BEGINNINGS OF CO-OPERATION 59 



ceptions to the rule, now outgrown, that crowding 

 is always harmful. 



The exploration of temperature relations is a 

 time-honored field. I prefer to take up a newer 

 though related area, that of the effects of ultra-violet 

 radiation, in which I shall present some evidence 

 so recently collected that it has never been reported 

 extensively before. A year ago Miss Janet Wilder 

 and I began exposing the common planarian worm 

 of this region to ultra-violet radiation, to find 

 whether there was any group protection from the 

 well-described lethal effect of ultra-violet light on 

 these worms. (12) 



In lots of twenty, worms of similar size and the 

 same history were placed together in a petri dish and 

 exposed to the action of the ultra-violet light long 

 enough so that they would disintegrate within the 

 next twelve hours. Half of them, that is, ten worms, 

 were then placed together in five cubic centimeters 

 of water and each of the other ten was put into five 

 cubic centimeters of similar water. Grouped and iso- 

 lated worms were treated alike in every way, except 

 that after irradiation together, half were grouped and 

 half were isolated. 



For one purpose or another we have repeated this 

 simple experiment a great many times with a variety 

 of waters, and with experimental conditions ade- 



