The notion of creatures being simply wiped from the face of the Earth was anathema to many early naturalists. When fossil remains were found that were unlike anything living at the time, some scientists argued that they were unusual examples of living creatures, or that animals known only from fossils must still survive in some unexplored part of the world.
There were a few naturalists who believed that animals or plants of which only fossil remains could be found did indeed represent forms that no longer existed. But it was only at the end of the 18th century that the great French paleontologist and anatomist Georges Cuvier was able to demonstrate convincingly that extinctions were real.
Much of Cuvier's research produced knowledge that would ultimately support Darwin's theory of evolution, although Cuvier himself did not realize it. He was the first to demonstrate that the different strata of rock in the Paris basin each had its own mammal fauna. Furthermore, he showed that the lower a stratum was, the more different its fossil animals were from species living in the present.
Yet Cuvier rejected the idea of organic evolution. He was an essentialist, convinced that plants and animals of all types were created for their particular roles and places in the world's environment, and that they were unchanging throughout their existence. There was no scale of perfection, in Cuvier's view, because each animal was perfectly adapted to its position in the natural world. He could see no evidence for a steady increase in complexity or perfection as claimed by those who believed in a "great chain of being." But in the course of history, he said, catastrophic events had killed off all members of some species, and their fossils would no longer be seen in the rocks. Subsequently, he believed, the old species were replaced by new ones that repopulated Earth.
Cuvier could be called the founder of comparative anatomy, and it was his knowledge in this field that accounted for his well-known and almost uncanny ability to reconstruct animals from only fragments of fossil remains. With elegant studies of the anatomy of large mammals such as elephants, Cuvier showed that fossil mammoths differed from any such creatures presently living. His many examples of fossils telling the stories of animals that lived and then disappeared were taken as incontrovertible proof of extinctions.