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.

We have seen that the wave of molecular disturbance in a living animal
tissue under stimulus is accompanied by a wave of electrical
disturbance; that in certain types of tissue the stimulated is
relatively positive to the less disturbed, while in others it is the
reverse; that it is essential to the obtaining of electric response to
have the contacts leading to the galvanometer unequally affected by
excitation; and finally that this is accomplished either (1) by
'injuring' one contact, so that the excitation produced there would be
relatively feeble, or (2) by introducing a perfect block between the two
contacts, so that the excitation reaches one and not the other.

Fur