HISTORICAL REVIEW 51 



from the source of stimulation. From these observations 

 he concluded that "orientation is produced indirectly by 

 following up these chance movements which bring respite 

 from the stimulation." 



This conclusion is in perfect harmony with that of Jen- 

 nings regarding the orientation of protozoa. The only 

 difference between the orienting reactions in the two classes 

 of animals mentioned is that the unicellular forms studied 

 by Jennings turn in different directions by means of the 

 avoiding reaction, i.e., they always turn toward a struc- 

 turally defined side, while the metazoa investigated by 

 Holmes are not thus limited in their direction of turning. 

 Not all protozoa however are limited in the direction of 

 turning. Lacrymaria olar, for example, swings its long 

 anterior proboscis-like appendage about in all directions 

 and there appears to be no limitation set to the direction 

 in which it may turn. 



Holmes contrasts the random movements w^ith forced 

 reflexes, and characterizes the former as " elements of 

 spontaneous, undirected activity." This statement natu- 

 rally leads to the conclusion that the direction of motion 

 in random movements is not definitely determined. It is 

 however hardly probable that Holmes intends to convey 

 such an idea, for it is undoubtedly true that the direction 

 in random movements is as definitely and absolutely deter- 

 mined as it is in the avoiding reaction or in forced reflexes. 

 The difference is merely that the factors involved are 

 different in the different methods of reaction. 



3. Summary of Historical Review 



(i) During the early periods of civilized man all living 

 things were held to be endowed with a soul which was 

 responsible for all activity. 



(2) Mechanical explanations of activity received but 

 little attention until early in the seventeenth century, the 

 period of Harvey, Descartes and Borelli. 



