Why did Darwin wait 20 years to publish "On the Origin of Species"?

From Nature (March 29, 2007):

Did Darwin delay publishing his theory of evolution by natural selection because he feared an outcry from the establishment? This has been a popular belief, and has been stoked by the fact that although Darwin began formulating the theory in 1837, he did not publish On the Origin of Species until 1859.

Now John van Wyhe, a science historian at the University of Cambridge, UK, says that after a painstaking trawl through the letters, notes and books written by, to or about Darwin, he can rule out the idea once and for all. But van Wyhe's work has irritated several prominent historians, who argue that he has gone too far in downplaying ideas about Darwin's reluctance to publish. "Portraying Darwin as having no feelings or reactions to the outside world warps the biographical picture," says David Kohn, editor of the Darwin Digital Library of Evolution.

This is all well and good. The problem is how van Wyhe established that there was no delay:

To carry out his study, Van Wyhe searched for the word "delay" in primary and secondary sources covering the period in which Darwin was working on Origin of Species. He says Darwin and those who knew him never unambiguously referred to a delay in publishing, or gave any explanation for the 20-year 'gap'. Indeed, in all the texts on Darwin, he says, the earliest reference to a delay appears in the 1940s. Only in a 1948 popular book, Darwin: Before and After, in which Robert Clark describes how Darwin was made ill by "an uncertainty that he allowed to haunt him for twenty years", do you see all the elements of the modern story, says van Wyhe.

According to van Whye, Darwin just didn't get to writing Origin of Species (hmm...the problem of not being under any tenure clock)

By documenting exactly what he was doing during the 'gap years', van Wyhe makes the case that Darwin just didn't get down to writing Origin of Species until he had completed other work in hand, including an eight-year study of barnacles. He was determined to build a formidable mass of documentation supporting his theory and to solve major stumbling blocks, such as that posed by non-reproductive castes of social insects. This, together with a busy personal life but poor health, filled the years. In other words, Darwin did not postpone publication; he just didn't publish until he was ready. "In my view, this settles the question once and for all," says van Wyhe.

The last statement notwithstanding, the "delay" analysis is not going to solve this issue easily:

But several Darwin scholars are not convinced. Kohn and others agree that the way in which cultural and social pressures influenced Darwin's decisions may have been overplayed, particularly in the public arena, with less attention being paid to the involved process of scientific discovery. But the consensus in the field is likely to remain that a multitude of factors underpinned Darwin's delay.

Kohn points out that searching for explicit references to a "delay" is a simplistic approach to the problem, and that other factors should be considered. For example, Darwin often criticized religion in his notebooks, which suggests that he would have been aware of the probable implications of his theory for religion.

This work of John van Wyhe is published in Notes and Records of the Royal Society (doi:10.1098/rsnr.2006.0171;2007)