The human soul—whatever we may hold as to its nature—undergoes a continual development throughout the life of the individual. This ontogenetic fact is of fundamental importance in our monistic psychology, though the “professional” psychologists pay little or no attention to it. Since the embryology of the individual is, on Baer's principle—and in accordance with the universal belief of modern biologists—the “true torch-bearer for all research into the organic body,” it will afford us a reliable light on the momentous problems of its psychic activity.

Although, however, this “embryology of the soul” is so important and interesting, it has hitherto met with the consideration it deserves only within a very narrow circle. Until recently teachers were almost the only ones to occupy themselves with a part of the problem; since their avocation compelled them to assist and supervise the formation of the psychic activity in the child, they were bound to take a theoretical interest, also, in the psychogenetic facts that came under their notice.