In the 1970s and early 1980s the Burgess fossils were largely regarded as evidence that the familiar phyla of animals appeared very rapidly in the Early Cambrian, in what is often called the Cambrian explosion. This view was already known to Charles Darwin, who regarded it as one of the greatest difficulties for the theory of evolution he presented in The Origin of Species in 1859. However, from the early 1980s the cladistics method of analysing "evolutionary family trees" has persuaded most researchers that many of the Burgess Shale's "weird wonders", such as Opabinia and Hallucigenia, were evolutionary "aunts and cousins" of present-day types of animal rather than a rapid proliferation of separate phyla, some of which were short-lived.