In his "Anthropogenie" (1874), Haeckel says: "The soul, or 'psyche' of man has evolved, as function of the cerebro-spinal nerve-chord simultaneously with the latter, and just as, even yet, brain and spinal column develop from the simple nerve-chord, so the human mind, or the soul-activity of the whole human race, has evolved, gradually and step by step, from the lower vertebrate soul. 'Spirit' and 'soul' are only higher and combined or differentiated powers of the same function which we designate with the general expression 'force.'"[34] In his essay on "Cell-souls and Soul-cells" (1878), Haeckel attributes to all animals the possession of soul, and adds that "we cannot wholly deny a soul to the plants also." The possession of soul he defines as the "capacity of sensibility in the organism to excitations of various sorts, and of reaction upon these excitations with certain movements." "This uniform character of protoplasm gifted with soul permits us the hypothesis that the ultimate factors of the soul-life are the plastidules, the invisible, homogeneous, elemental particles, or molecules, of protoplasm, which, in limitless multiplicity, compose the unnumbered cells." The soul connected with the higher developments of brain and spinal column is likewise a higher development, and differs from the soul connected with the uncentralized organization of lower species. In the latest edition of his "Natuerliche Schoepfungsgeschichte" (1889), he further asserts that all matter is possessed of soul, and that "the antithesis which we have assumed between living and dead nature does not exist. When a stone, thrown into the air, falls to the earth according to fixed laws, or when a crystal is formed in a solution of salts, or when sulphur and quicksilver combine to form cinnabar, these phenomena are not more and not less mechanical phenomena of life than the growth and bloom of the plants, than the propagation and sense-activity of animals, than the perception and thought-processes of human beings."[35] And both in this work and in his "Anthropogenie" he quotes the words of Goethe, that "matter can never exist and act without soul, the soul can never exist and act without matter." This last statement is, however, rather a metaphysical one, in distinction from Haeckel's other statements on this subject, which are properly naturalistic.