62 THE SOCIAL LIFE OF ANIMALS 



physical protection from the ill effects of ultra-violet 

 light. Finally (on the extreme right) is diagramed 

 the fact that worms radiated and tested singly lived 

 only 62 per cent as long as those radiated in a group 



RAOIATCO RAOIArCO RADIATES RADIATEO RADIATED RADIATED 



CROWDED SINGLY CROWDED CROUPtO GROUPED SINGLY 



TESTED TESTED TESTED TESTED TESTED TESTED 



Singly singly Singly singly singly singly 



517' 517' 107' 



WELL WATER 



Fig. 4. Planarian worms survive exposure to ultra- 

 violet radiation better if much crowded while being 

 radiated, or even partially crowded, even though all are 

 isolated after a few minutes of irradiation. 



of 20 per 20 cubic centimeters and also tested singly. 

 Again the figures give the number of pairs tested 

 and under "P" the statistical probability, which 

 shows that all these must be taken seriously even 

 though there is decreasing significance as the per- 

 centage of difference of average survival time de- 

 creases. 



In the two cases just outlined mass protection has 

 been demonstrated, first against the presence of toxic 



