DISCOVERY OF THE PROTOZOA 109 



already been indicated. Many interesting problems have 

 arisen in connection with recent studies of them. The 

 group embraces the very simplest manifestations of animal 

 life, and the experiments upon the different forms light the 

 way for studies of the vital activities of the higher animals. 

 Some of the protozoa are disease-producing; as the microbe 

 of malaria, of the sleeping sickness, etc., while, as is well 

 known, most diseases that have been traced to specific germs 

 are caused by plants — the bacteria. Many experiments of 

 Maupas, Caulkins and others have a bearing upon the dis- 

 cussions regarding the immortality of the protozoa, an idea 

 which was at one time a feature of \\ eissmann's theory of 

 heredity, Binet and others have discussed the evidences of 

 psychic life in these micro-organisms, and the daily activity 

 of a protozoan became the field for observation and record 

 in an American laboratory of psychology. The extensive 

 studies of Jennings on the nature of their responses to stim- 

 ulations form a basis for some of the discussions on animal 

 behavior. 



