According to the theory of spontaneous· generation (abiogenesis . or autogenesis), life originated from non-living things in a spontaneous manner. This concept was held by. early Greek_ philosophers like Thales, Anaximander, Xanophanes, Empedocles, Plato, Aristotle, etc. In ancient Egypt, it was believed that the mud of the Nile could give rise to frogs, toads, snakes, mice and even crocodiles when warmed by· the sun. Van Helmont (1577-1644) held that human sweat and wheat grains could give rise to organisms. These beliefs had no scientific grounds and hence were discarded.