The theory of descent, combined with anthropological research, has convinced us of the descent of our human organism from a long series of animal ancestors by a slow and gradual transformation occupying many millions of years. Since, then, we cannot dissever man's psychic life from the rest of his vital functions--we are rather forced to a conviction of the natural evolution of our whole body and mind--it becomes one of the main tasks of the modern monistic psychology to trace the stages of the historical development of the soul of man from the soul of the brute. Our "phylogeny of the soul" seeks to attain this object; it may also, as a branch of general psychology, be called _phylogenetic_ psychology, or, in contradistinction to _biontic_ (individual), _phyletic psychogeny_. And, although this new science has scarcely been taken up in earnest yet, and most of the "professional" psychologists deny its very right to existence, we must claim for it the utmost importance and the deepest interest. For, in our opinion, it is its special province to solve for us the great enigma of the nature and origin of the human soul. The methods and paths which will lead us to the remote goal of a complete phylogenetic psychology--a goal that is still buried in the mists of the future, and almost imperceptible to many--do not differ from those of other branches of evolutionary research. Comparative anatomy, physiology, and ontogeny are of the first importance. Much support is given also by palæontology, for the order in which the fossil remains of the various classes of vertebrates succeed each other in the course of organic evolution reveals to us, to some extent, the gradual growth of their psychic power as well as their phyletic connection. We must admit that we are here, as we are in every branch of phylogenetic research, driven to the construction of a number of hypotheses in order to fill up the considerable lacunæ of empirical phylogeny. Yet these hypotheses cast so clear and significant a light on the chief stages of historical development that we are afforded a most gratifying insight into their entire course.