1. According to one view, offspring are produced from their parents by process of reproduction, this is called concept of Biogenesis.
2. According to other view, living things are produced spontaneously from non-living things. This is called concept of A biogenesis.
Because at that time, there was neither so much advancement in science nor scientific tools like microscope and other instruments were invented, which could help in detailed observations about reality. According to some people, insects are produced from dewdrops, rats from debris, frogs from mud, and maggots from putrefied meat. Some scientists like Copernicus, Bacon, Galileo, Harvey, and Descartes also believed this concept.
From 16th to 18th century many scientists performed experiments to test this concept. They found some animals to be developed from non living matter. Therefore this concept seemed to be correct.
Later on, scientists performed experiments with more care. First of all an Italian scientists, Francesco Redi, (1668) proved that this concept was wrong.