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 timea feature of Weissmann’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. 
