[js1138]

Some things are irreversible. Breaking glass. burning books. Long stretches of evolution.

Once a series of mutations has happened in a certain order, it is unlikely that they will ever happen in exactly the reverse order.

Before a mutation happens there is no requirement for it and no specification. But if you are trying to reverse evolution experimentally, then you have a specification, all the probability calculations that the ID crowd is fond of are applicable.

For this and other reasons, it is unlikely that there will ever be radically new body plans evolving out of established species. New body plans can only emerge from rather simple body plans.

[jwalsh07]

For this and other reasons, it is unlikely that there will ever be radically new body plans evolving out of established species. New body plans can only emerge from rather simple body plans.

It is my understanding that all the phyla present today were present at the Cambrian. If that is correct, I am wondering if this was a prediction made by early Darwinists.

[b_sharp]

"For this and other reasons, it is unlikely that there will ever be radically new body plans evolving out of established species. New body plans can only emerge from rather simple body plans."

That depends on how you define radical. The therapsids mammals descended from evolved from reptiles. Is the body plan difference between reptiles and mammals radical?