1. “Stability-Lability A dimension of sensitivity to stimuli due to individual variations in autonomic nervous systems. Thus, a labile individual would be expected to react to a wider range of stimuli than a stable person” (Goldenson Robert M. (ed.), Longman dictionary of psychology and psychiatry (New York and London, 1984), 707).
2. The new English dictionary (Oxford, 1888), defines “knock up” as: “To overcome or make ill with fatigue; to exhaust, tire out.” Burkhardt Frederick, Smith Sydney (eds), The correspondence of Charles Darwin (Cambridge, 1985–; hereafter: Correspondence), i, 230; ii, 353. While I have previously explained Darwin's “violent fatigues” by the 1980 diagnosis of panic disorder (“To be an invalid, redux” (ref. 1), 211–13), I now believe that, although Darwin sometimes had the symptoms of this disorder, there were many occasions when his symptoms were either too few, or too numerous and too complicated, to fit the diagnosis of the disorder. Thus in this essay, instead of using the panic attack diagnosis, I have explained Darwin's “violent fatigues” as being different states of excitement, and my aims have been to describe the clinical symptoms and possible causations of each of these states.
3. Correspondence, i, 16, 22; Darwin Francis (ed.), The life and letters of Charles Darwin (New York, 1896; hereafter: Life and letters), i, 143.
4. Barlow Nora (ed.), The autobiography of Charles Darwin (London, 1958; hereafter: Autobiography), 44. Darwin cited his trembling after killing his first snipe as an example of the nervous system acting on the body independently of the will, or of habit, in his The expression of the emotions in man and animals (London, 1872: Third edn, London, 1998), 69–71, and footnotes on p. 71.