Archbishop James Ussher of Ireland, in the mid 1600's, calculated the age of the earth based on the geneologies from Adam and Eve listed in the biblical book of Genesis, working backward from the crucificxion. 4004 B.C. These calculations were part of Ussher's History of the World , and the chronology he developed was taken as factual, even being printed in the front pages of bibles. According to Hutton's work, certain geological processes operated in the past in much the same fashion as they do today, with minor exceptions of rates, etc. Thus many geological structures and processes cannot be explained if the earth is only 5000 years old. The Earth is thought older than 4.5 billion years, with the oldest known rocks being 3.96 billion years old.
Geologic time divides into eons, eroas, and smaller units.
Many of these categories are still used in biology, although the underlying thought concept is now evolution and not immutability of species. This was a major break from earlier concepts that species were created by a perfect creator and therefore could not change because they were perfect, etc. Buffon also provided evidence of descent with modification and speculated on various causative mechanisms. However, Buffon vacillated as to whether or not he believed in evolutionary descent, and professed to believe in special creation and the fixity of species. His writings on both botany and zoology contained many comments that suggested the possibility of common descent based on changes undergone by animals during development, artificial selection by humans, and the presence of vestigial organs.