[Sidenote: But the relations between parents and children must undergo profound modification as time passes] This is not to say however that the manifestations of this energy will not undergo considerable alteration as time passes. As children grow up and parents grow older, the former increase, the latter decrease in natural strength and ability of mind and body. In course of time therefore the attitude which parents and children naturally and reasonably adopt towards each other must gradually change to suit the varying conditions. At first children are dependent on the guidance and protection of their parents, who must make the necessary efforts to help and rear their offspring. Later on this differentiated relationship should give place to one in which parents and children are on equal terms. Finally, the original relationships may become to some extent reversed and, if parents and children are still within reach of one another, the former may come to look to the latter for some return of that help and protection that they themselves had previously afforded. [Sidenote: The care of the aged by their children] [Sidenote: is culturally very desirable] In this last situation, we see a form of the relationship, which appears to be peculiar to human society. Throughout the animal world and even in many primitive human communities there is no thought or care or tenderness devoted to old age. The increasing moralisation of human character (in which the relationship between parent and child has probably played a leading part) has brought it about that at least some degree of attention is given in all civilised societies to the needs--material and mental--of those who are no longer able fully to support themselves or to carry on their life without assistance. In any society in which the family is a permanent and firmly organised social unit, the duty of caring for the aged will naturally fall to some extent upon their children. This care of elderly, lonely or infirm parents by their children may perhaps legitimately be considered one of the most beautiful and touching expressions of specifically human morality--a point in which Man has definitely risen superior to the conditions of a brutal struggle for existence. As such it both deserves, and stands in need of, every encouragement and support which a developed and enlightened system of practical Ethics can afford. [Sidenote: though it has of necessity its limitations]