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Nature allows a seeming excess because the difficulties of life restrict survival of both young and old，and the result of this delicate balance is that populations rise and fall in small numbers; only rarely do they rise to disastrous millions or fall to complete zero. Both plants and animals may reach ages where there they are no longer able to reproduce, butusually before old age is reached sickness, storm, and slowed reactions to danger will have eliminated many individuals. Even a few strong and mature individuals die from disease or under the attacks of a predator.
大自然允许一种表面上的过量，因为生命的艰难竭制了幼小的和年老的生命的存活，这一微妙平衡的结果是，各物种数量上下浮动得很小，只偶尔才成百万地上升，造成灾难，或降跌至零。动植物都能活到不能再繁殖的年龄，但往往等不到年老，疾病、 暴风雨以及对危险的迟缓反应就已经使许多个体的生命消亡。甚至少数强壮成熟的个体生命也会死于疾病或食肉动物的袭击。
Life is the moving，ever-aggressive force. When any form of life dies，certain life processes take over and act out the saying "waste not，want not" Every atom and molecule of a livin plant or animal was molded by life into a form that is a certain species of life. Those very elements and compounds are needed to continue life, and nature is fairly efficient in recycling them.
生命是一种运动发展、永远进取的力量。任何一种生命形式消亡时，某种生命过程便会取而代之，实践起那句"不浪费，不会缺"的老话。活的动植物的每一个分子、原子都是经由生命塑造成某种生命物种形式的。那些元素和化合物正是延续生命所需要的，大自然卓有成效地将它们循环使用。
The controlling factor of life is really "supply and demand". A grass eating animal cannot exist without grass, a predator without prey, a plant without certain basic chemicals or certain kinds of soil. When prey species are rare，the population of predators declines; and when plants lack certain soil，chemical and weather conditions, they decline. Theoretically we would need only to replace those few chemicals filtered out of the soils by water and windsif each of our farm crops were plowed back under the ground, but then we wouldn't have any life-sustaining crops to eat. To raise our crops we need to fertilize by returning chemicals to the soil. It is part of life: through death and decomposing, nature keeps wildlife going.
生命的控制因素实际上是"供求关系"。食草动物没有草不能生存。食肉动物没有猎物不能存活，植物没有某些基本的化学物质或某种土壤不能生长。猎物稀少时，食肉动物的数量就减少；植物缺少一定的土壤、化学物质和气候条件就会凋零。从理论上来讲，如果我们把全部庄稼都犁入土中作肥料，那么我们只需再补上极少数由于水和风而从土壤中流失的化学物质就行了，但那样的话，我们就不会有赖以生存的粮食了。要种植庄稼，我们得向土壤施加化学物质，使土壤肥沃。大自然通过死亡和腐烂使得野生生物生生不已，这是生命的一部分。
