If we try to analyse such results as Fischer’s from a physicochemical point of view, we must realize that what we call life consists of a series of chemical reactions, which are connected in a catenary way; inasmuch as one reaction or group of reactions (_a_) (_e. g._, hydrolyses) causes or furnishes the material for a second reaction or group of reactions (_b_) (_e. g._, oxidations). We know that the temperature coefficient for physiological processes varies slightly at various parts of the scale; as a rule it is higher near 0° and lower near 30°. But we know also that the temperature coefficients do not vary equally for the various physiological processes. It is, therefore, to be expected that the temperature coefficients for the group of reactions of the type (_a_) will not be identical through the whole scale with the temperature coefficients for the reactions of the type (_b_). If therefore a certain substance is formed at the normal temperature of the animal in such quantities as are needed for the catenary reaction (_b_), it is not to be expected that this same perfect balance will be maintained for extremely high or extremely low temperatures; it is more probable that one group of reactions will exceed the other and thus produce aberrant chemical effects, which may underlie the colour aberrations observed by Fischer and other experimenters. It is important to notice that Fischer was also able to produce aberrations through the application of narcotics. Wolfgang Ostwald has produced experimentally, through variation of temperature, dimorphism of form in _Daphnia_. 5. Next or equal in importance with the temperature is the nature of the medium in which the cells are living. It has often been pointed out that the marine animals and the cells of the body of metazoic animals are surrounded by a medium of similar constitution, the sea water and the blood or lymph, both media being salt solutions differing in concentration but containing the three salts NaCl, KCl, and CaCl₂ in about the same relative concentration, namely 100 molecules NaCl: 2.2 molecules of KCl: 1.5 molecules of CaCl₂. This has suggested to some authors the poetical dream that our home was once the ocean, but we cannot test the idea since unfortunately we cannot experiment with the past. Plants, unicellular fresh-water algæ, and bacteria do not demand such a medium for their existence. Herbst had shown that when sea-urchin larvæ were raised in a medium in which only one of the constituents of the sea water was lacking (not only NaCl, KCl, or CaCl₂, but also Na₂SO₄, NaHCO₃, or Na₂HPO₄), the eggs could not develop into plutei; from which he concluded that every constituent of the sea water was necessary. This would indicate a case of extreme adaptation to all the minutiæ of the external medium.