76 THE SOCIAL LIFE OF ANIMALS 



They were. And the period after 1921 was en- 

 livened for some of us by denials from one first- 

 class laboratory after another that there was anything 

 significant in Robertson's data. Robertson himself 



ISOLATED PAIRED 



24 HOURS 20.5 92.4 



RATIO 1 2.2 



I 44 



Wf 



P = 0.0128 



Fig. 10. Robertson found that when two protozoans 

 were placed together each yielded over twice as many as 

 when the same number of similar protozoans were iso- 

 lated. 



rechecked and confirmed his results, though his ex- 

 planations of them tended to vary. For the moment 

 we are not concerned with the explanations; but 

 what are the facts? The first extensive corroboration 

 from outside Robertson's own laboratory came from 

 the work of Dr. Petersen at Chicago. When she cul- 

 tured the common Paramecium in small volumes of 

 liquid, she obtained the same results as had Rob- 

 ertson's critics, but when she used relatively larger 

 volumes of the same culture medium, a cubic cen- 



