THE PHYLOGENY OF THE SOUL. 153 



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 acro- 

 ganglion y 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 specialised 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 blastseades, 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 uni- 

 cellular 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. 



