Caricature of Richard Owen. "Old Bones" Vanity Fair, March 1st, 1873.

I named this blog after the concept of the archetype as articulated by the Victorian naturalist Richard Owen (1804-1892). For Owen, the archetype was a representation that summed the most basic, most generalized structural arrangement common to all the members of a given group of organisms. Owen’s well-known and most important contribution to modern biological thought is, however, not his archetype concept but the clear distinction he provided between the concepts of analogy and homology. On his words:

Analogue.- A part or organ in one animal which has the same function as another part or organ in a different animal.
Homologue.- The same organ in different animals under every variety of form and function. (Owen, 1843: 374, 379)¹

Homology is a concept that expresses the relationship between parts of organisms. It reflects the observation that we can identify a commonality of structure across the diversity of life. Homology thus forms the cornerstone of comparative biology.

Although the archetype is a forgotten concept, this abstract blueprint was central to Owen’s views on homology. During his lectures and in his publications, Owen distinguish between three kinds of relations of homology: special homology, serial homology and general homology. Special homology refers to the correspondence of body parts between species. Serial homology identifies the repeated elements within the body of an organism. General homology represents the correspondence of an element between a species and the archetype.

Both special homology and serial homology are in common use today, the former is particularly applied in systematics where it is translated into characters for phylogenetic reconstruction and is understood as similarity due to common ancestry. General homology, in contrast, together with the archetype are seldom mentioned today, if only when discussing the ideas of nineteen-century naturalists mostly in an historical context.

A crucial event in the history of these concepts is Charles Darwin’s famous transformation of Owen’s archetype into an ancestor:

If we suppose that the ancient progenitor, the archetype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they serve, we can at once perceive the plain signification of the homologous construction of the limbs throughout the whole class. (Darwin, 1859: 435)²

This move allowed Darwin to align the greatest achievement of morphology, the Unity of Type, under the umbrella of evolution as the most important evidence of species common decent: “On my theory, unity of type is explained by unity of decent.” ³ The switch from abstract archetype to material ancestor effectively relegated the archetype and the associated concept of general homology to proto-evolutionary historical curiosities.

Two contrasting readings of the historical development of the theory of evolution are important for us to understand the significance (or lack of) of the archetype. The received or classical view, reflected in the works of Mayr⁴, Ghiselin⁵ and Hull⁶ for example, equates the typological notions of pre-Darwinian authors (like the archetype) with a sort of essentialism that is antagonistic to evolutionary thinking: essentialism implies stasis, as opposed to change, and implies adherence to a type, as opposed to the realization that species are composed of variable populations (i.e., Mayr’s “population thinking”). Under this view, the commitment to essentialism through typological thinking displayed by authors like Richard Owen prevented them for thinking in evolutionary terms and accepting naturalistic origins of species. Under this view, the archetype is a vacuous concept, a dead-end.

The alternative view, articulated recently in the works of Winsor⁷, Amundson⁸ and Rupke⁹ for example, argues that while it is true that typological views are pervasive in the work of naturalists before and contemporaneous to Darwin, those views by no means equate to the sort of essentialism caricatured by proponents of the classical view. Instead, it is exactly this typological thinking, with its notions of shared morphological architectures and hierarchical arrangement of types, that allowed Darwin to recruit the findings of comparative anatomists and taxonomists as evidence for his evolutionary theory. Under this view, the archetype is not a dead-end but the result of valid inductive generalizations about reconstructed patterns of similarities between species, patterns that cry for explanation ¹⁰. Typology, they argue, is not antagonistic to evolutionary thinking but it is an important part of it.

I am more sympathetic with the second, alternative view. Though it is still not a popular one. But one thing is for sure. Whether the archetype is a vacuous concept or an important evolutionary precursor, it is the most elegant concept that came out of comparative anatomy from Victorian times.

References

Their main argument is an historical one, namely that the concept of homology predates the general acceptance of evolution, and hence its definition should remain non-evolutionary. Not doing so will be “whiggish history”. They seems not to realize, however, that the language of science, its concepts and definitions, behave like scientific hypothesis themselves, changing and adjusting as more knowledge about the world accumulates. Yes, surely Belon did not think in evolutionary terms and Owen was a closeted transmutationist, but they did realize that there was something behind those patterns that called for an explanation, they just didn’t know what it was (or were not ready to accept). Their pre-darwinian concepts and definition of homology reflected the  standards of their times, something that prepared Natural History for the big change in the way of viewing things that was about to come.

But today, objecting to the notion that the human five-fingered hand and the five-toed foot of a lizard, as homologous features, were both inherited from our common ancestor is equivalent to objecting at the definition of water as H2O. Water, and most of its properties, were know before the atomic theory of elements, but it is through this theory that we are now able to explain and predict the behavior of this compound. Today, we gain nothing by insisting on a non-atomic definition of water.

Like Ebach and Williams, I am fond of pre-darwinian concepts (as the title of this blog should reveal), and thus welcome that historians and philosophers of science are shifting focus away from Darwin an into the interesting nineteen century science that has been eclipsed by The Origin of Species. I also share the view that the discussion on just how much we need to assume about the evolutionary process in order to reconstruct evolutionary patterns is an interesting one. On a general level, I do think that questioning our epistemological limits is a healthy exercise. Nevertheless, I also share John S. Wilkins recent remark that one shouldn’t make of the pattern everything that there is.

Ebach and Williams close their post stating that “[t]here is a misconception in science that everything needs to be explained.” Well, in science everything does need to be explained. That the explanation into why we have a given paraphyletic group has to do with a taxonomist’s misjudgment or preference for non-natural groups rather than due to the existence of some evolutionary processes is another matter.

References
Anastasia Thanukos (2008). Bringing Homologies Into Focus. Evolution: Education and Outreach, 1 (4), 498-504 DOI: 10.1007/s12052-008-0080-5