We have seen that stimulus produces a certain excitatory change in living substances, and that the excitation produced sometimes expresses itself in a visible change of form, as seen in muscle; that in many other cases, however--as in nerve or retina--there is no visible alteration, but the disturbance produced by the stimulus exhibits itself in certain electrical changes, and that whereas the mechanical mode of response is limited in its application, this electrical form is universal. This irritability of the tissue, as shown in its capacity for response, electrical or mechanical, was found to depend on its physiological activity. Under certain conditions it could be converted from the responsive to an irresponsive state, either temporarily as by anaesthetics, or permanently as by poisons. When thus made permanently irresponsive by any means, the tissue was said to have been killed. We have seen further that from this observed fact--that a tissue when killed passes out of the state of responsiveness into that of irresponsiveness; and from a confusion of 'dead' things with inanimate matter, it has been tacitly assumed that inorganic substances, like dead animal tissues, must necessarily be irresponsive, or incapable of being excited by stimulus--an assumption which has been shown to be gratuitous. This 'unexplained conception of irritability became the starting-point,' to quote the words of Verworn,[21] 'of _vitalism_, which in its most complete form asserted a dualism of living and lifeless Nature.... The vitalists soon,' as he goes on to say, 'laid aside, more or less completely, mechanical and chemical explanations of vital phenomena, and introduced, as an explanatory principle, an all-controlling unknown and inscrutable "force hypermecanique." While chemical and physical forces are responsible for all phenomena in lifeless bodies, in living organisms this special force induces and rules all vital actions. 'Later vitalists, however, attempted no analysis of vital force; they employed it in a wholly mystical form as a convenient explanation of all sorts of vital phenomena.... In place of a real explanation a simple phrase such as "vital force" was satisfactory, and signified a mystical force belonging to organisms only. Thus it was easy to "explain" the most complex vital phenomena.' From this position, with its assumption of the super-physical character of response, it is clear that on the discovery of similar effects amongst inorganic substances, the necessity of theoretically maintaining such dualism in Nature must immediately fall to the ground. In the previous chapters I have shown that not the fact of response alone, but all those modifications in response which occur under various conditions, take place in plants and metals just as in animal tissues. It may now be well to make a general survey of these phenomena, as exhibited in the three classes of substances.