Analyzing archival documents, an evolutionary biologist at Queen Mary University of London, Professor Richard Baggs discovered that a few years before Darwin's death, he had a very determined opponent - the Scottish botanist William Carruthers.
Carruthers adhered to the creationist theory of the origin of flowering plants, believing that they arose through intervention from above, and trumpeted in the press that Darwin was not able to provide a scientific explanation on this issue.
A gap in Darwin's theory of evolution became public knowledge and threatened to undermine Darwin's position in the scientific world.
It was then, says Richard Buggs, that this phrase was born - the abominable mystery: a terrible or disgusting mystery.
What is Darwin's "terrible secret"?
For the first time this term was used by Charles Darwin in 1879 in a letter to his friend, researcher and botanist Joseph Hooker. In it, he wrote that the rapid development of the highest species of plants by geological standards is a terrible secret.
It was about flowers and flowering (or angiosperms) plants, the distinctive feature of which is the presence of organs of sexual reproduction. These include most of all plants on Earth - from water lilies and wildflowers to oaks and fruit trees.
Darwin could not explain the process of their origin and evolution. Flowering plants appeared on Earth relatively late in comparison with other species and very quickly acquired the widest variety of colors, sizes and shapes.
"Based on the so-called fossil record, flowering plants ( Angiospermae ) appeared suddenly - in the Cretaceous period, about 100 million years ago. They have no similarities with plants that existed before that. In addition, their appearance was marked by a variety of subspecies", - says Professor Baggs.
It was this suddenness that haunted Charles Darwin.
Why hasn't there been a consistent evolution? Where have the intermediate forms between conifers ( G ymnosperm ae ) and flowering ones gone ? And how is it possible that they appeared immediately in a great variety of options?
Darwin did not understand how these plants escaped the successive stages of development, unlike other vast species of flora and fauna, including mammals. All this contradicted one of the main principles of natural selection, which was that nature does not make sharp leaps.
For a long time, Darwin consoled himself with the idea that, perhaps, flowering plants originated and evolved on some as yet undiscovered island or continent.
In August 1881, just a few months before his death, he wrote to Hooker: “For me there is nothing more extraordinary in the history of the plant kingdom than the unexpected and rapid development of higher plants. Sometimes it seemed to me that for centuries somewhere near the South Pole there could be a distant and lost continent. "
In the library of the Royal Botanic Gardens, Kew, Professor Baggs came across a copy of a lecture that Scottish botanist William Carruthers was giving to members of the Association of Geologists in 1876.
In it, the Scotsman claims that Darwin is unable to understand and explain the emergence of flowering plants, because their appearance has a divine basis.
Carruthers attacks the entire Darwinian theory of evolution as a whole, which provokes heated debate not only in scientific circles, but also in society. His statements and conclusions were published in the Times newspaper, as well as in a number of scientific publications.
"Carruthers seized the moment to launch a campaign against Darwinian theory. He argued that angiosperms in the Cretaceous were created directly by God. For Darwin and his friends, this was complete heresy, but a problem arose: he could not explain this phenomenon in terms of evolution, "says Buggs.
According to the professor, it was this situation that prompted Charles Darwin to use the phrase "terrible secret." Baggs published his findings in the American Journal of Botany.
William Carruthers himself later became curator of the botany section of the British Museum and one of the foremost scientists in the field of paleobotany.
According to Richard Baggs, Charles Darwin's "terrible mystery" is akin to Fermat's theorem, formulated by the mathematician Pierre Fermat in 1637 - none of them could solve their own riddle during their lifetime.
"We got an idea of what was going on in Darwin's head in the last years of his life. This last riddle, attempts to solve it, occupied all of Darwin's thoughts until his death," says Professor Baggs.
Have scientists since then managed to solve the "terrible mystery"?
In a word, no.
Already 140 years have passed, and still no one can exhaustively explain the emergence of flowering plants.
“Of course, we have made significant progress in our understanding of evolution and in our knowledge of paleontology, but this mystery has not yet been solved,” says Richard Baggs.