As animals rise in the scale, as their brains become more subtle, more elaborate in structure, their actions become correspondingly more numerous and complicated, more varied, more individual. The nervous systems of such animals are characterised by an increasing complexity of development, and this provides the machinery necessary to the performance of an increasing number of muscular and mental co-ordinations; they can adapt themselves to unfamiliar surroundings and possess much enhanced advantages in the struggle for existence. _But_, associated with these advantages, is a much longer period of immaturity, because, where the capacity for flexibility and progress is great, the antenatal period is insufficient for the establishment of the necessary nervous connections or even for the development of the brain cells between which these connections will be formed. The chick will have its full plumage in ten weeks, but mentally it is far below a dog or a monkey, whose period of immaturity is much longer. Similarly, the dog attains his maturity long before the monkey, who is infinitely his superior in fertility of resource, power to learn through imitation, and capacity for attention. The infant in its turn is far longer in a dependent condition than the highest ape. Relatively large in bulk at birth, and reaching usually its full mass in the first fourteen years of life, the human brain possesses throughout childhood vast silent areas, big with future potentialities, areas in which the cells are slowly ripening to function. Even after full growth in size is reached, many more years must pass before capacity for the higher mental functions or for the complete control of such functions has developed. It must be borne in mind that, throughout this period of immaturity, errors of nutrition or defective stimulation may interfere with function. One of the most important duties of the home is to provide the suited environment for its child occupants during these long and anxious years. How long they are has been emphasised by Dr. Clouston,[106] who has said that, of all the periods of brain growth, the most important, as regards the development of our highest moral and mental potentialities, is that between eighteen and twenty-five years of age, when the capacity for self-control should be coming into function in its highest relations, and when failure to ripen in due course is fraught with most serious consequences for the future.