At the same time it is evident that a complete series of observations of the infant can hardly be carried out by a man alone. It is for the mother, or some other woman with a pass-key to the nursery, with her frequent and prolonged opportunities of observation to attempt a careful and methodical register of mental progress. Hence the importance of enlisting the mother or her female representative as collaborateur or at least as assistant. Thus supposing the father is bent on ascertaining the exact dates and the order of appearance of the different articulate sounds, which is rather a subject of passive observation than of active experiment; he will be almost compelled to call in the aid of one who has the considerable advantage of passing a good part of each day near the child.[7] ----- Footnote 7: The great advantage which the female observer of the infant’s mind has over her male competitor is clearly illustrated in some recent studies of childhood by American women. I would especially call attention to a study by Miss M. W. Shinn of the University of California (_Development of a child. Notes on the writer’s niece_), where the minute and painstaking record (_e.g._, of the child’s colour discrimination and visual space exploration) points to the ample opportunity of observation which comes more readily to women. ----- As the wee thing grows and its nervous system becomes more stable and robust more in the way of research may of course be safely attempted. In this higher stage the work of observation will be less simple and involve more of special psychological knowledge. It is a comparatively easy thing to say whether the sudden approach of an object to the eye of a baby a week or so old calls forth the reflex known as blinking: it is a much more difficult thing to say what are the preferences of a child of twelve months in the matter of simple forms, or even colours. The problem of the order of development of the colour-sense in children looks at first easy enough. Any mother, it may be thought, can say which colours the child first recognises by naming them when seen, or picking them out when another names them. Yet simple as it looks, the problem is in reality anything but simple. A German investigator, Professor Preyer of Berlin, went to work methodically with his little boy of two years in order to see in what order he would discriminate colours. Two colours, red and green, were first shown, the name added to each, and the child then asked: “Which is red?” “Which is green?” Then other colours were added and the experiments repeated. According to these researches this particular child first acquired a clear discriminative awareness of yellow. Preyer’s results have not, however, been confirmed by other investigators, as M. Binet of Paris, who followed a similar method of inquiry. Thus according to Binet it is not yellow but blue which carries the day in the competition for the child’s preferential recognition.