Parents, accordingly, will make no mistake in regarding any persistent manifestation of jealousy in their children as--like sulkiness--a danger-signal of real urgency and as indicating a special need for careful upbringing. Also, they should not be surprised if jealousy begins to show itself at an extremely early age. Some instances are on record of its appearance before the end of the first year. The naturalist Darwin noted its presence in his son at the age of fifteen and a half months. Arnold L. Gesell, one of the few scientists to make any extended research of jealousy, found that "infants will variously hold out their arms, fret, whine, or burst into violent crying, cover their face with their hands, or sulk, when their mothers caress or hold another baby." From the end of the second year jealousy is much in evidence, and is most variously motivated. Commonest of all, perhaps, is the jealousy occasioned by the advent of a little brother or sister, who is looked upon as a rival for the parents' affections. Or jealousy may be felt against one of the parents, little boys being frequently jealous of their fathers, and little girls of their mothers. Seemingly, they are unable to tolerate the love their parents feel for each other and would monopolise the affection of the parent of whom they are fonder. Again, there may be jealousy, sometimes of a violent sort, with regard to material possessions. Greatly to the profit of toy-makers, innumerable children have broken their toys to pieces in jealous rage at another child having been allowed to play with them. So, too, there may be jealousy with regard to food. A child will often eat food of which he is not really desirous, rather than see another gain pleasure from it. As the child grows older, other objects and situations cause in him the unpleasant reaction of jealousy. On this point--the shifting causes of jealousy, through later childhood into adolescence--I cannot do better than quote at some length the findings of Professor M. V. O'Shea, of the University of Wisconsin, as given in his "Social Development and Education," a book of great value to parents and teachers: