In the early 1700s, Linnaeus came up with a new system of classification of living things that eventually replaced the one that had been used since Aristotle developed it in ancient Greece about 2000 years earlier. Why was a change necessary? A new system was needed to include the new species that had evolved, since the original classification system was developed. So many new species were being discovered, it was impossible to arrange them all in a hierarchy based on relative complexity of species. A system based on differences in physical characteristics was no longer sufficient to describe all that was known about living things. There was no way in Aristotle's system to classify the microscopic organisms discovered after the development of the microscope.