We may instance, as a peculiarly interesting fact in the psychic life of the unicellular radiolaria, the extraordinary power of memory in them. The relative constancy with which the 4,000 species transmit the orderly and often very complex form of their protective flinty structure from generation to generation can only be explained by admitting in the builders, the invisible plasma-molecules of the pseudopodia, a fine "plastic sense of distance," and a tenacious recollection of the architectural power of their fathers. The fine, formless plasma-threads are always building afresh the same delicate flinty shells with an artistic trellis-work, and with protective radiating needles and supports always at the same points of their surface. The physiologist, Ewald Hering (of Leipsic), had spoken in 1870 of memory as "a general function of organised matter." I myself had tried to explain the molecular features of heredity by the memory of the plasma-molecules, in my essay on "The Perigenesis of the Plastidules" (1875). Recently one of the ablest of my pupils, Professor Richard Semon (of Munich, 1904), made a profound study of "Mneme as the principle of constancy in the changes of organic phenomena," and reduced the mechanical process of reproduction to a purely physiological base. From the cell-soul and its memory in the radiolaria and other unicellular protists, we pass directly to the similar phenomenon in the ovum, the unicellular starting-point of the individual life, from which the complex multicellular frame of all the histona, or tissue-forming animals and plants, is developed. Even the human organism is at first a simple nucleated globule of plasm, about 1/125 inch in diameter, barely visible to the naked eye as a tiny point. This stem-cell (_cytula_) is formed at the moment when the ovum is fertilised, or mingled with the small male spermatozoon. The ovum transmits to the child by heredity the personal traits of the mother, the sperm-cell those of the father; and this hereditary transmission extends to the finest characteristics of the soul as well as of the body. The modern research as to heredity, which occupies so much space now in biological literature, but was only started by Darwin in 1859, is directed immediately to the visible material processes of impregnation.