=Man’s Modification of his Environment.=--The energy of the world, whether of coal, waterfall, oil, forest, or rich soil, has the sun as its source. _All of these are being destroyed by man_, often with recklessness and wantonness. The promised land which “flowed with milk and honey” is now almost a desert. Other examples are Italy, Carthage, Spain. The destruction of forests causes floods which wash away the soil. _It is estimated that there are only one fourth as many song birds in the United States as there were fifteen years ago._ Insects and weeds or deserts replace rich soil, noble quadrupeds, singing birds, and stately trees. Many farmers, however, preserve the fertility of the soil. =To the erect posture= is due man’s free use of his hands and the _coöperation of hands and senses_. This has given man his intellectual development. The erect position has given greater freedom to the chest. _Man uses fewer organs of locomotion than any other animal._ The opossum has two hands, but they are on the hind limbs. The ape has four hands, but must use them all in locomotion. (What is a hand?) The erect position, however, makes _spinal deformity_ easier to acquire, and the _whole weight being upon one hip_ at each step man is liable to hip-joint diseases. In the horizontal trunk the organs lie one behind another; in man they _lie one upon another_, and are more liable to _crowding_ and _displacement_. The prone position in sickness helps to restore them. Large blood vessels at neck, armpits, and groins, which occupy protected positions in quadrupeds, are _held to the front and exposed_ to danger. The _open end_ of the vermiform appendix and of the windpipe are _upward_ in the erect trunk of man. Valves are lacking in some vertical veins and present where little needed in horizontal veins. But the _freedom of the hands_ more than makes up for all the disadvantages of erectness. =The Survival of the Fittest.=--_Those who do not work degenerate._ Those who overwork, or work with only a few organs, as the brain and nerves, degenerate. The workers survive and _increase in numbers_, the idle perish and _leave few descendants_.