Where do invertebrates come from?

For a long time it was thought that small invertebrate animals appeared spontaneously, from dirt and dust, and indeed even that small vertebrates, such as mice, came from woollen textiles. It was not until 1650 that the Italian physician Francesco Redi finally showed that maggots are the larvae of flies. He placed meat and fish heads in two containers, tied a cover over one of them, and left the other uncovered. After some days there were maggots in the open container which had been visited by bluebottles, but none in the closed container. This was a very simple experiment, but it demonstrated an extremely important biological principle, namely that even the smallest animals, do not arise spontaneously but are always produced by an individual of the same species. There is good reason to mention this principle, for many people still have a deep-rooted feeling that animals, such as lice and fleas, can arise from dirt. This is not true; although it is true to say that they may be more difficult to get rid of where the standard of hygiene is low.