Such expressions as that famous one of Linnæus, and which we often meet with in a more or less concealed form, that the characters do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent,—the only known cause of the similarity of organic beings,—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications (Darwin, 1859, p. 413f).

Wednesday, 21 November 2007

Urhomology and Perfection

Many of you may wonder why we have named the URL of our blog

The idea of a urhomology appeared to us when reading through Goethe’s scientific works on comparative biology. Goethe did not exactly discover homology, as the concept already existed. Vic D’Azyr and Geoffroy Saint Hilaire had already discovered serial and general homology respectively. Goethe was aware of the concept and closely followed the debate between Cuvier and Saint Hilaire.

Goethe did however have his own homology concept which he never coined nor referred to by name. We have therefore though it necessary to not only coin urhomology but also investigate its place in the history of homology.

Urhomology is undoubtedly influenced by the serial and general homology concepts. We chose to use the prefix ur in order to refer to an overall concept (i.e. urphenomenon) rather than to a functional homology concept (i.e. Remane's homology criteria).

Urhomology is a concept that states that two taxa are related by a third taxon by their characters. This does not appear to be remarkable at first glance until we discover what Goethe meant by characters.

In cladistics we refer to primitive and derived characters. The basal nodes of a cladogram contain the "primitive" or plesiomorphic characters and the terminal branches nested contain the apomorphic characters. The same terms are used for taxa. Plesiomorphic taxa represent "primitive" characteristics and so on. Goethe shunned the idea that a taxon or any organism can be primitive or derived. We may say that Archaeopteryx is a primitive bird. But from the point of view of the taxon it is perfect in its own right. Birds did not evolve to become "primitive" or "derived". Finches (part of the "derived" passerine clade) may appear to be derived today, but in a few 100 million years they too will be labeled “primitive” by future cladists. For Goethe primitive and derived were arbitrary terms, they meant nothing in classification.

Taxa however do possess general and specialized characteristics. A worm for instance has very similar looking organs where as a lemur has highly specialized organs. Goethe never used the terms “general” or “specialized” - possibly because they too are arbitrary and related to function rather than to structure – instead he used a "sliding scale" of "ideal" traits. A lemur is far more "ideal" in terms of structure than say a tardigrade. We however prefer to use the terms general and specialized.

Goethe's problem was how to compare different organisms that were not from the same group. How would one compare an echidna to a human when neither organism shares the same obvious structure? The answer is to use a third taxon. Goethe referred to this as an intermediary taxon, which should not be confused with a transitional form. As far as our reading of Goethe goes, he did not considered transformation between taxa. Instead Goethe saw that the same structure appears in different taxa. This was the key to relating taxa, by their same structures or homologues. In this sense echidnas can be related to humans based on the same structure, say a forearm, but it needs a third taxon in order to validate the relationship, such as a lemur. In short, Goethe's urhomology can be used to discover relationships between taxa no matter how general or specialized they appear, as long as they share the same structure.

Goethe did not go on to explore the concept of an urhomology further, but it stands a major contribution to science nonetheless - one that predates Owen and may have influenced the English anatomist's Special Homology. One concept that Owen did not correctly interpret from Goethe was that of the archetype or urphenomenon. But more of that in a later post.

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