The comparative psychology of man and the higher animals enables us to learn from the highest group of the placentals, the primates, the long strides by which the human soul has advanced beyond the _psyche_ of the anthropoid ape. The phylogeny of the mammals and of the lower vertebrates acquaints us with the long series of the earlier ancestors of the primates which have arisen within this stem since the Silurian age. All these vertebrates agree in the structure and development of their characteristic psychic organ--the spinal cord. We learn from the comparative anatomy of the vermalia that this spinal cord has been evolved from a dorsal _acroganglion_, or vertical brain, of an invertebrate ancestor. We learn, further, from comparative ontogeny that this simple psychic organ has been evolved from the stratum of cells in the outer germinal layer, the ectoderm, of the platodes. In these earliest flat-worms, which have no specialized nervous system, the outer skin-covering serves as a general sensitive and psychic organ. Finally, comparative embryology teaches us that these simple metazoa have arisen by gastrulation from blastæades, from hollow spheres, the wall of which is merely one simple layer of cells, the _blastoderm_; and the same science, with the aid of the biogenetic law, explains how these protozoic coenobia originally sprang from the simplest unicellular organisms. On a critical study of these different embryonic formations, the evolution of which from each other we can directly observe under the microscope, we arrive, by means of the great law of biogeny, at a series of most important conclusions as to the chief stages in the development of our psychic life. We may distinguish eight of these to begin with: I. Unicellular protozoa with a simple cell-soul: the infusoria. II. Multicellular protozoa with a communal soul: the catallacta. III. The earliest metazoa with an epithelial soul: the platodes. IV. Invertebrate ancestors with a simple vertical brain: the vermalia. V. Vertebrates without skull or brain, with a simple spinal cord: the acrania. VI. Animals with skull and brain (of five vesicles): the craniota. VII. Mammals with predominant development of the cortex of the brain: the placentals. VIII. The higher anthropoid apes and man, with organs of thought (in the cerebrum): the anthropomorpha. Among these eight stages in the development of the human soul we may further distinguish more or less clearly a number of subordinate stages. Naturally, however, in reconstructing them we have to fall back on the same defective evidence of empirical psychology which the comparative anatomy and physiology of the actual fauna affords us. As the craniote animals of the sixth stage--and these are true fishes--are already found fossilized in the Silurian system, we are forced to assume that the five preceding series of ancestors (which were incapable of fossilization) were evolved in an earlier, pre-Silurian age.