The ancient Greek , in order to explain the origin of the frogs , said that the pond models itself spontaneously in order to assume the shape of these animals. In a similar way they believed that the eels were formed from the river and that bugs, snails, leeches and other small animals were born in a similar way. This type of birth was called "spontaneous generation".

In the XVII century many naturalists continued to support the hypothesis of the spontaneous generation.
The naturalist Moffet (1553-1604) wrote, in a work about the bugs published after its death, that the bees were born from the putrefied body of a small bull.. Also the naturalist van Helmont (1577-1644) was sure that the frogs were born spontaneously from the mud of the ponds and asserted that the rats were born from a dirty shirt strewn in a vase with some seeds of wheat