Adaptation to Environment 331 



seen that the fish is naturally immune to a 9/8 m solu- 

 tion. It is also naturally immune to a 10/8 m or 1 1/8 m 

 solution if we give it time to compensate the injurious 

 effects of a 10/8 m solution by the repairing action of 

 its blood or kidneys. Beyond this no rise is possible. 

 In reality adaptation does not exist in this case. 



In former experiments the writer had shown that a 

 pure NaCl solution of that concentration in which this 

 fish naturally lives kills it very rapidly, while it lives 

 in such a solution indefinitely if a little CaCl 2 is added. 

 The explanation of this fact is that the pure NaCl solu- 

 tion is able to diffuse into the tissues of the animal 

 while the addition of a trace of CaCl 2 renders the mem- 

 brane practically impermeable to NaCl. The question 

 then arose whether it was possible to make the fish more 

 resistant to a pure NaCl solution of sufficiently high 

 concentration and how this could be done. On the basis 

 of the idea of an adaptive effect of the environment we 

 should expect that by gradually raising the concentra- 

 tion of a pure NaCl solution the latter would gradually 

 alter the animal and 'make it more resistant. The 

 method of procedure suggested was therefore to put 

 the fish first in low and gradually into increasing con- 

 centrations of NaCl. This method was tried and found 

 futile for the purpose. Fundulus when put from sea 

 water (after having been washed) into a 6/8 m NaCl 

 solution die in about four hours. When kept previ- 

 ously in a weaker NaCl solution they die if anything 



