33 6 Adaptation to Environment 



perature they could render them resistant to a tem- 

 perature of 39 . The remarkable fact was that fish 

 if once made resistant to a high temperature (35 ) did 

 not lose this resistance when kept for four weeks at 

 from io° to 14 C. Control fish taken from the same 

 temperature died in from two to four minutes; im- 

 munized fish taken from io° and put directly to 35 C. 

 lived for many hours or indefinitely. They will even 

 retain this immunity when kept for two weeks at a 

 temperature of 0.4 C. 



Why is it that an animal can in general resist a high 

 temperature better if the latter is raised gradually 

 than when it is raised suddenly? Physics offers us 

 an analogy to this phenomenon in the experience that 

 glass vessels which burst easily when their temperature 

 is raised suddenly, remain intact when the temperature 

 is raised gradually. Glass is a poor conductor of heat 

 and when the temperature is raised suddenly inside a 

 glass cylinder the inner layer of the cylinder expands 

 while the outer layer on account of the slowness of 

 conduction of heat does not expand equally and the 

 cylinder may burst. We might assume that the sud- 

 den increase in temperature brings about certain changes 

 in the cells (e. g., an increase in permeability or destruc- 

 tion of the surface layer?). If the rise of temperature 

 occurs gradually the blood or lymph or the cell sap 

 may have time to repair the damage, and this repair 

 seems to be irreversible, at least for some time, as the 



