In one field quite remote from what is usually considered natural science, that of physiognomy and expression, Darwin’s observation is especially interesting, though of course he connected this line of research, as so many others, with his general scientific theory. His book on ‘The Expression of the Emotions’ is one of the most entertaining and profitable of all for the general reader, and it is instructive to note, how early, how persistently, and how faithfully he collected memoranda on this comparatively collateral issue. The use of what was immediately about him, of his own personal experience in daily living, is especially significant in this regard. For the study of expression he felt that unconsciousness in the subject was a prime requisite. Hence the study of infants, who were perfectly indifferent to your investigations, was peculiarly profitable, and almost from the moment his children were born, Darwin began to make notes on their expressions of pain and pleasure, all the little subtle indications of desire and need, which mothers use instinctively but which fathers are not commonly apt to register as scientific data. The curious paper, published in _Mind_, called ‘The Biography of a Child,’ gives many of Darwin’s notes on this subject, and repeated references in ‘The Expression of the Emotions’ show what fruitful use he made of those notes at a later period. The minuteness with which he observed and reflected is well shown in this passage on childrens’ crying: ‘I ought to have thought of crying children rubbing their eyes with their knuckles, but I did not think of it, and cannot explain it. As far as my memory serves, they do not do so whilst roaring, in which case compression would be of no use.... I wish I knew more about the knuckles and crying.’[17] The observation not only records the larger and more violent manifestations of passion, but is constantly on the watch for those trifling signs of feeling which appear and flit away in trivial social intercourse. Take this account of an animated conversation: ‘Another young lady and a youth, both in the highest spirits, were eagerly talking together with extraordinary rapidity; and I noticed that, as often as the young lady was beaten, and could not get out her words fast enough, her eyebrows went obliquely upwards, and rectangular furrows were formed on her forehead. She thus each time hoisted a flag of distress; and this she did half-a-dozen times in the course of a few minutes.’[18]