Attention is here requested to the last clause in the last sentence. As civilization has progressed and various inventions have been made, the whole field of possible future invention has been narrowed, but a field of clear though limited opportunity has been mapped out. Each invention narrows the field by removing the opportunities for making that especial invention: after the printing press had been invented, for instance, the number of possible inventions was reduced by one; but see what a field for future invention was mapped out, and what immeasurable opportunities were suggested! Nevertheless, opportunity does not produce inventions, it merely invites them; and we have occasionally noted in this book that the opportunity to make a certain invention had existed for ages before it was realized: for instance, the sewing-machine and the little stethoscope. In 1868 Sholes invented what is usually considered the first practical typewriting machine. The machine that Thurber had invented in 1843 had never been developed to a practical stage, and, consequently, it was not itself a direct contribution to the Machine. Whether it paved the way for Sholes's is a debatable point; if it did, it was an indirect contribution, like Hero's engine. Not for several years after 1868 did the typewriter take its place in the Machine: but now it plays an exceedingly useful, if not conspicuous, part in making it operate day after day. In the same year Nobel contributed another of his notable inventions, and called it dynamite. It was the development of an exceedingly brilliant and original idea; and, as often happens with conceptions of that kind, it was easily developed into a concrete, usable and useful thing. It consisted merely in mixing nitro-glycerin with about an equal quantity of very finely divided earth. The resulting mixture was much less sensitive to shock and therefore much safer to handle than nitro-glycerin. It supplied the factor needed to render the utilization of nitro-glycerin possible, and therefore it was a valuable contribution to the Machine. In the same year, Mege invented oleomargarine, a comparatively inexpensive substitute for butter, and therefore an important factor in furthering the health and comfort of the poorer classes and a considerable forward step.