33 Adaptation to Environment 



fish is naturally immune to a 9/8 m NaCl+KCl+CaCl 2 

 solution, by the method of slowly raising the concen- 

 tration it may be made to tolerate a 10/8 m or n/8 m 

 solution, but not more. These fish when once adapted 

 to a 10/8 m solution can be put suddenly into a very 

 weak solution, e. g., a m/8o NaCl, without suffering and 

 when brought back into a 10/8 m solution of NaCl+ 

 KCl+CaCl 2 they will continue to live. If they remain 

 for several days in the weak solution their power of 

 resistance to 10/8 m NaCl+KCl+CaCl 2 solution is 

 weakened. 



What change takes place when the fish is made more 

 resistant and why is its normal resistance so great? 

 The answer based on the wTiter's experiments seems 

 to be as follows: Fundulus is comparatively resistant 

 to sudden changes in the concentration of the sea water 

 between m/8o and 9/8 m because it possesses a com- 

 paratively impermeable skin whose permeability is 

 not seriously altered by sudden changes within these 

 limits of concentration; while if these limits are ex- 

 ceeded and the fish are brought suddenly into too high 

 a concentration the skin becomes permeable and the 

 fish dies, the gills becoming unfit for use or nerves 

 being injured by the salt which diffuses into the fish. 



The fact, that by slowly raising the concentration 

 to 10/8 m the fish may resist this limit, is in reality 

 no adaptation. There is no sharp limit between the 

 injurious and non-injurious concentration. We have 



