1. The term environment in relation to an organism may easily assume a mystic rôle if we assume that it can modify the organisms so that they become adapted to its peculiarities. Such ideas are difficult to comprehend from a physicochemical viewpoint, according to which environment cannot affect the living organism and non-living matter in essentially different ways. Of course we know that proteins will as a rule coagulate at temperatures far below the boiling point of water and that no life is conceivable for any length of time at temperatures above 100° C., but heat coagulation of proteins occurs as well in the test-tube as in the living organism. If we substitute for the indefinite term environment the individual physical and chemical forces which constitute environment it is possible to show that the influence of each of these forces upon the organism finds its expression in simple physicochemical laws and that there is no need to introduce any other considerations. We select for our discussion first the most influential of external conditions, namely temperature. The reader knows that there is a lower as well as an upper temperature limit for life. Setchell has ascertained that in hot springs whose temperature is 43° C., or above, no animals or green alga are found.[247] In hot springs whose temperature is above 43° he found only the _Cyanophyceæ_, whose structure is more closely related to that of the bacteria than to that of the algæ, inasmuch as they have neither definitely differentiated nuclei nor chromophores. The highest temperature at which _Cyanophyceæ_ occurred was 63° C. Not all the _Cyanophyceæ_ were able to stand temperatures above 43° C., but only a few species. The other _Cyanophyceæ_ were found at a temperature below 40° C., and were no more able to stand higher temperatures than the real algæ or animals. The _Cyanophyceæ_ of the hot springs were as a rule killed by a temperature of 73°. From this we must conclude that they contain proteins whose coagulation temperature lies above that of animals and green plants, and may be as high as 73°. Among the fungi many forms can resist a temperature above 43° or 45°; the spores can generally stand a higher temperature than the vegetative organs. Duclaux found that certain bacilli (Tyrothrix) found in cheese are killed in one minute at a temperature of from 80° to 90°; while for the spores of the same bacillus a temperature of from 105° to 120° was required.[248] [247] Setchell, W. A., _Science_, 1903, xxvii., 934. [248] Duclaux, E., _Traité de microbiol._, 1898, i., 280.