Historians of science speculate widely on the reasons why Charles Darwin waited so long to publish On the Origins of Species. (For those not familiar with the story, Darwin had conceived the theory of natural selection in the years after his voyage on the HMS Beagle ended in 1836, but he did not publish until 1859 and was largely motivated by the discovery that Alfred Russel Wallace had conceived a nearly-identical theory.)
One of the most convincing arguments I have heard was espoused by Stephen Jay Gould is that Darwin waited because he knew his theory would challenge the dualist theory of mind. Descartes is famous for suggesting the human brain comprises material elements (the neurons and other cells in our brains where electrical and chemical signals) and spiritual elements (some unknown “thing” that emerges from the brain). According to Gould, Darwin realized his theory explained the origins of Homo sapiens in purely material or natural terms. This aspect of his theory challenged the dominant assumptions of society, and–in particular–challenged the beliefs of his wife.
In reading Darwin, the 1991 biography by Adrian Desmond and James Moore, I encountered the story of the Plinian Society, a collection of young people with strong interest in science and natural history, and their meeting on March 27, 1827 (in the year before he set sail on the HMS Beagle). At that meeting, Darwin shared his discovery about local sea life; this is thought to be his first professional presentation. After Darwin’s presentation, Dr. Robert Edmond Grant made a presentation in which he argued that consciousness is the product of material processes. Grant’s presentation was so controversial, even among the radical members of the Plinan society, that it was struck from the record of the meeting.
This story seems to support the conclusion that Darwin may have been reluctant to publish his theory of natural selection. Early in his career, he had seen the difficulties that would arise when he challenged the dominant assumption.