rather the resultant of the conditions given in the chemo-physical predispositions of the organism itself, and in the architecture of its parts, _plus_ the stimulus. Directly associated with this property of irritability is another form of spontaneity and freedom in living beings—the power of adapting themselves to changed conditions of existence. Some do not show this at all, while others show it in an astonishing degree, helping themselves out by new contrivances, so to speak. Thus the organism may protect itself against temperature and other influences, against injury, making damages good again by self-repairing processes, “regenerating” lost organs, and sometimes even building up the whole organism anew from amputated parts. The mechanical interpretation must here proceed in the same way as in dealing with the question of stimuli, applying to the development of form the same explanations as are there employed. And just because this domain does not lend itself readily to mechanical explanation, we can understand that confidence in the sufficiency of this mode of interpretation grows rapidly with each fresh conquest, when this or that particular process is shown to be actually explicable on mechanical principles. Processes of development or morphogenesis—which are among the most intricate and difficult—are attacked in various ways. The processes of regeneration, for instance, are compared with the similar tendencies observed in crystals, which when they are injured have the capacity of restoring their normal form. This capacity therefore obtains in the realm of the inorganic as well as among organisms, and is referred to the tendency of all substances to maintain a definite state of equilibrium, conditioned by their form, and, if that is disturbed, to return to a similar or a new state of equilibrium. Or, the procedure may be to reduce the processes of a developmental or morphogenetic category to processes of stimulation in general, and then it is believed, or even demonstrated, that chemo-physical analogies or explanations can be found for them.