2 Man, being a distinct species of animal, has particular laws that apply only to him, and therefore we should be careful not to judge him too closely by facts regarding other forms of animal life 3 Man has changed very materially in the few thousand years of his civilization, in his external habits and customs, but very little in his fundamental physiological processes; therefore we should be able to judge what will be best suited for his needs by studying the process of the development of his organs during the millions of years that preceded the historic period. This plane of life is best seen today in the case of savages unacquainted with fire, and in the case of anthropoid apes. With this general survey of evolution, and a clear understanding of the principles involved, I trust the reader will consider the facts here presented in the unprejudicial spirit of the true scientist. LESSON XX SEX AND HEREDITY THE ORIGIN OF SEX That part of human life and living that is associated with the functions of sex and reproduction is at once the cause of the world's greatest misery and the world's greatest happiness. It is the subject of the greatest popular ignorance and superstition, and at the same time the field of the most wonderful of all scientific knowledge. For the origin of sex we must look back into the remote ages of creation in the early stages of organic evolution. [Sidenote: Fundamental function of the cell] The first essential property of matter that makes life possible is the power of nutrition, which means the ability of the living cell to transform other chemical substances into its own protoplasm or living substances. But this world would have remained a barren mass of igneous rock if nutrition had been the only function with which the earlier forms of life were endowed. Not only must the living cell be enabled to grow by absorbing other substances, but it must reproduce itself, or multiply the number of living individuals. [Sidenote: First form of reproduction] The first method by which this was accomplished was undoubtedly one of simple division; that is, the living cell grew by absorbing other substances and when sufficient size had been attained, divided, forming two daughter-cells. This division process of reproduction is the form by which all bacteria (so-called dis-ease germs) and many other lower forms of life increase their numbers. [Sidenote: Second form of reproduction] [Sidenote: Strength in fusion of cells]