Darwin, however, reviving another ancient metaphor, the tree of life, hypothesized that the groups of the Linnaean classification (today's taxa), descended in a tree structure over time from simplest to most complex. The Linnaean hierarchical tree was synchronic; Darwin envisioned a diachronic process of common descent. Where Linnaeus had conceived ranks, which were consistent with the great chain of being adopted by the rationalists, Darwin conceived lineages. Over the decades after Darwin it became clear that the ranks of Linnaeus' hierarchy did not correspond exactly to the lineages. It became the prime goal of taxonomy to discover the lineages and alter the classification to reflect them, which it did under the overall guidance of the Nomenclature Codes, rule books kept by international organizations to authorize and publish proposals to reclassify species and other taxa. The new approach was called phylogeny, the "generation of phyla," which devised a new tree metaphor, the phylogenetic tree. One unit in the tree and all its offspring units were a clade and the discovery of clades was cladistics.