Did Darwin respond to Wallace regarding pitcher plants?

UPDATE (9/14): It dawned on me yesterday that while I have provided here at The Dispersal of Darwin many examples of anti-evolutionists claiming Darwin said something when he did not (quote-mining), this post is an example of Darwin having written something and then it being claimed that he did not. Interesting.

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In 1875 Darwin published his book about plants that eat insects, Insectivorous Plants. It was rather technical in nature, so did not receive the popular readership as did his Journal of Researches (1839, later The Voyage of the Beagle), On the Origin Of Species (1859), or the later (and last book) The Formation of Vegetable Mould, through the Action of Worms (1881). Like many of his books, Insectivorous Plants was a continuation of Darwin’s theory of transmutation project. Specifically, the book is a study of the adaptations of such plants to impoverished conditions. Darwin wrote of it in his autobiography:

During subsequent years, whenever I had leisure, I pursued my experiments, and my book on Insectivorous Plants was published July 1875,—that is sixteen years after my first observations. The delay in this case, as with all my other books, has been a great advantage to me; for a man after a long interval can criticise his own work, almost as well as if it were that of another person. The fact that a plant should secrete, when properly excited, a fluid containing an acid and ferment, closely analogous to the digestive fluid of an animal, was certainly a remarkable discovery.

A remarkable discovery indeed, but a fellow naturalist, whom Darwin shared the discovery of the theory of natural selection with, was concerned that some would not find natural selection a suitable explanation for the adaptations of carnivorous plants. In a letter to Darwin on July 21, 1875, Alfred Russel Wallace wrote:

Dear Darwin,–Many thanks for your kindness in sending me a copy of your new book [Insectivorous Plants]. Being very busy I have only had time to dip into it yet. The account of Utricularia is most marvellous, and quite new to me. I’m rather surprised that you do not make any remarks on the origin of these extraordinary contrivances for capturing insects. Did you think they were too obvious? I daresay there is no difficulty, but I feel sure they will be seized on as inexplicable by Natural Selection, and your silence on the point will be held to show that you consider them so! The contrivance in Utricularia and Dionaea, and in fact in Drosera too, seems fully as great and complex as in Orchids, but there is not the same motive force. Fertilisation and cross-fertilisation are important ends enough to lead to any modification, but can we suppose mere nourishment to be so important, seeing that it is so easily and almost universally obtained by extrusion of roots and leaves? Here are plants which lose their roots and leaves to acquire the same results by infinitely complex modes! What a wonderful and long-continued series of variations must have led up to the perfect “trap” in Utricularia, while at any stage of the process the same end might have been gained by a little more development of roots and leaves, as in 9,999 plants out of 10,000!

Is this an imaginary difficulty, or do you mean to deal with it in future editions of the “Origin”?–Believe me yours very faithfully,
ALFRED R. WALLACE.

Letters to and from Darwin of 1875 are not yet available through the Darwin Correspondence Project, but this letter can be found on pages 233-34 of Alfred Russel Wallace: Letters and Reminiscences (edited by James Marchant, New York: Harper & Brothers, 1916). Wallace’s words in this letter have been taken up by intelligent design proponents as a way to criticize Darwin. Remember, Wallace is the new poster boy for the Discovery Institute. In “Carnivorous plants eat Darwin” (August 18, 2011), Denyse O’Leary (also blogging about this at The ID Report) writes for Uncommon Descent:

University of Bonn geneticist Wolf-Ekkehard Lönnig will soon have a new book out, on the 200-year-old headache that carnivorous plants pose for Darwinism. Briefly, how does a plant evolve in slow, Darwinian steps, toward making insects part of its normal diet? Like the pitcher plant, for example.

O’Leary quotes Granville Sewall in the post:

In every family of the plant and animal kingdoms there are species whose sudden appearances and whose irreducibly complex features pose problems for neo-Darwinism. But certain carnivorous plants pose these problems in such a spectacular way that they are a focal point of the Darwinism debate, ever since Alfred Wallace warned Darwin about the problems posed by Utricularia, saying “I feel sure they will be seized on as inexplicable by Natural Selection” and implored him to address these difficulties in a future edition of his book “On the Origin of Species.”

These words are indeed from Wallace, in the letter to Darwin above. The way they are being used, however, seems to imply that Wallace finds natural selection an unconvincing explanation, whereas he is only stating that others might criticize Darwin for this (Wallace remarked, “I daresay there is no difficulty”). Moreoever, O’Leary writes in her post, in response to Wallace imploring Darwin “to deal with it in future editions of the Origin,” that “Darwin never did.” To state that Darwin never responded to Wallace’s question in a later edition is to imply that Darwin gave no response at all.

If one were to look in the historical record more deeply, they would find that Darwin did indeed respond to Wallace. On July 22, 1875, one day after Wallace’s letter about Utricularia, Darwin wrote to Wallace that he had “thrown some light on the acquirement of the power of digestion in Droseraceae,” another group of carnivorous plants (unfortunately there is no full text of the letter available until the DCP publishes the 1875 letters; they are currently readying 1871 for print). Darwin is referring to pages 361-63 of Insectivorous Plants:

The six genera of the Droseraceae very probably inherited this power from a common progenitor, but this cannot apply to
Pinguicula or Nepenthes, for these plants are not at all closely related to the Droseraceae. But the difficulty is not nearly so great as it at first appears. Firstly, the juices of many plants contain an acid, and, apparently, any acid serves for digestion. Secondly, as Dr. Hooker has remarked in relation to the present subject in his address at Belfast (1874), and as Sachs repeatedly insists, the embryos of some plants secrete a fluid which dissolves albuminous substances out of the endosperm; although the endosperm is not actually united with, only in contact with, the embryo. All plants, moreover, have the power of dissolving albuminous or proteid substances, such as protoplasm, chlorophyll, gluten, aleurone, and of carrying them from one part to other parts of their tissues. This must be effected by a solvent, probably consisting of a ferment together with an acid.† Now, in the case of plants which are able to absorb already soluble matter from captured insects, though not capable of true digestion, the solvent just referred to, which must be occasionally present in the glands, would be apt to exude from the glands together with the viscid secretion, inasmuch as endosmose is accompanied by exosmose. If such exudation did ever occur, the solvent would act on the animal matter contained within the captured insects, and this would be an act of true digestion. As it cannot be doubted that this process would be of high service to plants growing in very poor soil, it would tend to be perfected through natural selection. Therefore, any ordinary plant having viscid glands, which occasionally caught insects, might thus be converted under favourable circumstances into a species capable of true digestion. It ceases, therefore, to be any great mystery how several genera of plants, in no way closely related together, have independently acquired this same power.

So when asked by Wallace how to account for the evolution of one particular group of carnivorous plants, Darwin responded that his thoughts about another group should answer the question, it is understandable that Darwin need not have addressed this issue in a future edition of On the Origin of Species.

In his book, available as a PDF here, Lönnig quotes Wallace on page 145, and states (this is a Google translation from German), “I am not aware that Darwin has replied…” Well, to set the record straight, he did reply.

As I am not one to go into the actual biology of this issue, see Nick Matzke’s comments on O’Leary’s post and two others about carnivorous plants. The Darwin Correspondence Project has many letters to and from Darwin on carnivorous plants (Drosera and Utricularia), and some from Mary Treat about Utricularia have been published online ahead of print as part of the project’s Darwin and Gender initiative.