Thus living response in all its diverse manifestations is found to be only a repetition of responses seen in the inorganic. There is in it no element of mystery or caprice, such as we must admit to be applied in the assumption of a hypermechanical vital force, acting in contradiction or defiance of those physical laws that govern the world of matter. Nowhere in the entire range of these response-phenomena--inclusive as that is of metals, plants, and animals--do we detect any breach of continuity. In the study of processes apparently so complex as those of irritability, we must, of course, expect to be confronted with many difficulties. But if these are to be overcome, they, like others, must be faced, and their investigation patiently pursued, without the postulation of special forces whose convenient property it is to meet all emergencies in virtue of their vagueness. If, at least, we are ever to understand the intricate mechanism of the animal machine, it will be granted that we must cease to evade the problems it presents by the use of mere phrases which really explain nothing. We have seen that amongst the phenomena of response, there is no necessity for the assumption of vital force. They are, on the contrary, physico-chemical phenomena, susceptible of a physical inquiry as definite as any other in inorganic regions. Physiologists have taught us to read in the response-curves a history of the influence of various external agencies and conditions on the phenomenon of life. By these means we are able to trace the gradual diminution of responsiveness by fatigue, by extremes of heat and cold, its exaltation by stimulants, the arrest of the life-process by poison. The investigations which have just been described may possibly carry us one step further, proving to us that these things are determined, not by the play of an unknowable and arbitrary vital force, but by the working of laws that know no change, acting equally and uniformly throughout the organic and the inorganic worlds. FOOTNOTES: [21] Verworn, _General Physiology_, p. 18. INDEX Action current in metal, 88 in nerve, 8 in plant, 19 After-images and their revival, 177 After-oscillation in photo-sensitive cell, 159, 163 Anaesthetics, effect on response in nerve, 72 in plant, 30, 73, 74, 75 Annealing, effect on response in metal, 101, 138 Binocular alternation of vision, 175 Block method, advantages of, 28, 77 for obtaining response in metal, 82 in plant, 28 Chloral, effect on plant response, 75 Chloroform, effect on nerve response, 72 plant response, 74 Compensator, 22 Current of injury in nerve, 7 Curves, characteristics of response, 3 Death-point, determination of, in plants, 61, 63 Depressants, effect on inorganic response, 142 Depression, response by relative, 87 Dewar on retinal current, 149