Non-circular Definition of Homology/Analogy

In discussing (read: debating) evolution with one of my classmates, I used homologous structures as an example/proof of evolution. I know it's not the best argument, but I think it can work.Anyway, the problem was that both of us were a little rusty in the nomenclature of evolutionary biology, and he didn't remember what homology was, and I couldn't give an explanation that didn't assume evolution to be true (which I know is often pointed out by creationists, as it's on the EvoWiki claims page).So, I ask, what is a good definition of homology and analogy?I propose this:Analogy: similarity between two organisms that is due to adaptation to perform a function
Homology: similarity between two organisms that is not due to adaptation to perform a functionSo, for example, the shape of the wings of a penguin are similar to the flippers of a seal, as both have adapted to perform the same function, so can be called analogous. But, the bones in the wings of a penguin are still similar to that of other birds, for no functional reason, indicating homology.I think I accurately prevented circularity in my argument here. I seem to be using adaptation, which is pretty similar to evolution, so I worry I may not have the best wording. If I put the word 'design' in there instead, it doesn't change it much, so I think it's pretty good. I'm aiming for it to address the idea of God reusing designs when there was no functional need to do so.However, I fear these example aren't as precise as they could be. And also, I'm not sure whether a structure could be considered both analogous and homologous to the same structure. I don't know whether they are usually, but my definitions don't seem to allow it.So, do you agree with my definitions? If not, can you help me fine-tune these definitions, in any aspects?(Evolution section would be a good place, but it could also fit in the ID section I guess)

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That's a good definition Justin. Personally, I would have put 'adaptation' instead of 'explanation'. On the other hand, that might not be the wisest thing... The guy I was talking to is a biochemistry student. He, like me, has also done a first year subject on evolutionary biology. I'm confident he will do some meditation on it.

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I wonder that as well. One of the explanations given by AiG for why ratites have wings and related structures (such as a broad sternum) was "design economy": Missing Link | Answers in Genesis

quote:

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It is a result of ”design economy’ by the Creator. Humans use this with automobiles, for example. All models might have mounting points for air conditioning, power steering, etc. although not all have them. Likewise, all models tend to use the same wiring harness, although not all features are necessarily implemented in any one model. In using the same embryological blueprint for all birds, all birds will have wings.

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Why an omnipotent being, to whom nothing can be difficult, would need to take shortcuts is beyond me.

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