I begin with the simple, incontrovertible fact that the elements composing many of the simple structures, although they gravitate to each other, are generally mobile and therefore changeable in their mutual positions and proportion. We do not need a hypothesis either to conceive the nature of the impression or action of all the agents which these structures exhibit in their own action, or to understand how and why the impression is transmitted at different speeds along the nerves, muscle fibres and membranes to a section of cellulose material in order to set the whole of a part, and even many parts, into immediate, simultaneous action. The impression consists in the change of mutual position or proportion of the elements of living molecules as a result of any imperceptible impulse or attraction produced by stimuli. The speedy diffusion depends on the change in position or proportion of the elements of the first molecule so that it acts differently from the molecule adhering to it. And this action not only produces a similar or corresponding change in the elements of the adjacent molecule; it also changes the degree of its adherence. If, then, an interval elapses before the molecules return to their former state, an alternating contraction and dilation of volume is visible, as in the case of muscles, membranes and cellulose formations. On the other hand, if the molecules return instantaneously, the diffusion of the impression can be judged only from the corresponding sensation, or from the contraction and dilation produced in distant parts by the cause of the impression, as in the case of the nerves.

If we compare all the prominent properties of simple structures we find that just as the first three properties (hardness, elasticity and flexibility) are different gradations of a force dependent on the preponderant reciprocal gravitation of the elements, the other properties (sensibility, irritability, contractility and dilation) are different gradations of a force dependent on the preponderant mobility of the elements. In this case, however, the elements maintain their reciprocal gravitation not only by forming a solid structure but also by returning promptly to their former state.

The second force is exclusively present to living bodies for the duration of their life. The living animal fibres and molecules of an organic body differ only in the following respect from fibres and molecules that do not have, and have never had life: the first are easily impressionable and return promptly to their position after an impression, whether this is received directly from external bodies or indirectly from the impression of adjacent molecules; the others, which are forced to change the position and proportion of their elements (in which I said an impression consists) form new molecules, and do not return their elements to their former position and proportion.

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