152 ANIMAL LIFE AND SOCIAL GROWTH 



in the deep shadow of an overhanging brook- 

 bank vibrate in unison their pea-sized bodies on 

 their long thread-like legs. The initiation of the 

 vibration appears to come from some part of the 

 aggregation which is excited by the near approach 

 of a stimulating object such as the finger of an 

 inquisitive naturalist. Now one, now another 

 part of the aggregation may assume the lead in 

 setting up and maintaining the vibration rhythm. 

 The stimulus to vibration in this instance is 

 passed from body to body through the interlocking 

 legs of the harvestmen. 



With animals somewhat higher in the evolu- 

 tionary scale than the Amoeba or jellyfishes, 

 one end becomes set apart as the anterior region. 

 This end meets the new and stimulating elements 

 of the environment and comes to carry the greatest 

 localization of sense-organs and of nervous tissue. 

 In other words it becomes the head of the animal. 

 For some reason the head develops at the end of 

 the animal which has the highest rate of metabo- 

 lism and as a consequence of its structure and 

 physiological activity, it is the dominating region 

 of the animal. With the evolution of a definite 

 head, individual animals have developed definite 

 and localized leadership for the rest of the body. 



Similar localized leadership frequently occurs in 

 groups of wild or of domesticated animals. In 



