370 ELEMENTS OF BIOLOGY 



bristle (Fig. 219). It is unthinkable that ever before in all preceding 

 time the genus Hydra had ever been impaled in this fashion; its 

 response under these wholly unique and exceptionally severe cir- 

 cumstances constitutes a most extraordinary demonstration of the 

 ability of protoplasm to solve new situations as they are encoun- 

 tered. Such behavior also furnishes food for thought for the scientist 

 in his attempts to interpret the reactions of protoplasm in terms of 

 physical and chemical phenomena. 



The hydra does not accomplish this solution by moving directly 

 toward the only possible successful method but by a series of more 

 or less random trials. Similarly, in finding a path around an obstacle 

 a protozoon also makes a number of unsuccessful attempts. This 

 so-called "trial and error" method (Fig. 19) is characteristic of the 

 attempts of all animals to reduce the difficulties they encounter. A1 

 moment's thought will call to mind that the reader himself fre- 

 quently solves difficulties by trial and error. 



Behavior. Among the more complex animals the nervous sys- 

 tem together with the muscles and other effectors, that is to say, 

 receptor-transmittor-efifector systems, are the mechanisms of emer- 

 gency and temporary adjustment to the environment. The behavior 

 of an animal is the sum of its responses to stimuli. Attention has 

 been largely centred on motor responses and the whole subject of 

 animal behavior deals almost exclusively with muscle contractions 

 that transport the animal from place to place or cause the change in 

 position of a part of the body in response to some environmental 

 condition. 



Animal behavior exhibits two diflferent aspects, namely, responses 

 that are reflex, automatic, independent of experience, regular in oc- 

 currence, and always of the same sort when the stimuli are identical, 

 commonly called instincts; the second type of response is variable 

 and associated with real or imaginary experience. It is in some way 

 associated with, and controlled by that as yet undefined property of 

 the brain known as the mind. Controlled behavior as exhibited by 



