Among mental traits we can safely ascribe to heredity only general distinctions. Intellectual parents are more likely to give birth to intellectual children than are parents whose natural mental faculties are less developed. There is also no doubt that certain natural characteristics of mind, such as quick temper, musical ability, etc., may be inherited. The belief, however, in the inheritance of many less distinct features, both physical and mental, is not well established by scientific investigation. Strength of muscle, control of the nervous system, susceptibility to, or freedom from dis-ease, etc., are more matters of nutrition and environment than of inheritance. The idea that consumption, alcoholism, etc., are inherited, or that the education or training of parents along certain lines will result in children with faculties adapted to similar education, is not in accordance with scientific knowledge. SUMMARY OF FACTS REGARDING SEX AND HEREDITY 1 The function of sex has been developed in the process of evolution for the purpose of perpetuating life. 2 The sexual functions are very closely related to the life of the individual, and can be normal only when the laws of nutrition and of general hygiene are observed. 3 The idea of prenatal culture as commonly taught is a delusion; the only method that the mother can employ to control the growth of her unborn child is to live a wholesome, normal life, physically and mentally, and thus supply her own body and that of the child with perfect material for the building of living cells. 4 The powers of heredity are often overestimated, and many of the weaknesses and disorders of life supposed to be inherent can be overcome by proper nutrition and environment. All life, whatever be the inherited tendencies, will be developed to the highest possible capacity by obeying the laws of individual growth, for in the individual, as in the race, Nature is always striving to bring the products of her work to the highest degrees of perfection. LESSON XXI REST AND SLEEP REST [Sidenote: Opposing forces in nature] Throughout all nature we observe the phenomena of universal rhythm, manifested in opposing forces, such as heat and cold, light and darkness, construction and destruction, etc. The human body is as much affected by this rhythm as is any other form of life. [Sidenote: Opposing forces in human system] There are two forces continually at work within us, one toward destruction and disintegration, and the other toward construction and upbuilding. The common physiological terms for these activities are "waste" and "repair," and we observe them as one of the distinct manifestations of the universal laws of growth, progress, and evolution. History moves in cycles. Even the life of nations depends somewhat upon this same principle of the interplay of the positive and negative forces of life.