1896.] “ A New Factor in Evolution.” 703 
likely to be pain. We are not certain what the appropriate 
form of stimulus is for the pain-nerves, but assuming it to be 
mechanical pressure, then any unusual stretching or tension, 
whether in the capillaries or the surrounding tissues, as caused 
by congestion, or from undue secretion of any of the glands, 
or from any other disorder, would perfectly explain the attend- 
ance of pain. That this should explain the characteristic 
pains of exhaustion, weakness, disease, and all other abnormal- 
ities, rather than the mere loss of general bodily strength, to 
which the common tradition more directly attributes them, no 
scientist should doubt. For, first, there is no evidence that 
mere weakness, independently of the physiological derange- 
ments which are the co-results of its cause, are at all painful. 
A man’may bleed to death, and suffer no pain. Again, a frail 
invalid may fade away with weakness, and suffer no trace of 
pain; indeed, may depart with gladness. Ora sprinter may 
drop with exhaustion and, perhaps, suffer no pain at all; or if 
any, none save what is unmistakably due to the abnormal dis- 
turbances of circulation already referred to. Secondly, all 
causes of weakness are likely to produce disorders which, in 
turn, shall produce disturbances likely to excite the pain- 
nerves in the way above indicated. This is so evident that 
it need not be discussed. Third, when so excited, even during 
general bodily weakness, there is still every evidence that the 
pain discharges are characteristically strong above other ner- 
vous activities, and relatively so proportionally to the lowering 
of the general level of strength. It would seem, therefore, that 
every known phenomena of pain, on the one hand, receives 
perfect explanation on the basis of pain-nerves, that every 
analogy demands such nerves, and that finally they have been 
conclusively demonstrated. And, on the other hand, it is 
strikingly manifest that every evidence we possess flatly contra- 
dicts the assumption that pain discharges are feeble. 
The corresponding assumption that the neural discharge of 
pleasure is “ excessive ” equally fails of corroboration when con- 
fronted with the facts. Here, again, we can measure the dis- 
charge only by its psychic accompaniment, its stimulus, and its 
motor effect. That pleasures, among psychic states, are charac- 
