This fish seemed a favourable object from which to find out how far an adaptation to the environment really existed; and the result was surprising. By changing the concentration of the sea water gradually it is possible to raise the natural resistance of the fish only a trifle, not much over ten per cent. The concentration of the natural sea water is a little over that of a m/2 solution of NaCl+KCl+CaCl₂ in the proportion in which these three salts exist in the sea water. When adult _Fundulus_ are put into a 10/8 m solution of NaCl+KCl+CaCl₂ in the proportion in which these salts occur in sea water they die in less than a day, but when put from sea water directly into a 8/8 m or 9/8 m solution they can live indefinitely. It was found[281] that if the concentration of the sea water was raised gradually (by m/8 a day) the fish on the fifth day could resist a 10/8 m solution of NaCl+KCl+CaCl₂ for a month (or possibly indefinitely; the experiment was discontinued after that period). When a 10/8 m solution was allowed to become more concentrated slowly by evaporation (at room temperature) all the fish died rapidly when the concentration was 12/8 m or even below. In higher concentrations they can live only a day or two. These experiments show that while the fish is naturally immune to a 9/8 m NaCl+KCl+CaCl₂ solution, by the method of slowly raising the concentration it may be made to tolerate a 10/8 m or 11/8 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/80 NaCl, without suffering and when brought back into a 10/8 m solution of NaCl+KCl+CaCl₂ 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+CaCl₂ solution is weakened. [281] Loeb, J., _Biochem. Ztschr._, 1913, liii., 391. What change takes place when the fish is made more resistant and why is its normal resistance so great? The answer based on the writer’s experiments seems to be as follows: _Fundulus_ is comparatively resistant to sudden changes in the concentration of the sea water between m/80 and 9/8 m because it possesses a comparatively impermeable skin whose permeability is not seriously altered by sudden changes within these limits of concentration; while if these limits are exceeded 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.