In 1668 the Italian naturalist, Redi, demonstrated that putrefying flesh does not "spontaneously breed" maggots. He showed that if a piece of flesh is protected by a wire network cover from the access of flies, no maggots appear in it, and that the flies attracted by the smell of the meat lay their eggs on the wire network, unable to reach the meat, whilst if the wire cover is removed they lay their eggs on the meat, and from them the maggots are hatched. It took a long time for this demonstration by Redi to affect popular belief, and there are still country folk who believe in the spontaneous generation of maggots.[4] [4] See the chapter, "Primitive Beliefs about Fatherless Progeny," in "Science from an Easy Chair," Second Series. But few, if any, persons of ordinary intelligence or education now believe that these sudden productions of living things, without regular and known parentage, take place. The spontaneous generation of large, tangible creatures having ceased to be an article of general belief, the conviction nevertheless persisted for some time that at any rate minute microscopic living things were generated without parentage. This theory was more difficult to test on account of the need for employing the microscope in the inquiry, which was not brought to a high state of efficiency until the last century. By experiments similar to those of Redi, it was shown in the first half of last century by Theodor Schwann that even the minute bacteria do not appear in putrescible material when those already in it are killed by boiling that material, and when the subsequent access to it of other bacteria is prevented by closing all possible entrance of air-borne particles, or insect carriers of germs. It took another fifty years to thoroughly establish by observation and experiment the truth of Schwann's refutation of the supposed "spontaneous generation" of the minutest forms of life.