ii6 evolution: the ages and tomorrow 



the pressure of the weight of food particles in the digestive 

 vacuoles in its protoplasm, provided that the habitat water 

 is slightly acid. In neutral water there is no gravity reaction 

 in Farameciiim, indicating that here, as elsewhere on the 

 scale up to man, the condition of the habitat has a bearing 

 on the behavior of organisms. These simple reactions are 

 basic in the behavior of Farauiecium, and would seem to be 

 sufficient since it swims around in a bacterial soup. 



It was long thought that these protozoa had no way of 

 profiting by experience, did not have the least rudiment of 

 the property we call memory, were without the least degree 

 of consciousness, and acted entirely automatically. These 

 beliefs have been shaken recently by the results of psycho- 

 logical experiments carried out on Varamecium and other 

 protozoa. H. S. Jennings fed harmful substances to protozoa 

 and found that they learned to reject them after a number 

 of trials. J. W. French studied trial-and-error learning in 

 Farameciwn where he introduced a factor of escape from a 

 capillary tube; he found some evidence of improvement 

 with practice. F. Bramstedt taught paramecia to swim in a 

 triangular path and then placed them in a circular enclosure. 

 He reports that they continued to swim in the triangular 

 path for some time. If he taught first the circular and then 

 substituted the triangular path, they continued for a time in 

 the circular. Beatrice Gelber, in what is probably one of 

 the best controlled of these types of studies, taught para- 

 mecia to remain in a definite area for feeding. Under cer- 

 tain conditions she found, and could "report with a high 

 degree of confidence," that behavior was changed by her 

 procedure. These and other less conclusive studies would 

 make it seem likely that life at the level of protozoa has 

 something of the higher talent for learning by experience. 



How far below the level of unicellular life the capacity 

 for learning extends is problematic, but there is evidence 

 that certain rhythms are acquired on some sort of vague 

 learning pattern. In some plants, for instance, rhythms such 



