ILLUSTRATIONS FIGURE PAGE 1. Seasonal variations in health and energy in Connecticut and 5. Decline of yellow fever in the Western Hemisphere during the last quarter-century 219 6. Death rate from tuberculosis in the United States registra tion area 226 7. Death rate from all causes in the United States registration area 232 & Land capable of use for crops 9. Utilization of the land area of the United States 248 249 10. Potential uses of land area of the United States 251 II. 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 improved land in the United States, 1850-1920 256 14. Average annual removal of standing timber in the United States through waste, destruction, or use 257 15. Average annual removal of standing timber from the forests of the United States assigned to various types of use or causes of destruction 258 16. Continuous variation. Discontinuous variation 301 17. Color inheritance 304 18. Suggested explanation of color inheritance in guinea pigs 305 FIGURE PAGE 27. Neurotic inheritance 336 28. Examples of the inheritance of insanity drawn from hospital 33. The inheritance of ability, in the Wedgwood, Darwin, and 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 -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 |