Meanwhile, however, the conception of life as something material had been discarded by Paracelsus for the belief that the soul, or as he called it, the ‘Archaeus’, by which the chemical processes of the body are governed, is not a material but a spiritual force, a view restated by Stahl more than a hundred years afterwards. ‘The events of the body’, says this author, ‘may be rough-hewn by chemical and physical forces, but the soul will shape them to its own ends, and will do that by its instrument, motion.’ This, of course, is vitalism, and vitalism in its extreme or ‘animistic’ form. The idea recurs later on in the biology of Treviranus. To be living is to have a soul, he tells us, and the conscious _Lebenskraft_ employs the forces of the material world to form the organism. ‘Das Weitzenkorn hat allerdings Bewusstsein dessen, was in ihm ist und aus ihm werden kann, und träumt wirklich davon.’ Though he adds quaintly enough, ‘Sein Bewusstsein und seine Träume mögen dunkel genug sein’. It is curious to observe the revival, at the beginning of the twentieth century, of this mediaeval mysticism in the speculative writings of so accomplished an experimentalist as Hans Driesch. Driesch is an embryologist who in his earlier days had enunciated an invaluable analytical theory of development, a theory which suggests that while the formation of the first or elementary organs that appear in the embryo or larva--such structures as the larval gut or sense-organ, or the germ-layers--depends upon the presence in the germ of certain specific organ-forming substances (and this is a fact which has since been abundantly demonstrated by experiment), the origin of parts that appear later in development may be accounted for by the action of the first-formed structures upon one another, these actions being in the nature of physiological responses to stimuli; and for this also some evidence has been produced. On this view differentiation is a mechanical process, set in motion by fertilization or some other cause, and, given a certain initial structure of the germ or ovum, given the presence in it of a certain number of parts or substances capable of acting upon one another with a fixed co-ordination or harmony of the stimuli and the responses, given further a proper constitution of the external environment, then a definite result must follow, the production of an organism which is like the parents that gave it birth. But in his later treatises this hypothesis has been repudiated, and, by a remarkable _volte-face_, replaced by a dogma of a wholly different kind. For now it is urged that no merely material factors can possibly account either for the harmony of development--the due co-ordination of mutually reacting parts; or for the secondary harmony of composition--the formation of complex organs by the union of tissues; or for the functional harmony seen in the activities of the adult.