Ralph Colp, Jr., a physician and psychiatrist, became interested in Darwin's case and, in 1959—a century after Darwin's book,—began researching everything he could find on Darwin. For the next 18 years he exhaustively studied into the matter, and in 1977 published a book on his conclusions.

BACKGROUND OF THE PROBLEM
Why would a strong, healthy young man develop such a condition?
"A few years after returning to England from his five-year voyage of exploration, Charles Darwin became a semi-invalid who suffered daily for the rest of his life. Doctors were baffled; they could find neither cause nor cure.

"As a young man Darwin had uncommon strength and endurance. During the Beagle expedition, he endured rough seas, primitive conditions on overland treks and rode spirited horses with the roughest gauchos in Argentina. Whenever he encountered a mountain on his inland treks, he usually climbed it. Yet a few years later, he was afflicted with almost daily weakness, vomiting, and chronic fatigue."—*R. Milner, Encyclopedia of Evolution (1990), p. 113.

Various theories about Darwin's health problem have been devised, but none have been as thoroughly researched as Colp's. Indeed, there are oddities about Darwin that lend strong credence to Colp's ideas. You will recall statements by Darwin, quoted elsewhere in these Encyclopedia articles, that he did not like to think about the human eye because it disturbed him and the sight of a peacock's feather made him sick. Why would those thoughts and sights so deeply disturb him? Because he knew, deep down, that he was on the wrong track in his theories.
He also wept frequently over a letter his wife gave him early in their marriage.

"In 1839, Darwin married his first cousin Emma Wedgwook, whose traditional religious beliefs were opposed to his unorthodox inquiries into the origin of species. Soon after their marriage, she wrote him a letter, begging him to reconsider challenging the Bible's account of Creation, lest they be separated for eternity in the hereafter. All his life he cherished her touching letter (many times I have kissed and cried over this), but remained committed to his scientific career."—*R. Milner, Encyclopedia of Evolution (1990), p. 110.

Why would Darwin weep over that letter, if he did not believe what it said? He wept over it—and repeatedly—during his life, because it was telling him something he believed—yet emotionally did not want to accept. For the same reason it made him feel sick when he thought of evidences for Creation which were unanswerable, such as the complex structure of the eye or the orderly pattern of a peacock's feather. Those evidences make him feel sick, for he knew they were not true.

The depth of Colp's research clearly provides solutions.
According to Colp, Darwin's weakness, nausea, inability to work, depression, insomnia, and other symptoms were all part of a complex psychosomatic condition brought on by deep conflicts about his lifework.
As Colp sees it, Darwin's theorizing about evolution injured his health because he saw too many conflicts in his theories.
Colp says that Darwin even experienced an "identity crisis" as a result of his emotional turmoil.
Colp decided that the physical problems started when Darwin began his theorizing, and worsened thereafter.
Colp believes it was this guilt and ambivalence that kept Darwin for years from writing his book, until he did it to keep Wallace from obtaining prior credit ahead of himself.