Recapitulation : Yes or No? Revisiting von Baer  (2005.10.1[Sat])

 

It's not uncommon to see one's own theories oversimplified, misunderstood and cited in potentially misleading ways. For scientists, perhaps that just comes with the territory. The age of information overload has made it particularly important for us to keep pitching away to ensure that our messages don't get overlooked or lost in the fog. But is it any better to be the victim of misconstruing citations that result in being going down in history as a leading proponent of the opposite of one's true views? That precisely such a thing can happen is witnessed by the case of Karl Ernst von Baer, the renowned German comparative embryologist, whose work is summarized in his 1828 treatise Über die Entwicklungsgeschichte der Thiere, Beobachtung und Reflexion.

 When we hear the word "recapitulation," the tendency is to think of Haeckel, but von Baer preceded his thinking on the question by a half century, and various other theories of recapitulation date even further back still. But prior to the acceptance of the theory of evolution in Haeckel's time, the development of the embryo was thought to recapitulate the natural order or scala naturae, rather than the evolutionary process. From Aristotle onward, the order of the world was believed to begin with the basest worm, rising through frogs and lizards and past mammals on the way to the human pinnacle, a linear ascendancy that was thought to be paralleled in both the embryogenesis of man and evidence from the fossil record. It went so far as to adduce comparisons of the mammalian embryo to the fish and amphibian adult. It was Von Baer that strove to bring this classical theory of recapitulation in line with the state of the science of his day, propounding in the Entwicklungsgeschichte his law of developmental physiology, which stated that embryos tend to resemble the embryos, not the adults, of other species, and that general animal traits appear earlier in development than do specialized ones. Indeed, his views stood as a direct refutation of the theories of recapitulation prevalent at the time.

 It must also be noted, however, that von Baer clung to the simple categorical hierarchy in which embryonic development invariably progresses from invertebrate to tetrapod to amniote to mammal to primate, a line of thinking that, if refitted for lineages, is fundamentally indistinguishable from Haeckel's. And so today we can think of von Baer as both one of the fathers of the theory of recapitulation, and one of its greatest critics. For his part, he undoubtedly would have considered himself the latter. But with the perspective afforded by hindsight, we are also obliged to remember that he was the founder of the modern version of the theory of recapitulation.We would also do well to remember that von Baer's Entwicklungsgeschichte is the first work to note the three germ layers of the chicken embryo (the germ layer theory), an advance that gave comparative morphology a solid developmental foundation and simultaneously helped to deflate rival preformationist theories. For the day, it was a revolutionary scientific achievement.