Another biological law, that of periodicity,[100] or of rhythmic alternations of activity and rest, has hitherto often suffered among human beings more in the breach than in the observance of its tenets; though unquestionably conformity to its requirements makes for health and stability. Throughout nature habits of rhythmic, organic activity are too familiar to attract attention. Of these, the periodic return of the seasons, for instance, or the daily tides, the flowering of plants and the ripening of fruit, the migrations of birds and the hibernation of certain insects and animals, are obvious examples. These rhythms have been proved by experience to be advantageous in the world. They make for efficiency and economise energy, and, from their high degree of development in man's nature, it may be fairly assumed that to him their observance is of great consequence. Many of them are beyond his control; such, for example, as the diurnal variations of his body temperature, the beating of the heart, the call of hunger, or the rhythm of growth. Others he can observe or abuse according to his pleasure; sleep, for instance, or the rhythm of work, or the daily discharge from his body of its waste products.[101] It is the work of hygiene to demonstrate how to combine obedience to all these laws with the demands of modern existence, and it is the duty of man to conform reasonably to modes of life based on these demonstrations. More especially does responsibility for the establishment of certain rhythms, such as sleep, devolve upon the organiser of a child's early life. VI. THE ORIGIN OF FAMILY LIFE AND ITS RELATIONSHIP TO SANITARY SCIENCE Further researches into the records of the past, and a closer study of the underlying principles upon which humanity has formulated many generally adopted customs, indicate how unexpectedly intimate is the relation between the growth of a social organisation and the origin of primitive efforts after the preservation of life and health. The world at large is so accustomed to the widespread existence of family life that curiosity is rarely aroused as to its origin, intention and worth; consequently to ignorance of its significance must be attributed the assertion that the custom is well-nigh obsolete and the proposal of some would-be reformers to abolish the institution and to instal the State _in loco parentis_. Professor McDougall[102] assures us that such is the social importance of the family that all who have given serious attention to the question are agreed that the stability of the family is the prime condition of a healthy state. This opinion is supported by other writers,[103] who have emphasised their conviction that the healthful development of the individual--even the possibilities of racial progress--depend to a large degree upon maintaining intact the integrity of family life. Their conclusions are based upon recent researches into the sciences of biology, sociology, and economics.