PREFATORY NOTE. 



The investigations the results of which are herein set forth were 

 carried out by the aid of certain grants from the Carnegie Institution 

 of Washington. The author desires to express his deep sense of obli- 

 gation for the aid thus rendered. The first five papers were prepared 

 at the Zoological Laboratory of the University of Michigan, and were 

 submitted to the Carnegie Institution for publication August i, 1903. 

 To the third paper some additions were made in February, 1904. 

 The sixth and seventh papers were prepared at the Naples Zoological 

 Station, while the writer was acting as Research Assistant of the 

 Carnegie Institution, and were transmitted for publication in January 

 and March, respectively, 1904. 



LIST OF PAPERS. 



1. Reactions to Heat and Cold in the Ciliate Infusoria. 



2. Reactions to Light in Ciliates and Flagellates. 



3. Reactions to Stimuli in Certain Rotifera. 



4. The Theory of Tropisms. 



5. Physiological States as Determining Factors in the Behavior of Lower 



Organisms. 



6. The Movements and Reactions of Amreba. 



7. The Method of Trial and Error in the Behavior of Lower Organisms. 



