Organisms are classied according to their shared characteristics. Initially, the Greeks classied organisms based on their gross morphological characters and, therefore, at that time there appeared to be only two major groupings: plants and animals. Within each group, there were recognizable entities that could breed to produce more of the same kind of organism. Eventually, these became known as species, but this was not formalized until the eighteenth century by Carl Linnaeus. In the same century, scientists began to use microscopes to add additional characters and groups to taxonomic classications. Higher order groupings also were formalized to logically classify organisms for study as well as for commerce. Eventually, electron microscopes aided classications based on microscopic characters. During the past two decades, molecular biology methods have been at the forefront of taxonomy, since often they are more direct measures of genetic relatedness among the taxa. Additionally, they allow accurate phylogenetic reconstructions of the taxa that can yield precise measures of evolutionary events and the timing of those events.