68 THE SOCIAL LIFE OF ANIMALS 



were inoculated did steady and regular growth oc- 

 cur. With the goldfish spoken of earlier, the mass 

 protection was largely or wholly inoperative when 

 the group of ten was exposed to ten times the amount 

 of toxic colloidal silver to which a single fish was 

 exposed. With these bacteria, however, such quanti- 

 tative limitations did not hold; thirty organisms 

 were found to fix at least two hundred times the 

 amount of poison normally neutralized by an iso- 

 lated bacterium. This difference between the change 

 which thirty bacteria can effect together as compared 

 with what they can accomplish if isolated has been 

 called an expression of the communal activity of 

 bacteria. There is a fairly large and growing litera- 

 ture on this subject which indicates that when only 

 one or a few bacteria, even if strongly pathogenic, 

 gain access to the human body, they are likely to 

 be killed by various devices which aid in resisting 

 infection. It is fortunate for their victims that bac- 

 terial infections normally tend not to take unless 

 the inoculum is somewhat sizable or unless a smaller 

 dose is frequently repeated. 



Mass protection is known to occur among sper- 

 matozoa. Many animals, especially those that live in 

 the ocean, shed their eggs and spermatozoa into the 

 sea water, and fertilization takes place in that me- 

 dium. Dilute suspensions of such spermatozoa lose 



