wow, this thread is complicated, isn't it?
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say animal A has no eyes. animal D has a pair of them.
to get from A to D there may be as many animals in between. but what would they be like? would there be animals with partly formed eyes? if so, that is like saying that they began to form this new organ in anticipation that somewhere along the line, animal D would come about and be able to make use of them.
the partly formed organs situation doesn't make sense. say we're to have an arm in the middle of back in the future - say we'll have use for them under the circumstances. how would this change come about? would we start by having a bone come under the skin around that area and develop a lump there that over generations becomes an arm? those first few generations would have a defunct lump that has no purpose. if you stop evolution right there, that would be seen as a mutation i suppose and completely useless really. so why would the mutation continue this way?
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Well, haven't ever seen an animal with some useless organs or something, so supposedly changes happen faster.
An example about organ changes, considering the theory that most life forms came from water... Fish lives in water, it has fins. Those are good in water, but would be useless on earth. Then there are animals like seals, that have something between fins and legs, and since they are most of the time in water and spend some time on shore, this part legs/part fins are good for them. Then there are amphibians, who actually have webbed feet and feel themselves good both in water and on earth. Then there are animals who spend almost all the time on surface and they no longer have webbed feet or fins, because they are useless for them. Then they are humans, who have their front legs changed to what we call hands, so that we could use them for purposes other than walking. If you follow the changes, you can see some evolution here.
The problem i see here is how those changes actually accured. I haven't ever seen or heard of a lifeform giving life to some different life forms that would have different organs or something like that (ohh, so many times using the word life in one sentence...). Or maybe those changes appear so slow that is difficult for us to notice. Like if you live with somebody for 10 years, you don't notice how he changes getting older, but if you meet somebody whom you haven't seen for 10 years, you see a lot of changes.
Another example would be about cockroaches. If you try to poison them with some chemical, but some of them survive, they will develop immunity to it in later generations. Isn't that a kind of evolution, trying to adopt to the changes around? So, maybe for a life form to have some drastic changes like new organs, there needs to be a big change in the world around them like some cataclysm or something?
Well, maybe i'm all wrong about those examples, but to be honest i am a bit lost on the subject myself. But nevertheless it is a very interesting thread.