It must always be remembered that while the study of genius is highly interesting, and even, it is probable, not without significance for the general laws of heredity, we must not too hastily draw conclusions from it to bear on practical questions of eugenics. Genius is rare and abnormal; laws meant to apply to the general population must be based on a study of the general population. Vaerting, who is alive to the practical character which such problems are to-day assuming, realises how inadequate it is to confine our study to genius. Marro, in his valuable book on puberty, some years ago brought forward interesting data showing the result of the age of the parents on the moral and intellectual characters of school-children in North Italy. He found that children with fathers below twenty-six at their birth showed the maximum of bad conduct and the minimum of good; they also yielded the greatest proportion of children of irregular, troublesome, or lazy character, but not of really perverse children who were equally distributed among fathers of all ages. The largest number of cheerful children belonged to young fathers, while the children tended to become more melancholy with ascending age of the fathers. Young fathers produced the largest proportion of intelligent, as well as of troublesome children, but when the very exceptionally intelligent children were considered separately they were found to be more usually the offspring of elderly fathers. As regards the mothers, Marro found that the children of young mothers (under twenty-one) are superior, both as regards conduct and intelligence, though the more exceptionally intelligent children tended to belong to more mature mothers. When the parents were both in the same age-group the immature and the elderly groups tended to produce more children who were unsatisfactory, both as regards conduct and intelligence, than the intermediate group.[4]