In metals I have found exactly parallel instances. In tin, so little liable to fatigue, the top of the curve is horizontal or ascending; or it may exhibit a slight decline. But the record with platinum shows the rapid decline due to fatigue (fig. 73). [Illustration: FIG. 73 (_a_) The top of response-curve under continuous stimulation in tin is horizontal or ascending as in figure. (_b_) In platinum there is rapid decline owing to fatigue.] Taking any of these instances, say that in which fatigue is most prominent, it is found that short period of rest restores the original intensity of response. This affords additional proof of the fact that fatigue is due to overstrain, and that this strain, with its sign of attendant fatigue, disappears with time. #Staircase effect.#--We shall now discuss an effect which appears to be the direct opposite of fatigue. This is the curious phenomenon known to physiologists as 'the staircase' effect, in which successive uniform stimuli produce a series of increasing responses. This is seen under particular conditions in the response of certain muscles (fig. 74, _a_). It is also observed sometimes even in nerve, which otherwise, generally speaking, gives uniform responses. Of this effect, no satisfactory theory has as yet been offered. It is in direct contradiction to that theory which supposes that each stimulus is followed by dissimilation or break-down of the tissue, reducing its function below par. For in these cases the supposed dissimilation is followed not by a decrease but by an increase of functional activity. This 'staircase effect' I have shown to be occasionally exhibited by plants. I have also found it in metals. In the last chapter we have seen that a wire often falls, especially after resting for a long time, into a state of comparative sluggishness, and that this molecular inertness then gradually gives place to increased mobility under stimulation. As a consequence, an increased response is thus obtained. I give in fig. 74, _b_, a series of responses to uniform stimuli, exhibited by platinum which had been at rest for some time. This effect is very clearly shown here. So we see that in a substance which has previously been in a sluggish condition, stimulation confers increased mobility. Response thus reaches a maximum, but continued stimulation may afterwards produce overstrain, and the subsequent responses may then show a decline. This consideration will explain certain types of responses exhibited by muscles, where the first part of the series exhibits a staircase increase followed by declining responses of fatigue. [Illustration: FIG. 74.--'STAIRCASE' EFFECT (_a_) in muscle (after Engelmann). (_b_) in metal.]