Editing On the Origin of Species (section)

===Geological record===
Chapter IX deals with the fact that the [[geological record]] appears to show forms of life suddenly arising, without the innumerable [[transitional fossil]]s expected from gradual changes. Darwin borrowed [[Charles Lyell]]'s argument in ''[[Principles of Geology]]'' that the record is extremely imperfect as [[fossil]]isation is a very rare occurrence, spread over vast periods of time; since few areas had been geologically explored, there could only be fragmentary knowledge of [[Formation (stratigraphy)|geological formations]], and fossil collections were very poor. Evolved local varieties which migrated into a wider area would seem to be the sudden appearance of a new species. Darwin did not expect to be able to reconstruct evolutionary history, but continuing discoveries gave him well-founded hope that new finds would occasionally reveal transitional forms.<ref name=bowl182>{{harvnb|Bowler|2003|p=182}}</ref><ref name=punkeek>{{Citation |url=http://www.talkorigins.org/faqs/punc-eq.html#pe-vs-pg |title=Punctuated Equilibria |author=Wesley R. Elsberry |author-link=Wesley R. Elsberry |year=1996 |access-date=30 April 2009}}</ref> To show that there had been enough time for natural selection to work slowly, he cited the example of [[Weald Basin|The Weald]] as discussed in ''Principles of Geology'' together with other observations from [[Hugh Miller]], [[James Smith of Jordanhill]] and [[Andrew Ramsay (geologist)|Andrew Ramsay]]. Combining this with an estimate of recent rates of sedimentation and erosion, Darwin calculated that erosion of The Weald had taken around 300 million years.<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=300 282–287]}}</ref> The initial appearance of entire groups of well-developed organisms in the oldest fossil-bearing layers, now known as the [[Cambrian explosion]], posed a problem. Darwin had no doubt that earlier seas had swarmed with living creatures, but stated that he had no satisfactory explanation for the lack of fossils.<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=324 306–308]}}</ref> Fossil evidence of [[pre-Cambrian]] life has since been found, extending the history of life back for billions of years.<ref>{{Harvnb|Schopf|2000}}</ref>

Chapter X examines whether patterns in the fossil record are better explained by common descent and branching evolution through natural selection, than by the individual creation of fixed species. Darwin expected species to change slowly, but not at [[phyletic gradualism|the same rate]] – some organisms such as ''[[Lingula (genus)|Lingula]]'' were unchanged since the earliest fossils. The pace of natural selection would depend on variability and change in the environment.<ref name=darX/> This distanced his theory from [[Lamarckism|Lamarckian]] laws of inevitable progress.<ref name=bowl182/> It has been argued that this anticipated the [[punctuated equilibrium]] hypothesis,<ref name=punkeek/><ref>{{harvnb|Rhodes|1987}}</ref> but other scholars have preferred to emphasise Darwin's commitment to gradualism.<ref>{{harvnb|Darwin|Costa|2009|p=108}}</ref> He cited [[Richard Owen]]'s findings that the earliest members of a class were a few simple and generalised species with characteristics intermediate between modern forms, and were followed by increasingly diverse and specialised forms, matching the branching of common descent from an ancestor.<ref name=bowl182/> Patterns of extinction matched his theory, with related groups of species having a continued existence until extinction, then not reappearing. Recently extinct species were more similar to living species than those from earlier eras, and as he had seen in South America, and [[William Clift]] had shown in Australia, fossils from recent geological periods resembled species still living in the same area.<ref name=darX>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=330 312–345]}}</ref>