The word "conservation" has a thrifty meaning. To conserve is to save and protect, to leave what we ourselves enjoy in such a good condition that others may also share the enjoyment. Our forefathers had no idea that human population would increase faster than the supplies of raw materials: most of them, even until very recently, had the foolish idea that the treasures were "limitless" and could "last forever". Most of the citizens of earlier generations knew little or nothing about the complicated and delicate system that runs all through nature, and which means that, as in a living body, an unhealthy condition of one part will sooner or later be harmful to all the others. Fifty years ago, nature study was not part of school work; scientific forestry was a new idea; wood was still cheap because it could be brought in any quantity from distant woodlands; soil destruction and river floods were not national problems; nobody had yet studied long-term climatic cycles in relation to proper land use; even the word "conservation" had nothing of the meaning that it has for us today. For the sake of ourselves and those who will come after us, we must now set about correcting the mistakes of our forefathers. Conservation should be made part of everybody's daily life. To know about the water table   in ground is just as important to us as a knowledge of the basic math formulas . We need to know why all watersheds   need the protection of plant life and why the running current of streams and rivers must be made to give their full benefit to the soil before they finally escape to the sea. We need to be taught the duty of planting trees as well as of cutting them. We need to know the importance of big, grown trees, because _ In a word, it should be our goal to restore as much of the original beauty of nature as we can. According to the author, the greatest mistake of our forefathers was that  _  .
they had no idea about scientific forestry they had little or no sense of environmental protection they were not aware of the importance of nature study they had no idea of how to make good use of raw materials