A colleague suggested I post in several installments my past response to critics of Icons of Evolution. The series begins here, which includes the full reviewer citations.
Part3
(c) Haeckel’s embryos
Aware of the problems with the fossil record, Darwin thought that the best evidence for his theory came from embryology. He believed that early vertebrate embryos “are closely similar, but become, when fully developed, widely dissimilar.” He concluded that this was not just evidence for common ancestry--it was “by far the strongest single class of facts in favor of” his theory. In the 1860s, German Darwinist Ernst Haeckel made drawings of vertebrate embryos to illustrate these “facts.” Yet (as his contemporaries pointed out) Haeckel faked his drawings:
Vertebrate embryos actually start out looking very different, then converge somewhat in appearance midway through development before becoming more different again as adults. Haeckel misrepresented the midpoint of development as the first stage, then he distorted the embryos at this point to make them look much more similar than they really are. [9]
Reviewer Jerry Coyne focuses most of his remarks (at least, most of the few that actually deal with science) on my treatment of vertebrate embryos--a daring move on his part, since I’m a vertebrate embryologist and he’s a fruit fly geneticist. Coyne begins by re-stating the standard view: “As Darwin first realized, some aspects of vertebrate development--especially transitory features--provide strong evidence for common ancestry and evolution. Embryos of different vertebrates tend to resemble one another in early stages, but diverge as development proceeds, with more closely related species diverging less widely. This conclusion has been supported by 150 years of research.” Coyne then takes me to task for foolishly trying “to refute this mountain of work.” (Coyne, p. 745)
Naturally, I would be grateful to Coyne for correcting me about this--if he were right. But his claim that vertebrate embryos are most similar in their early stages is dead wrong. As British zoologist Adam Sedgwick wrote in 1894, the claim is “not in accordance with the facts of development.” Comparing a dogfish with a chicken, Sedgwick wrote: “There is no stage of development in which the unaided eye would fail to distinguish between them with ease.” It is “not necessary to emphasize further these embryonic differences,” Sedgwick continued, because “every embryologist knows that they exist and could bring forward innumerable instances of them. I need only say with regard to them that a species is distinct and distinguishable from its allies from the very earliest stages all through the development.” (Emphasis in the original) [10]
Many other vertebrate embryologists have noted the same thing. In 1976, Dartmouth College embryologist William Ballard wrote that it is “only by semantic tricks and subjective selection of evidence,” by “bending the facts of nature,” that one can argue that the earliest stages of vertebrate embryos “are more similar than their adults.” And in 1987, Canadian embryologist Richard Elinson wrote that early developmental patterns in frogs, chicks and mice are “radically different.” [11]
So the “mountain of work” Coyne invokes actually buries his claim. But that doesn’t seem to bother him, because (in a cover-all-your-bases move worthy of Padian and Gishlick) he acknowledges that vertebrate embryos are not most similar in their early stages: “Wells also notes that the earliest vertebrate embryos (mere balls of cells) are often less similar to one another than they are at subsequent stages when they possess more complex features.” Like other evolutionary biologists, Coyne argues that the dissimilarity of early vertebrate embryos can be explained in the light of Darwin’s theory, since “the earliest stages of vertebrate embryos show adaptation” to the conditions of their existence. Coyne even regards this as evidence for the theory: “Wells repeatedly fails to grasp the evidential value of phenomena [i.e., dissimilarities in early embryos] that can be understood only as the result of a historical process.” (Coyne, p. 745)
So let me get this straight. Some of the strongest evidence for Darwin’s theory is that vertebrate embryos are most similar in their early stages--except that they’re not. But if we just interpret the embryos’ dissimilarities in the light of Darwin’s theory, they then have “evidential value.”
Oh, now I get it! Darwin’s theory wins no matter what the evidence shows. Apparently I was just ignorant of how evolutionary biology works.
NOTES:
[9] Charles Darwin, The Origin of Species, Chapter XIV; The Descent of Man, Chapter I. The quotation calling embryology “by far the strongest” evidence is from a September 10, 1860, letter to Asa Gray, in Francis Darwin (editor), The Life and Letters of Charles Darwin (New York: D. Appleton & Company, 1896), Vol. II, p. 131; the letter is cited in Ernst Mayr, The Growth of Biological Thought (Cambridge, MA: Harvard University Press, 1982), p. 470, and in Stephen Jay Gould, Ontogeny and Phylogeny (Cambridge, MA: Harvard University Press, 1977), p. 70.
[10] Adam Sedwick, “On the Law of Development commonly known as von Baer’s Law; and on the Significance of Ancestral Rudiments in Embryonic Development,” Quarterly Journal of Microscopical Science 36 (1894): 35-52.
[11] William W. Ballard, “Problems of gastrulation: real and verbal,” BioScience 26 (1976): 36-39, p. 38; Richard P. Elinson, “Change in developmental patterns: embryos of amphibians with large eggs,” pp. 1-21 in R. A. Raff & E. C. Raff (editors), Development as an Evolutionary Process, vol. 8 (New York: Alan R. Liss, 1987), p. 3. See also Jonathan Wells, “Haeckel’s Embryos and Evolution: Setting the Record Straight,” The American Biology Teacher 61 (1999): 345-349.
