The Reception of Karl Ernst von Baer's Law

On September 10, 1860, Charles Darwin wrote to his friend, the Harvard biologist Asa Gray, "Embryology is to me by far the strongest class of facts in favor of change of forms." This statement is remarkable in that it had been assumed that embryology provided evidence against evolution, and another Harvard biologist, Louis Agassiz, indeed was using embryology against Darwin's hypothesis. How could Darwin say that embryological evidence supported evolution? The key was the embryological law of Karl Ernst von Baer, a law that was supposed to be against the transformation of species.

Figure 1 Karl Ernst von Baer

When von Baer proposed his law of animal development in 1828, it was thought of as anti-evolutionary. Evolution, in those pre-Darwinian days, did not have the mechanism of natural selection to solidify it. Rather, evolution was seen as equivalent to embryonic development (and was often called either "development" or "transformation"). Those theories of transformation were linear, wherein the embryos of advanced ("higher") animals represented the adults of primitive animals. Thus, the protists were like the one-celled egg, the hydra was like the gastrula stage and so forth. As one progressed, human embryos (and humans were obviously considered the highest grade of all animals) passed through a stage where they looked (supposedly) like adult fish (i.e., human embryos had rudimentary gill slits) (see Gould, 1977 for further history of transformationism).

Von Baer disproved this transformationist notion, showing instead that human and fish embryos shared a common stage, but then both diverged from it. "The embryonic vertebrate, at every stage, is an undeveloped and imperfect vertebrate, it can represent no adult animal whatever." Like other biologists of the early nineteenth century, von Baer argued for four basic animal groups, defined by the forms of symmetry found in their body plans. Each body plan was seen to be created by a particular kind of developmental organization—the Type. Following Cuvier, von Baer recognized four Types—radiates (cnidarians, ctenophores, echinoderms), articulates (arthropods), vertebrates, and molluscs (nearly everything else). Von Baer thought that the transformationists made a major error in equating the "type of organization" with the "grade of development." The grade of development was how specialized each organ was. Homogeneity was the primitive condition, heterogeneity (specialization) was the higher condition. The animals could not be compared between the four types. Thus, in the early stages of "higher animal" embryos there should be structures that are similar to the structures seen in more primitive embryos within the same type. But nowhere does an early mammalian embryo resemble a fish, since the fish embryo has acquired structures that the mammal never acquires.

Von Baer was not averse to using his lively wit and sense of irony to ridicule transformationists. He wrote (1828) a paragraph of his book as if a bird were a transformationist biologist writing such a volume. This bird, he said, would note that adult humans lack beaks, just as embryonic birds do. Their adult extremities are very much alike (having not differentiated into wings and legs) just as is the case of embryonic birds. Furthermore, we mammals show only the most primitive form of feather production. Therefore (this avian scientist would conclude), birds are far more advanced than humans, and the adult human resembles the embryonic bird.

Thus, von Baer argued against transformationism, the evolutionary hypothesis of his day. Indeed, to his dying day (and he died long after Darwin wrote The Origin of Species), he did not reconcile himself to evolution. (Even when Kowalewsky discovered the notochord in tunicate larva, von Baer refused to believe tunicates to be allied to chordates. He thought them to be molluscs, and he never appreciated that his embryological observations actually broke the notion of the four Types.)

However, his work became central to Darwin's evolutionary biology. As historian Dov Ospovat (1981) concluded:

Von Baer perceived nature as a combination of diversity and underlying unity, with stringent limits placed on the extent to which such unity could be traced. In this perception, which received its clearest expression in his description of of development, von Baer laid down the main lines of the branching conception of the organic world.

This branching conception of the animal world, where there is a common relatively undifferentiated origin (the egg in development, the primitive representative of the type ["archetype"] in the animal kingdom) from which more derived forms emerge, was critical for Darwin's ordering of nature. Von Baer's ideas entered British biology through Milne Edwards and Martin Barry, both of whom used von Baer's law as the key to animal classification. Barry (1837), in particular, saw embryological structures as being more important for classification than adult structures. He saw that if von Baer were correct, then all animals of a type started off following similar developmental trajectories and then veered away from each other as their development proceeded. The earliest developmental stages should show the most homologies. Thus, all animals had an egg, and had three germ layers which did pretty much the same thing in every animal. Both Edwards and Barry use the branching tree metaphor, and Darwin wrote to Edwards saying how much he appreciated his linking of development with evolution (see Ospovat 1976, 1981).

In his sketch of the evolutionary theory in 1842, Darwin (quoted in Gould 1977) wrote, "It is not true that one passes through the form of a lower group, though no doubt fish are more nearly related to foetal state," and he would later state explicitely his evolutionary interpretation of von Baer's law: " The less difference of foetus &mdash that has obvious meaning on this view: otherwise how strange that a horse, a man, and a bat should at one time have arteries running in a manner that is only intelligible in a fish! The natural system being on theory geneological, we can see at once why foetus, retaining traces of the ancestral form, is of highest value in classification."

Von Baer's law of development provided Darwin with three essential pieces of his evolutionary theory. First, it offered Darwin a natural mechanism for a branched tree-like pattern of evolutionary divergence. Darwin's evolutionary theory would not have to be confined to the linear view of the transformationists before him. Second, von Baer's observations of vertebrate embryos offered Darwin the notion that homologous structures could be explained by common descent. Third, and conversely, it gave him the insight that embryonic structures could provide the basis for biological classification. Thus, he would conclude in the Origin of Species, (1859, p. 449) "Community of embryonic structure reveals community of descent." Even before natural selection came to dominate his thinking about the mechanism of evolution, Darwin saw changes in development as the way that nature could produce descent with modification. No wonder he wrote to Gray that embryology was first among the sciences to provide facts in favor of his theory of evolution.

So strongly were von Baer's views taken up by evolutionary biologists, that certain evolutionary biologists presented their data in such a way as to try to make their non-Baerian embryological ideas fit into von Baer's framework. Nowhere was this more apparent than in the work of Ernst Haeckel, a German evolutionary biologist who attempted to synthesize Darwin's biology with the old linear-chain transformationism. Although his ideas were precisely the ones ridiculed and disproven by von Baer, Haeckel praised von Baer and actually listed von Baer as one of his supporters! For more about this episode, click here.

Literature Cited

Baer, K. E. von. 1828. Entwicklungsgeschichte der Thiere: Beobachtung und Reflexion. Bornträger, Königsberg. (Gould, 1977, op. cit., translates the paragraph written as if by a bird.)

Barry, M. 1837. On the unity of structure in the animal kingdom. Edinburgh New Phil. J. 22: 116-141, 345-364.

Darwin, C. 1860. Letter to A. Gray. Quoted in Gould, op. cit., 1977.

Darwin, C. 1859. The Origin of Species. John Murray, London.

Gould, S. J. 1977. Ontogeny and Phylogeny. Harvard University Press, Cambridge.

Ospovat, D. 1976. The influence of Karl Ernst von Baer's embryology, 1828-1859: A reappraisal in the light of Richard Owen's and William B. Carpenter's paleontological application of von Baer's law. J. History of Biology 9: 1-28.

Ospovat, D. 1981. The Development of Darwin's Theory: Natural History, Natural Theology, and Natural Selection, 1838-1859. Cambridge University Press, New York.

© All the material on this website is protected by copyright. It may not be reproduced in any form without permission from the copyright holder.

HOME :: CHAPTER 1 :: THE RECEPTION OF VON BAER'S PRINCIPLES :: THE RECEPTION OF KARL ERNST VON BAER'S LAW

PREVIOUS :: NEXT