At once the old questions arise. Are these processes fundamentally peculiar to the life of organisms? Does the capacity of the organism thus to adjust itself to its environment involve factors not found in the operations of inorganic nature? Our answers will be determined essentially by the nature of our interest in the organism--whether we regard its existence as the END or merely an incidental EFFECT of its activities. The first alternative is compatible with thoroughgoing vitalism. The second, emphasizing the nature of the processes rather than their usefulness to the organism, relieves biology of the embarrassments of vitalistic speculation, and allies it at the same time more intimately than ever with physics and chemistry. This alliance promises so well for the analysis of adaptations, as to demand our serious attention. Physiologically, the living organism may be thought of as a physico-chemical system of great complexity and peculiar composition which varies from organism to organism and from part to part. Life itself may be defined as a group of characteristic activities dependent upon the transformations in this system under appropriate conditions. According to this definition, life is determined not only by the physical and chemical attributes of the system, but by the fitness of its environment, which Henderson has recently done the important service of emphasizing.[1] Relatively trifling changes in the environment suffice to render it unfit, however, that is, to modify it beyond the limits of an organism's adaptability. The environmental limits are narrow, then, within which the transformations of the organic system can take place that are associated with adaptive reactions. The conditions within these limits are, further, peculiarly favorable for just such transformations in just such physico-chemical systems. [1] "The Fitness of the Environment." The essential characteristic of the adaptive reaction appears to be that the organism concerned responds to changing conditions without losing certain attributes of behavior by which we recognize organisms in general and by which that organism is recognized in particular. It exhibits stability in the midst of change; it retains its identity. But this stability, let us repeat, is the stability of a certain type of physico-chemical system, with respect to certain characters only, and exhibited under certain circumscribed conditions. In so far as the problem of adaptation is thus restricted in its application, it remains a question of standards, a taxonomic convenience, a problem of the organism by definition only, empty of fundamental significance.