Biological similarity is an obsession with me, ever since my graduate studies. Anyone who has read this blog for any length of time knows about my obsession. I talk about it again and again (my favorite was "Transitional Forms").
While recently browsing reactions to my blog, I noticed that some creationists out there were ... let's say they were scoffing at my obsession. To them, there is no problem. Common form implies a common designer. QED. I thought I'd fire back with yet another explanation of the issue, but then I realized that there's really little point. If I haven't yet convinced certain creationists that similarity is a real issue, I'm not likely to change there minds at this point. So I'll save my breath.
Instead, I thought I'd give some attention here to other nonevolutionary explanations of similarity. I'll start with Richard Owen. Owen isn't really very popular. T.H. Huxley hated him. Darwin didn't like him either. He was evidently a rigid fellow, vainly impressed with his own work. He imagined that he had resolved the old form/function debate, at least as he understood it. Crudely put, the problem was which factor was more important in determining organismal form? The obvious conserved structural patterns or the necessities of life? Was the appearance of an organism determined by some kind of law of shape? Or was it governed by habits, behaviors, or ecology?
In England, the answer was usually that function determined form. This was Paley's emphasis. Critters have amazing contrivances given them by the Creator to fulfill some function. On the continent, abstract structural principles had become more common as explanations of form. The German poet Goethe famously asserted that all plant form were just variations on the leaf. Owen embraced both approaches, in some sense baptizing the idealistic structuralism of Germany and France with the language of English natural theology.
Modern historians question whether his solution to the form/function debate was any answer at all, but I think that was the point, at least to Owen. There was not a single solution to "the problem" because there was not a single problem. Owen's eclectic embracing of functionalism and structuralism were answers to different questions: 1. Why are organisms so well-adapted? and 2. Why are there homologies? I think that was his point.
To Owen, homology was the result of the vertebrate archetype (pictured above), which he described very, very briefly as a sort of platonic ideal, a design pattern for creating organisms. Organismal similarity was to Owen based a natural law of the archetype. The differences Owen attributed to functional requirements. (Thus he saw two answers for two different questions.) In his book On the Archetype and Homologies of the Vertebrate Skeleton, Owen wrote,
... the general type of the vertebrate endo-skeleton is rightly represented by the idea of a series of essentially similar segments succeeding each other longitudinally from one end of the body to the other, such segments being for the most part composed of pieces similar in number and arrangement, and though sometimes extremely modified for special functions, yet never so as to wholly mask their typical character... (p. 7)Darwin pulled the rug out from under Owen by interpreting the archetype as an ancestor. You know the rest of that story.
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