 However, it was later shown that the alleged hybrids were simply sea urchins.Williamson's hypothesis has been reviewed in the companion website for the eighth edition of the textbook "Developmental Biology".In an article published in the esteemed journal "Proceedings of the National Academy of Sciences" in 2009, Williamson claimed that the body plan of the adult butterfly and its caterpillar larval stage would have evolved separately in different organisms; then, their phylogenies would eventually merge by hybridisation at a more recent point of their evolutionary history.