It is to be expected that systems differing widely in composition and structure will differ in their responses to given conditions. This will be true whether the systems compared thus are organic, or inorganic, or representative of both groups. The compounds of carbon, of which living substance is so characteristically composed, exhibit properties and reactions that distinguish them at once in many respects from the compounds of lead or sulphur. They also differ widely among themselves; compare, in this connection, serum albumen, acetic acid, cane sugar, urea. No vitalistic factor is needed for the interpretation of divergencies of this kind. But there are many significant similarities between organisms and inorganic systems as well. These are so frequently overlooked that it will now be desirable to consider a few illustrative cases. For the sake of brevity, they have been selected as representative of but two types of adaptation commonly known under the names of ACCLIMATIZATION and REGULATION. Let us first consider the case of organisms which become acclimatized by slow degrees to new conditions that, suddenly imposed, would produce fatal results. Hydra is an organism which becomes thus acclimatized finally to solutions of strychnine too strong to be endured at first. Outwardly it appears to suffer in the process no obvious modifications. Yet modifications of a physiological order take place, as is shown, first, by the necessary deliberation of the acclimatization, second, by the death of the organism if transferred abruptly back to its original environment. In other forms the structural changes accompanying acclimatization may be far more conspicuous. For example, the aerial leaves of Limnophila heterophylla are dentate, while those grown under water are excessively divided. Again, the helmets and caudal spines of Hyalodaphnia vary greatly in length with the seasonal temperature. In these and the large number of similar cases that might be cited, stability of the physiological system under changed conditions is only obtained by changes in the system itself which are often exhibited by striking structural modifications. Compare with such phenomena of acclimatization the responses of sulphur, tin, liquid crystals and iron alloys to changes of temperature. The rhombic crystals that characterize sulphur at ordinary temperatures and pressures, give place to monoclinic crystals at 95.5 degrees C. Sulphur thus exists with two crystalline forms whose stability depends directly upon the temperature.