sibly be entirely based on mechanics, nor can we meet the difficulty by substituting for "mechanics" the highest physics and chemistry, including consummate molecular mechanics. For physics treats only of phenomena dependent on motion—that is, changes in space, the forces, the causes of motion, and the causes of changes in motion, as formulated by human intelligence, are its proper domain. Chemistry, which ascribes to Nature one single force—that of affinity—because it does not need any other, deals with substances alone, with the various elements of matter. Processes which can not be attributed to either changes of forces, i. e., change of one force into another, or to changes of substances, i. e., the transformation of the latter through separation and combination, are not physical and not chemical; consequently, neither physics nor chemistry is engaged in their investigation. Such processes, however, take place in animate bodies—the feeling of hunger, for instance, which the most thorough physical and chemical inquiry fails to explain, though its necessary conditions and consequences may be ascertained.
During the fertilization and division of the ovum, during the differentiation of embryonic forms, during the gradual transformation of fetal and aimless into functionary movements of muscles and nerves, and during the perfection of the organs of sense and of the nerve-centers after birth, a series of processes occur which are entirely outside the domain of both the physicist and the chemist. They are not called upon to deal with the problems of heredity and psychogenesis, as they do not present themselves in the physical and chemical world.
For the very reason that there is one world only, all dualistic systems are untenable. The mechanical theory asserts, though illogically, that, by means of its one-sided principles, all things will in time be understood. The dualistic or vitalistic theory starts by assuming the existence of a contrast in the world, the forces in the living body being different from those in the crystal, those in the brain not the same as those in the material of which the brain is composed, those in young protoplasm not the same as in the old. Now it devolves upon physiology to show how to eliminate this "vitalism" which encumbers its path. Physiological inquiry must attach more importance to the conception of evolution. Morphology, in applying the method of evolution in all its departments, has gained a higher repute than ever. The anatomical phylogenetic history of man, in connection with his embryological and later history of development, may indeed furnish an explanation of the marvelous fitness and natural evolution of the human body.
It is, therefore, surprising that in that domain of the science treating of living bodies, in physiology, or the science of the functions, it hitherto has not been applied at all, or only occasionally and reluctantly. The neglect is due partly to the erroneous opinion that not the physiological function, but only its substratum the bodily organ, is
