THE HIGHER SOCIAL LEVELS 151 



organ-bearing radius of the animal. These differ- 

 ent sense-organs act to originate nervous impulses 

 which cause the rhythmical pulsations by means 

 of which the jellyfish moves. Any one of these 

 marginal sense-organs may temporarily become 

 dominant over all the others and so control the 

 rate of rhythmic pulsations. The dominant 

 organ is the one which is acting most rapidly at 

 the time; it becomes the pace setter for the whole 

 pulsating region of the animal's body. When it 

 slows down, another of the potentially equal 

 sense-organs with a higher rate of action becomes 

 in turn the pace-setter and hence is the leader in 

 the pulsations. 



With animal aggregations a similar degree of 

 development of leadership can be recognized. 

 For example, there are aggregations of fire-flies 

 that flash in unison. In one such lot which occu- 

 pied a valley near Ithaca, N. Y., the observer 

 was able to recognize the pace-setting center of 

 flash origin for the fire-flies of the whole valley and 

 by using his pocket flashlight, was able himself 

 to assume control of the rate of flashing, as shown 

 by his ability to speed the flash-rate above the 

 normal period shown when the fire-flies were left 

 to their own pace. 



Similarly, the great aggregations of harvestmen 

 often called daddy-long-legs, which may be found 



