Nevertheless it can be shown that the results at which we arrived in our experiments on _Fundulus_ are of a general application. Osterhout[268] has shown that plants which grow in the soil or in fresh water are readily killed by a pure NaCl solution of a certain concentration, while they can resist the same concentration of NaCl if some CaCl₂ is added. Wo. Ostwald[269] has shown the same for a species of _Daphnia_. We, therefore, come to the conclusion that the injurious action following an alteration in the constitution of the sea water is in some of the cases due to an increase in the permeability of the membranes of the cell, whereby substances can diffuse into the cell which when the proper balance prevails cannot diffuse. For this balance the ratio of the concentration of the salts with univalent cation Na and K over those with bivalent cation Ca and Mg C_{Na+K salts}/C_{Ca+Mg salts} is of the greatest importance. [268] Osterhout, W. J. V., _Bot. Gazette_, 1906, xlii., 127; 1907, xliv., 257; _Jour. Biol., Chem._, 1906, i., 363. [269] Ostwald, Wo., _Arch. f. d. ges. Physiol._, 1905, cvi., 568. 6. The importance of this quotient appears in the so-called “behaviour” of marine animals. We have mentioned the newly hatched larvæ of the barnacle in connection with heliotropism. These larvæ swim in a trough of normal sea water at the surface, being either strongly positively or negatively heliotropic. They collect as a rule in two dense clusters, one at the window and one at the room side of the dish. If such animals are put into a solution of NaCl+KCl (in the proportion in which these salts exist in the sea water), they will fall to the bottom unable to rise to the surface. They will, however, rise to the surface and swim energetically to or from the window if a certain quantity of any of the chlorides of a bivalent metal, Mg, Ca, or Sr, is added, but these movements will last only a few minutes when only one of these three salts is added; and then the animals will fall to the bottom again. If, however, two salts, _e. g._, MgCl₂ and CaCl₂, are added the animals will stay permanently at the surface and react to light as they would have done in normal sea water. These animals also can resist comparatively large changes in the concentration of the sea water, and it seemed of interest to find out whether the quotient C_{NaCl+KCl}/C_{MgCl₂+CaCl₂}, which just allowed all the animals to swim at the surface, had a constant value. The MgCl₂+CaCl₂ solution was 3/8 m and contained the two metals in the proportion in which they exist in the sea water; namely, 11.8 molecules MgCl₂ to 1.5 molecules CaCl₂. The next table gives the result.[270] Since these experiments lasted a day or more each, usually two different concentrations of NaCl+KCl of the ratio 1:2 or 1:4 were compared in one experiment. [270] Loeb, J., _Jour. Biol. Chem._, 1915, xxiii., 423. TABLE XVIII