PROTECTION FROM TOXIC REAGENTS 217 



different problem from that found in aquatic animals. Discussion 

 of experimentation in this connection will be reserved until chapter 

 xiv, which deals with aggregations and insect survival. 



GROUP PROTECTION FROM HIGH TEMPERATURES 



Not all adverse conditions in nature are the result of the presence 

 of toxic materials in aerial or watery solutions where the types of 

 protection we have just been considering might operate. One other 

 adverse condition is that furnished by the physical factor of high 

 temperature. Concerning the ratio between the mass of exposed 

 animals in relation to volume of medium with respect to this physi- 

 cal factor, Robertson (192 1) records observations upon the Aus- 

 tralian infusorian Enchelys farcimen. 



A temperature of 30° C. prevents subcultures of this protozoan 

 from multiplying, and the isolated individual almost inevitably dies. 

 But shade temperatures of 30° C. are known in pools of South 

 Australia in which wild Enchelys live. Wild infusorians brought 

 into the laboratory are similarly killed by such temperatures if 

 isolated; but if the culture shdes are populated by 20-30 individuals, 

 they can successfully resist exposure to temperatures of 33-34° C. 

 for as long as 7 days in succession without apparent injury or abnor- 

 mality. Also, a similar number of individuals put into fresh hay 

 infusion at this temperature survive and multiply, while isolated 

 individuals perish. 



Typhoid bacilli can be rendered abnormally susceptible to hydro- 

 gen peroxide by a degree of heat which is just sublethal; and the 

 growth inhibition so caused can be neutralized, as in bacteria natu- 

 rally susceptible to peroxide, by the accumulation of certain prod- 

 ucts of bacterial growth (Burnet, 1925a). These peroxide-neutraliz- 

 ing materials are more rapidly produced the larger the colony of 

 bacteria and the more vigorous their growth. This subject will re- 

 ceive further attention in chapter xvi. 



Drzewina and Bohn (1926, 1928) report similar results with sper- 

 matozoa. Although these will be considered at greater length else- 

 where, it should be stated that both of these observers interpret 

 the greater protection which they find to be given by the presence 



