Problem:
One of the tenets of Darwin's theory is the survival of the fittest, i.e. adaptation of features that allow a species to adapt better to its surrounding environment. I am wondering that given the right conditions.
Question: Is there a possibility of a more "complex" species, say humans, back-evolving into a less "complex" species, say a dog?
Question: Is there anything in Darwin's theory or some experimental proof that excludes such a possibility?
Please explain your answers for the given questions.