Successful innovations have driven many older technologies to extinction and have resulted in higher productivity, greater consumption of energy, increased demand for raw materials, accelerated flow of materials through the economy and increased quantities of metals and other substances in use each person. The history of industrial development abounds with examples. In 1870, horses and mules were the prime source of power on U. S. farms. One horse or mule was required to support four human beings—a ratio that remained almost constant for many decades. At that time, had a national commission been asked to forecast the horse and the population for 1970, its answer probably would have depended on whether its consultants were of an economic or technological turn of mind. Had they been 'economists' , they would probably have projected the 1970 horse or mule population to be more than 50 million. Had they been 'technologists', they would have recognized that the power of steam had already been harnessed to industry and to land and ocean transport. They would have recognized further that it would be the prime source of power on the farm. It would have been difficult for them to avoid the conclusion that the horse and mule population would decline rapidly. According to the passage, what supplied most of the power on U. S. farms in 170?