Charles Darwin Paraphrases His Conclusion of “Origin of Species” in Praising a Scientist for Embracing Evolution

He also is fascinated to learn of variations in species of insects and suggests an experiment to test it

Charles Darwin's On the Origin of Species was one of the most important books of all time. When it was published in 1859, there was skepticism within (as well as without) the scientific community about the central premise, that species changed through time by a process Darwin labeled natural selection.  The idea that human beings evolved from other, previous species struck at the heart of the generally accepted belief that humans were different from other animals, and that species were fixed or immutable. Everyone had learned the story of creation from the Bible, and Darwin's ideas presented a challenge to those who looked at the question through a literal, religious perspective. But Darwin hoped that scientists as well as others would be able to accommodate, adopt an essentially new view and accept evolution.  Yet, at the same time, he realized that many who had grown up in the previous scientific school would have a hard time doing so. It seemed that perhaps a new generation of scientists might have to rise before his work was generally accepted.

At the end of Chapter 14, the final chapter of the groundbreaking work, Darwin summed up his theory and specifically dealt with whether his findings would be accepted.  "Although I am fully convinced of the truth of the views given in this volume under the form of an abstract, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as the 'plan of creation,' 'unity of design,' &c., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject my theory. A few naturalists, endowed with much flexibility of mind, and who have already begun to doubt on the immutability of species, may be influenced by this volume. But I look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality. Whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction; for only thus can the load of prejudice by which this subject is overwhelmed be removed."

Perhaps the type of man he was waiting for was Robert MacLachlin, a naturalist nearly 30 years younger than Darwin, who had only recently emerged on the scientific scene in England.  In December 1865, before he reached the age of 30, he published a work noting evolution or "development" in a population of moths called Sterrha.  "A certain divergence will gradually become wider, until eventually they develop into what is considered as a species."  While MacLachlin was not prepared to embrace "development theory to the full extent," he felt it "a reasonable way of accounting for phenomena which otherwise cannot be satisfactorily settled." He went on to define the debate in much the same way Darwin would.  "A faithful description of a new 'form' or 'species' is as useful to a naturalist holding the one view as to another who prefers to adhere to old ideas; it is only that one looks upon the origin of that species in a different light."   MacLachlin would go on to a great career in science and serve as President of the Entomological Society of London.

Early in 1866, he wrote to Darwin, whose theory had not yet gained wide acceptance, and who was in the process of publishing the fourth edition of his work, sending the results of his study, which professed a belief that species can change, the basis of Darwin's natural selection.  Darwin was also working at the time on a book about the Variations of Plants and Animals.  MacLachlan cited two separate insect organisms, Neuroptera and Eupithecia, who showed variations consistent with "development theory," as MacLachlin called it.  In the case of the insect Eupithecia, MacLachlin noted how the larva could change color based on the food it was eating, an adaptation that might allow insects to hide in their natural habitat.  They used "mimicry" of color to change.  Darwin must have been moved to see his theory put into action in just the way he had hoped, and responded with this letter in which he very closely paraphrased his final chapter of Origin of Species.  He also used it as an opportunity to learn and to advise the young man.

Letter signed, Kent, 23 March, 1866, to MacLachlin, on mourning stationery. "I am very much obliged to you for your paper on Sterrha, which I have read with very great interest. Many of your observations are quite new to me.  I was particularly glad to read about the varying organs in the Neuroptera; should you ascertain similar cases in the Trichoptera, I would should be grateful for information.  Would it not be well worth while to get some of your friends to breed Eupithecia from eggs of the same female and to feed them on different plants and thus ascertain positively how the mimicry is effected?

"I have been very glad to see (whether or not you have been influenced by my writings) that you have given up to a great extent the belief in the immutability of species; & I feel sure that as you attend to other subjects besides the discrimination of species you will ultimately go further in your belief; at least I have hitherto found this to be the case with those who have doubted to a limited degree. "

Compare this to his conclusion in Origin: "A few naturalists, endowed with much flexibility of mind, and who have already begun to doubt on the immutability of species, may be influenced by this volume."

A remarkable and very rare letter relating directly to his greatest work, and showing that despite opposition, he was confident that his theories would prevail. It also is significant to see him mentor a young scientist who would go on to a fine career.