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 seen that the fish is naturally immune to a 9/8 m solution. It is also naturally immune to a 10/8 m or 11/8 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 CaCl₂ is added. The explanation of this fact is that the pure NaCl solution is able to diffuse into the tissues of the animal while the addition of a trace of CaCl₂ renders the membrane 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 concentration 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 concentrations 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 previously in a weaker NaCl solution they die if anything more quickly. But it is possible to make them live longer in a 6/8 m solution of NaCl; we have to proceed, however, by a method which is in contrast with the ideas of the adaptive influence of the environment. When the fish are first treated with sea water (or with a mixture of NaCl+KCl+CaCl₂) of a higher concentration so that they become adapted to a 10/8 m solution of NaCl+KCl+CaCl₂ or to 10/8 m sea water, they become also more resistant to an otherwise toxic solution of NaCl. Fish taken directly from sea water were killed in less than four hours when put into a 6/8 m NaCl solution, while fish of the same lot previously adapted to 10/8 m sea water in the manner described above lived two or three days in a 6/8 m NaCl solution.[282] [282] Loeb, J., _Biochem. Ztschr._, 1913, liii., 391. It is not impossible that it was the high concentration of calcium in the 10/8 m sea water which rendered the fish more immune to a subsequent treatment with NaCl. We know why a pure NaCl solution kills them and we also know why the addition of CaCl₂ protects them against this pernicious effect. It is rather strange that where the conditions of the experiments are clear we find nothing to indicate an adaptive effect of the environment.