Literature

Robert Deyes

Historically the field of evolutionary biology has been plagued with a fundamental move to fit or adapt scientific data into the picture of organic continuity that Darwin painted in The Origin Of Species (Ref 1). So strong has been this move that on occasions scientific objectivity has been lost and replaced by, what could only be described as a 'force of fit' to ensure that the validity of Darwinian theory was maintained (Ref 1). The unity of form observed during embryonic development was according to Darwin indicative of "the progenitor [. . .] of all the members of the same great class" (Ref 2, p.600). Darwin's conviction over the importance of the embryological picture to his theory was made plainly clear in his autobiography where he wrote that,

"hardly any point gave me so much satisfaction when I was at work on The Origin, as the explanation of the wide difference in many classes between the embryo and the adult animal, and of the close resemblance of the embryos within each class" (Ref 3, p.125)

Haeckel's embryo drawings (source here)

Darwin was not an embryologist and so relied very heavily on the viewpoints of his contemporaries. One of these was Professor Ernst Haeckel who, famous for his comparative drawings of embryos including those of fish and amphibians, brought seemingly strong confirmatory evidence to the Darwinian thesis. Darwin had a high regard for Haeckel's perspective and saw Haeckel's study of embryonic characters as "a great beginning" that showed us "how classification will in the future be treated" (Ref 2, p. 579). And yet Haeckel was overzealous and in making his embryo drawings, depicted a number of important details incorrectly (Ref 4). As Michael Richardson and his colleagues pointed out in a letter to Science, significant variations in embryonic size and form were missed (Ref 4). What Haeckel's inaccuracies demonstrated was a zeal to make the data 'fit' the theory. After all, these inaccuracies tended towards supporting the idea of a common progenitor rather than negating it. Richardson and his colleagues revealed the degree of 'supportive approximation' that Haeckel employed in his drawings when they published photographic evidence of progressive stages of development amongst different vertebrate species (Ref 4). Their results showed that, while there was a close correlation between embryonic developmental sequences between humans and other mammals, there was a very weak correlation between humans and other classes of vertebrates. (For Richardson's photos, go here).

Today Haeckel is remembered for his biogenetic law in which he proposed that "ontogeny recapitulated phylogeny" (Ref 5, p. 353). Haeckel theorized that animals climbed up their own family tree during embryonic development and as such presented us with a vision of how things had once been. Armed with his inaccurate representations of embryonic development and the convincing arguments of his biogenetic law, Haeckel created the phylogenist's 'dream' (Ref 6, pp. 246-247). However, by the beginning of the twentieth century, the shine of recapitulation had lost its luster. Gould recounts the disappointment that ensued with a quote from E.B Wilson's description of the "exact" experimental method:

It is a ground of reproach to morphologists that their science should be burdened with such a mass of phylogenetic speculations and hypotheses, many of them mutually exclusive, in the absence of any well-defined standard of value by which to estimate their relative probability. The truth is that the search [. . .] has too often led to a wild speculation unworthy of the name of science; and it would be small wonder if the modern student, especially after a training in the methods of more exact sciences, should regard the whole phylogenetic aspect of morphology as a kind of speculative pedantry unworthy of serious attention." (Ref 6, p. 247)

Paleontologists David Raup and Steven Stanley were equally emphatic about the inaccuracies of Haeckel's claims:

"During the eighteenth and early nineteenth centuries, many students of Mesozoic ammonites attempted to apply Haeckel's recapitulation theory to all ammonite species, believing that the course of ammonite evolution could thus be read from ontogenetic changes in shell ornamentation and suture patterns. In 1901, Pavlow invalidated the strict recapitulation concept by showing that in certain Jurassic lineages of ammonites new evolutionary features arose in the early stages of ontogeny; not until later in the evolutionary history of their respective lineages were these changes retained in the adult stages [. . .] ontogenetic development of the new features was retarded, relative to time of reproductive maturation." (Ref 7, p.275)

In other words, changes in the adult form arose after changes in the embryo, not before. It could no longer be convincingly argued that ontogenetic changes were a reflection of some evolutionary past. In light of such findings, it is paradoxical that prominent zoologists such as Richard Dawkins still maintain the importance of the so called 'embryological continuum'. Dawkins wrote, for example, that science, "can point out that the (embryological) continuum that seamlessly joins a non-sentient foetus to a sentient adult is analogous to the (evolutionary) continuum that joins humans to other species" (Ref 8, p.34). Likewise science writer David Quammen emphasized the classical Darwinian perception of the embryo not only as "the animal in its less modified state" but also the embryo as revealing, "the structure of its progenitor" (Ref 9, p. 13). Obviously, Dawkins and Quammen are at odds with the alternative, more persuasive assessment that was built on empirical evidence.

"The assumption that ancestral reminiscences could always be distinguished from recent embryonic adaptations had not been sustained. Too many stages were missing, too many others discombobulated. The application of Haeckel's law produced endless, unresolvable, fruitless argument, not an unambiguous tree of life." (Ref 6, p. 246)

These findings are in themselves preliminary indicators that the different classes of vertebrate are discontinuous. The data does not fit Darwin's premise of a few common progenitors from which all of life had originated.

REFERENCES

1. Gareth Nelson (1998), Colin Patterson (1933-98): Paleontologist-reformer of the fossil record, Nature, Volume 394 p626

2. Charles Darwin (1859), The Origin of Species By Means of Natural Selection Or The Preservation of Favored Races In the Struggle For Survival, Modern Library Paperbacks Edition (1998), New York

3. Charles Darwin, The autobiography of Charles Darwin, Copyright held by Nora Barlow in 1958, W.W. Norton and Company Inc, New York

4. Michael K. Richardson, James Hanken, Lynne Selwood, Glenda M. Wright, Robert J. Richards, Claude Pieau, Albert Raynaud (1998), Haeckel, Embryos, and Evolution, Science Vol 280 p293

5. Stephen Jay Gould (2002), The Structure of Evolutionary Theory, Belknap Press of Harvard University Press, Cambridge, Massachusetts

6. Stephen Jay Gould (1992), The Panda's Thumb- More Reflections In Natural History W.W Norton and Company, New York

7. David Raup and Steven Stanley (1971), Principles of Paleontology, W. H. Freeman and Company, San Francisco

8. Richard Dawkins (2003), A Devil's Chaplain, Published by Weidenfeld and Nicolson, London

9. David Quammen (2004), Was Darwin Wrong? National Geographic Magazine, November 2004 pp4-31