A Duck Folded in Half

Armand Marie Leroi

  • Before the Backbone: Views on the Origins of the Vertebrates by Henry Gee
    Chapman and Hall, 346 pp, £35.00, August 1996, ISBN 0 412 48300 9

The evening of 22 August 1799 – the eve of his departure from Egypt – was surely one of the less happy that Napoleon Bonaparte had known. Unusually mindful of the mortality of empires, he is said to have declared to the mathematician Gaspard Monge, one of a collection of savants he had brought to Egypt, that he would far rather be a Newton than an Alexander. To which Monge replied that no one could attain again to the glory of Newton for there was only one world to discover. Not so, said Napoleon: there is still the ‘world of details’ and the laws that govern them – a sentiment curiously apposite to his expedition. For trailing him out of Egypt came not only the Rosetta Stone, but a young professor from the Muséum d’Histoire Naturelle in Paris: Etienne Geoffroy St-Hilaire, a man whose mind was torrid with details and with the desire to find the laws governing them.

The British took the Rosetta Stone, but allowed France to keep Geoffroy, as well as his collection of crocodiles, ichneumons, and mummified ibises, which he had threatened to burn rather than cede to the victors. Geoffroy was one of nature’s romantics: ostensibly a descriptive anatomist, he investigated the devices by which puffer fish inflate themselves, but did not shy away from larger problems, such as the relationships between the ‘imponderable fluids’ of the universe (light, electricity, nervous energy etc), his deductive theory of which never saw print. More usefully, it was also in Egypt that he had the first glimmerings of what would become his philosophie anatomique: a grand scheme to demonstrate the fundamental structural unity of all animals.

Initially, the goal was a modest one. Geoffroy attempted to show that structures which appear in mammals were the same, only modified, as those which appear in other vertebrates, such as fish, reptiles and amphibians. In other words, he attempted to identify what we now call homologues, arguing, for example, that the opercular bones of fish (which cover the gills) were essentially the same as the tiny bones that make up the middle ears of mammals (the malleus, incus and stapes). Today this is a familiar idea, usually presented in school textbooks as the notion that the wing of a bat, the flipper of dolphin, the hand of a chimpanzee and so on are all ‘the same’. But opercular bones were small beer for a truly synthetic thinker: Geoffroy went on to find homologies between the most wonderfully disparate structures in the most wildly different creatures. Confronted with the exoskeleton of an insect and the vertebrae of a fish, he proposed that they were one and the same structure. To be sure, insects have an exoskeleton (meaning all their guts are inside their hard parts) while fish have an endoskeleton (bones are surrounded by tissue), but where other anatomists saw this as ample reason to keep them distinct, he explained with the simple confidence of the visionary that ‘every animal lives within or without its vertebral column.’ Not content with this, he went on to show how the anatomy of a lobster was really very similar to that of a vertebrate – if only you flipped it upside down. Where lobsters carry their major nerve cord on their ventral sides (bellies) and their major blood vessels on their dorsal sides (backs), the reverse is true for vertebrates. And then there was the curious case of cephalopods: if one took a duck and folded it in half backwards so that its tail touches its head (an exercise performed only on paper, I believe), did its anatomy not remarkably resemble that of a cuttlefish?

It did not. Geoffroy’s speculations attracted the wrath of Baron Cuvier, greatest of living anatomists, and his powerful rival at the Muséum. The result was a debate in front of the Académie Française in 1829 which Geoffroy lost – a duck doesn’t look like a cuttlefish no matter how you bend it; even the homologies between fish opercula and the mammalian middle ear didn’t bear serious scrutiny. Yet if the particular homologies that he proposed sometimes seemed absurd, even in his day, his general method was not. Different organisms do have structures that are somehow similar yet modified, and the similarities are well worth studying. Indeed, the idea of homology is today so commonplace in biology (we speak of homology among DNA sequences as easily as among tetrapod fore-limbs) that it is easy to read into Geoffroy’s claims an evolutionary meaning he did not intend. The homologies that he saw, or thought he saw, were as far as he was concerned, placed there by the Creator. It was the age of what would be called Transcendental Anatomy.

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