Sold – Darwin Confirms Information Proving His Thesis on Evolution in Plants

After he had stunned the world with Origin of the Species in 1859, Darwin turned his attention to encouraging colleagues around the world to research and publish in the new field of evolution, and to keep him informed of their progress. For example, he wrote Julian de Haast, a New Zealand government geolist, “I have been extremely much interested by what you say about the tracks of supposed mammalia. Might I ask, if you succeed in discovering what the creatures are, you would have the great kindness to inform me…I would venture to suggest to you to urge on some of the capable members of your institution to observe annually the rate and manner of spreading of European weeds and insects…” Thus, he became a focal point for newly discovered information, which was sent to him by correspondents around the world.

Darwin did not neglect his own work. In 1860 he began a new book on variations in plants and animals, deciding that studying domesticated varieties would particularly illustrate his point by showing changes in a more concentrated period of time. The resulting book, The Variation of Animals and Plants Under Domestication, was published in 1868. Darwin sets forth his purpose in the introduction: “The object of this work is…to give under the head of each species only such facts as I have been able to collect or observe, showing the amount and nature of the changes which animals and plants have undergone whilst under man's dominion, or which bear on the general principles of variation.” Chapter XI is entitled “On Bud Variation, and on Certain Anomalous Modes of Reproduction and Variation.” Darwin’s purpose in this chapter was to show that the variations were evolutionary in nature, and had evolutionary characteristics, as shown by the fact that “The modifications which arise through bud-variation can generally be propagated to any extent by grafting, budding, cuttings, bulbs, etc., and occasionally even by seed.” To prove the point, he needed to establish that the changes were not the result of other non-evolutionary  factors. “These cases,” he said, “prove that those authors who…attribute all variability to the crossing either of distinct races, or of distinct individuals belonging to the same race but somewhat different from each other, are in error; as are those authors who attribute all variability to the mere act of sexual union. Nor can we account in all cases for the appearance through bud-variation of new characters by the principle of reversion to long-lost characters.”

In Chapter XI, Darwin gave specific examples supporting his thesis, writing in a section on flowers, “Many cases have been recorded of a whole plant, or single branch, or bud, suddenly producing flowers different from the proper type in colour, form, size, doubleness, or other character. Half the flower, or a smaller segment, sometimes changes colour.” He was particularly interested in Hibiscus (Paritium tricuspis), and spent a great deal of time researching that species. His discussion in the chapter states, “A seedling of this plant, when some years old, produced, at Saharunpore [India], 21 some branches which bore leaves and flowers widely different from the normal form. The abnormal leaf is much less divided, and not acuminated. The petals are considerably larger, and quite entire. There is also in the fresh state a conspicuous, large, oblong gland, full of a viscid secretion, on the back of each of the calycine segments." (Calycine refers to petals divided into thirds.) This must be, clearly, an evolutionary rather than sexual variation.

In early January 1871, Darwin received a letter from George King, physician, botanist, and Superintendent of the Royal Botanic Garden in India; King included samples of an aberrant Paritium tricuspis. Darwin recognized the subject of King’s letter as likely the same plant he had written about, and considered King’s report so important that when The Variation of Animals and Plants Under Domestication went into a second edition, he revised the book to reflect the information King had provided. His response to King’s letter follows.

Letter Signed on Darwin’s letterhead, three pages, January 27 [1871], to King in India.  “I am much obliged for your letter & for the great trouble which you have taken in sending me the specimens of the Paritium. I have no doubt this is the case to which I referred in my book. The difference in the leaves of the sport & parent tree is most surprising, but perhaps essentially not more than between a fern-leaved & comm[on] hornbeam. If you should succeed in proving that the characters of the sport are inherited, & if you will compare the flowers & calycine glands of the two forms, I think your M.S. [manuscript] would be well worth publication & I would with pleasure send it to the Linnean Society or to some Journal.”  The Linnean Society was [and remains] a prestigious organization dedicated to advances in biology, and particularly biological diversity and evolution. Only manuscripts of significance would have been submitted there. A sport is an offshoot of a parent; a hornbeam is a sturdy deciduous tree superficially resembling a beech.

Darwin is seen here providing words of encouragement, as he so liked to do. However, he apparently became convinced that King had, in fact, sufficiently proven his point, as he went back to the section of Chapter XI on Paritium tricuspis, and for his book’s next edition added this statement: “Dr. King who subsequently had charge of these Gardens, informs me that a tree of Paritium tricuspis (probably the very same plant) growing there, had a branch buried in the ground, apparently by accident; and this branch changed its character wonderfully, growing like a bush, and producing flowers and leaves, resembling in shape those of another species, viz., P. tiliaceum. A small branch springing from this bush near the ground, reverted to the parent-form. Both forms were extensively propagated during several years by cuttings and kept perfectly true.” Darwin, The Variation of Animals and Plants Under Domestication. The Papers of Charles Darwin indicate that he carried on a lengthy correspondence with King until shortly before his death. We are pleased to offer a letter confirming information that Darwin considered so significant that he inserted it in his book.