We all know that Darwin missed at least one important finding during his lifetime: the work of Gregor Mendel. It’s unlikely that he actually knew of it, or that he would have appreciated its significance if he had (it would have solved the dilemma, pointed out by the engineer Fleeming Jenkin, that Darwin’s own theory of blending inheritance inexorably erodes the genetic variation necessary for natural selection to work), but the lack of a Darwin-Mendel conjunction is often mourned by evolutionists. Now a trio of scholars have found another “missed opportunity” for the Sage of Downe.
In a new communication to Current Biology, Adam Hart et al. report the discovery of a letter written to Darwin by the British entomologist Albert Brydges Farn (1841-1921). Farn lays out in his letter the evidence that color variation and change in the moth Gnophos (now Charissa) obscurata, called the “annulet,” reflected the action of natural selection. It’s the peppered-moth story in a different species. Had Darwin followed this up, say Hart et al., he would have apprehended a crucial piece of evidence missing from his theory: observation of natural selection in action. Here’s Farn’s letter, sent to Darwin on November 18, 1878.
My dear Sir,
The belief that I am about to relate something which may be of interest to you, must be my excuse for troubling you with a letter.
Perhaps among the whole of the British Lepidoptera, no species varies more, according to the locality in which it is found, than does that Geometer, Gnophos obscurata. They are almost black on the New Forest peat; grey on limestone; almost white on the chalk near Lewes; and brown on clay, and on the red soil of Herefordshire.
Do these variations point to the “survival of the fittest”? I think so. It was, therefore, with some surprise that I took specimens as dark as any of those in the New Forest on a chalk slope; and I have pondered for a solution. Can this
It is a curious fact, in connexion with these dark specimens, that for the last quarter of a century the chalk slope, on which they occur, has been swept by volumes of black smoke from some lime-kilns situated at the bottom: the herbage, although growing luxuriantly, is blackened by it.
I am told, too, that the very light specimens are now much less common at Lewes than formerly, and that, for some few years, lime-kilns have been in use there.
These are the facts I desire to bring to your notice.
I am, Dear Sir, Yours very faithfully,
A. B. Farn
In fact, the idea of industrial melanism as evidence for natural selection was not explicitly suggested until 1896 (14 years after Darwin’s death) by the British entomologist James William Tutt.
There’s no evidence that Darwin answered Farn’s letter, but of course he had voluminous correspondence with hundreds of people, and it’s too much to fault the old man for failing to follow up on this one suggestion. And even had he done so, that might not have hastened the general acceptance of natural selection, which after all was not widely embraced by evolutionists until around 1930. Nevertheless, it’s interesting to contemplate Darwin’s reaction as he read this letter. Did he blow it off? Or did it simply get lost in the welter of his correspondence?
Hart et al. imply that this is the one crucial missing link in Darwin’s “chain of evidence,” but of course there’s another: the fossil record. Archaeopteryx, a transitional fossil spanning reptiles and birds, was known in Darwin’s time—and he even mentions it in a later edition of The Origin*—but he didn’t seem to have grasped its significance. However, the evidence for common ancestry that could have been provided by Archaeopteryx was also supplied by several other areas of biology, notably embryology and the study of vestigial features.
Fig. 1. The annulet, Charissa obscurata. Looks a lot like Biston betularia, no?
*p. 367 of the fourth edition: “Until quite recently these authors might have maintained, and some have maintained, that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of Solenhofen. Hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world.”
Hart, A. G. R. Stafford, A. L. Smith, and A. E. Goodenough. 2010. Evidence for contemporary evolution during Darwin’s lifetime. Current Biology 20:R95.
h/t: Matthew Cobb