Why is it that an animal can in general resist a high temperature better if the latter is raised gradually than when it is raised suddenly? Physics offers us an analogy to this phenomenon in the experience that glass vessels which burst easily when their temperature is raised suddenly, remain intact when the temperature is raised gradually. Glass is a poor conductor of heat and when the temperature is raised suddenly inside a glass cylinder the inner layer of the cylinder expands while the outer layer on account of the slowness of conduction of heat does not expand equally and the cylinder may burst. We might assume that the sudden increase in temperature brings about certain changes in the cells (_e. g._, an increase in permeability or destruction of the surface layer?). If the rise of temperature occurs gradually the blood or lymph or the cell sap may have time to repair the damage, and this repair seems to be irreversible, at least for some time, as the experiments on _Fundulus_ seem to indicate. If the temperature rises too rapidly the damage cannot be repaired quickly enough by the cell or body liquids. It is also to be considered that substances might be formed in the body at a higher temperature which do not exist at a lower temperature, and _vice versa_, and this might explain results like those of Schottelius or Dieudonné and many others. 6. The theory of an adapting effect of the environment has often been linked with the assumption of the inheritance of acquired characters. The older claims of the hereditary transmission of acquired characters, such as Brown-Séquard’s epilepsy in guinea pigs after the cutting of the sciatic nerve, have been shown to be unjustified or have found a different and more rational explanation. Recently P. Kammerer has claimed to have proven by new experiments that by environmental changes, hereditary changes can be produced.