, and it will then make no further trials, but will 

 move in the direction of the stimulus. 



Jennings maintains that the very existence of movements 

 of this trial and error sort show that we have in a simple 

 way the same kind of an element interposed between stimula- 

 tion and motor response as in an animal wuth a nervous 

 system, which receives the stimulus from without and then 

 (within the nervous system) itself brings about another and 

 different stimulation which is the direct cause of the muscular 

 contractions. The protozoa have no nervous system and yet 

 according to Jennings the organism acts as though it had 

 received the stimulus and then in turn stimulated its par- 

 ticular parts, as if the same kind of thing were here, in a 

 simple sort of way, as in a more complex form where structur- 

 ally defined units are connected with the several steps of the 

 process. 



It is perhaps too early for a decision between these two 

 views ; the truth is perhaps not all on one side. We need 

 more work and more agreement upon the use of our terms. 

 The study appeals to zoologists because w^e believe this is 

 the right end at which to begin in an attempt to explain the 

 reactions of the higher organisms. In this connection, some 

 have sought to explain all of the more complex activities as 

 being built up of the simpler tropisms, and this is sometimes 

 called the tropism theory, but of this our time will not permit 

 a discussion. 



Regarding the question of the beginnings of intelligence 

 in higher forms, the experimental zoologist believes that he 

 and the psychologist are steadily accumulating facts which lend 

 support to the conception of these activities as differing in 

 degree rather than in kind from one end of the animal scale 

 to the other, and of the more complex and elaborate as 

 having arisen by evolution from the more simple and ele- 

 mentary. 



