56 THE SOCIAL LIFE OF ANIMALS 



idea that the other was in Europe. With no pre- 

 arrangement, what is the probability that an Ameri- 

 can Negro from Chicago will meet his aunt in the 

 Louvre? Yet it did happen this once without in any 

 way shaking the probability principle. 



Perhaps the digression is not so great as might ap- 

 pear at first glance, for we need a slight common 

 understanding of the practical working of statistical 

 probability; all of modern science, the more as well 

 as the less exact, is built on it. 



To get back to our goldfish: those in the groups 

 of ten lived decidedly longer than their fellows ex- 

 posed singly to the same amount of the same poison; 

 and significantly so. But why? Others had made that 

 experiment w^ith smaller animals, and had decided 

 that the group gave off a mutually protective secre- 

 tion which would protect that particular species and 

 none other. One reason that we were working with 

 goldfish was because they are large enough so that 

 we could use approved methods of chemical analysis 

 in finding where the silver went. The balance sheet 

 from such tests showed that we could account for 

 all the silver present. With the suspensions which 

 had held ten fish the silver was almost all precipi- 

 tated, while in the beakers that had held but one fish 

 almost all the silver was still suspended. 



When exposed to the toxic colloidal silver the 



