From the results of these carefully carried out scientific investigations, Weichardt has come to the conclusion that fatigue is due to a definite toxin, analogous to that of diphtheria and tetanus, and he believes that the explanation of the phenomena of training lies in the fact that in the body of the athlete there must be a specific “antibody,” which neutralizes the “fatigue toxin” as soon as it is formed. In the animals undergoing these experiments of extreme fatigue there was a fall of temperature. A practical use of this fact could be made for the individual, by noting the fact that a subnormal temperature was a grave danger-signal. Other observers concede that fatigue is due to chemical substances, produced in the body as the result of brain and muscle activity, and find that these toxins produce a depressing effect, especially on the muscular system, and that the sensation of fatigue is in large part the manifestation of this depression. The action of toxins is not confined to the tissues in which they arise; excessive activity of one tissue can cause fatigue of others. The facts of acid intoxication are noticed as analogous to fatigue phenomena, so far as the latter are due to toxic substances. As antidotes, only rest and sleep can be relied upon. Observations in the electric experiments on nerve-cells have shown a remarkable shrinking of the nerve-cells, and especially of their nuclei. After five hours’ continuous work, the cell nucleus was only half its normal size, and twenty-four hours of rest was necessary in order to restore it to its normal size, but half that amount of work does not require half that amount of time for its recovery. The mental symptoms of normal fatigue are loss of memory; the sense of perception is less acute; the association centers act less spontaneously and therefore slower; the vocabulary diminishes; the emotional tone is lowered; the attention is unstable and flickering. All these are marked symptoms that the individual is far below her best. All kinds of perceptions are more acute in the morning. Fatigue is not, therefore, merely physically uncomfortable; it is intellectually, physically, and morally dangerous. =Overwork.=—A surplus nervous energy must be persistently aimed at—what Emerson calls “plus health.” It must be indelibly impressed on the intelligence of every one that no fatigued individual can be at her best; she is doomed to do inferior work, to be mentally depressed, and to be morally weakened. Hope and courage ooze away, and all sense of proportions and perspective are lost. The amount of work that can be performed without fatigue is a matter of individuality, and the only safe gauge of overwork are the danger-signals sent out by nature—loss of appetite, insomnia, increasing exhaustion from day to day without increase in the amount of work done, mental depression, lack of interest and initiative. No one can afford to disregard these danger-signals.