FIGURE ILLUSTRATIONS 1. Seasonal variations in health and energy in Connecticut and Pennsylvania 2. Distribution of energy on the basis of climate 3. Food relations of aquatic animals 4. Mortality from yellow fever in Havana 5. Decline of yellow fever in the Western Hemisphere during the last quarter-century PAGE 80 83 ΙΟΙ 218 6. Death rate from tuberculosis in the United States registration area 219 226 7. Death rate from all causes in the United States registration area & Land capable of use for crops 9. Utilization of the land area of the United States 232 248 249 10. Potential uses of land area of the United States 251 11. Average yields per acre, 1909-1913, of seven important crops in four European countries expressed in percentage of average yields in the United States 253 12. Yearly averages of cotton production, consumption, and exports in the United States, 1920-1924 256 13. Per capita acreage of forest land contrasted with that of 15. Average annual removal of standing timber from the forests of the United States assigned to various types of use or causes of destruction 16. Continuous variation. Discontinuous variation 17. Color inheritance 256 257 258 301 304 18. Suggested explanation of color inheritance in guinea pigs 305 19. The relations between successive generations 20. The inheritance of hæmophilia 21. The inheritance of polydactylism FIGURE 27. Neurotic inheritance 28. Examples of the inheritance of insanity drawn from hospital PAGE 336 36. The families of 461 leading American scientific men 32. The inheritance of low ability 33. The inheritance of ability, in the Wedgwood, Darwin, and Galton Families 34. "The typical American boy" 35. The declining birth rate in Europe 348 349 350 406 THE PHYSICAL BASIS CHAPTER I THE EARTH AND ITS ELEMENTS Listen, first, while I sing the fourfold root of Fire, and water, and earth, and the boundless height of the æther, For therefrom is begotten what is, what was, and what shall be. -EMPEDOCLES How little the average man knows is apparent to every one except himself. How much he can know is a matter of uncertainty about which he is not disturbed. That the wisest man knows but little is the conviction of every ignoramus. On one thing all are agreed; complete knowledge is far from us. Yet the story of the growth in human understanding is the most fascinating as well as the most important thing in human history. The stars shone on the curious animals of ages past but we do not know that the animals saw the stars. The primitive savage, peeking upwards through the leaves of the trees, got glimpses of them but we do not know what he thought about them. At some far later time the "watchman upon the mountain tops," "the shepherds keeping watch over their flocks by night," began the careful observations which led to the science of astronomy. THE SOLAR UNIVERSE By the middle of the second century B.C. a Greek writer, Hipparchus, was able to list some 1,080 stars. He knew the |