38 THE SIGNS OF LIFE [LECT. 



conformity with almost every series of observations we are able 

 to carry out on living matter. The familiar logarithmic curve 

 of sensation is concave to its abscissa, sensation Y increases by 

 diminishing increments for equal increments of stimulation X. 

 Muscular contraction, the negative variation of a nerve-current 

 (as I have shown previously), give the same type of curve within 

 at least a certain range of observation. And other cases might 

 be quoted illustrating the general law, that the response of living 

 matter to the excitation of its environment increases by dimish- 

 ing increments, giving a cause/effect curve that is concave towards 

 its abscissa. And we need not attempt to see whether or no 

 this curve can be neatly fitted with a logarithmic formula, for 

 the fit or the misfit of a curve, constructed from such a formula, 

 would not afford much information beyond what is to be learned 

 from inspection of the empirical curve, and comparison with 

 other curves. 



Moreover, one may convince himself at once by somewhat 

 closer observation, that the entire range of any given cause/ 

 effect curve cannot be fitted by a logarithmic formula. Above 

 the range of moderate stimulation, we have no further increment 

 of effect, but, on the contrary, a decrement, attributable to fatigue 

 or to shock, or to actual injury by excessive stimulation. And 

 below the range of moderate stimulation, the curve does not 

 spring suddenly from its abscissa and rise by increments diminish- 

 ing from the very outset, but it rises gradually from zero by 

 increments smallest at first then increasing, so that a first part 

 of the curve instead of concave is convex to the abscissa, and the 

 whole curve S-shaped, convex at first, concave at a higher range. 

 I have urged elsewhere that this S-shaped character of the 

 cause/effect curve is general, and that any well-studied case, 

 where we can plot physical cause along an abscissa and physio- 

 logical effect along ordinates, will probably be found to yield 

 such an S-shaped curve.* 



20. Ancesthetics. In conclusion of this chapter I will 

 briefly remark on the two last headings of the syllabus in 



* WALLER." On the Excitability of Nervous Matter." Presidential 

 Address to the Neurological Society, 1900. Brain, 1900. 



