[填空题]The word conservation has a thrifty meaning. To conserve is to save and
protect, to leave what we ourselves enjoy in such good condition as others (53)______
may also share the enjoyment. Our forefathers had no idea that human population
would increase faster than the supplies of crude materials. Most of them, even (54)______
until very recently, have the foolish idea that the treasures were "limitless"
and "inexhaustible". Most of the citizens of early generations knew little or (55)______
nothing about the complicated and the delicate system that runs all through the
nature, which means that, as in a life body, an unhealthy condition of one part (56)______
will sooner or later be harmful to all others. (57)______
Fifty years ago nature study was not part of the school work. Scientific (58)______
forestry was a new idea; timber was still cheap