The whole subject of the reproduction of animals and plants was, until the introduction of the microscope, involved in obscurity and mystery. The Greeks and Romans had necessarily very imperfect and erroneous notions on the subject, and it was not until 300 years ago that William Harvey, the discoverer of the circulation of the blood, declared, as a general law, that every living thing is born from an egg. During that 300 years his conclusion has been examined and modified, corrected and expanded, and the microscope has at last enabled us to see and follow the excessively minute particles and structures by which sexual reproduction is effected. Harvey's dictum was a step in advance when it was made, for previously the belief was current that living things were "bred" in all sorts of queer ways. It was supposed that the putrefying flesh of a dead animal actually was converted by a sudden process into maggots, and that rotten wood would breed, out of its own substance, ships' barnacles and even young geese and mice--an opinion contested only 200 years ago by Sir Thomas Browne! No difficulty was felt in admitting that whole swarms of insects, fishes, and even herds of larger beasts were spontaneously generated from mud, from putrid matter, or from the waters of the sea. That, indeed, was the popular notion set forth by the poet, John Milton, as to the mode in which living things were "miraculously" brought into existence at the beginning of things by the "fiat" of the Creator. What more probable than that such a creation should still be, here and there, at work? However, not three centuries ago, actual experiment gradually convinced the learned that maggots are bred in a dead body only from the eggs laid by parent flies, as shown by the Italian Redi in 1668 who found that no maggots were bred when he simply excluded the flies from access to the dead body by covering it with wire gauze, but that the blow-flies swarmed on the gauze and vainly laid their eggs on it! It was only gradually recognised that birth by means of eggs or germs extruded from parental organisms of the same history and character as their offspring is the explanation of all such swarms of flies, worms, and even mushrooms and moulds as had been formerly ascribed to a mysterious power of breeding these organisms possessed by inanimate dirt and refuse.