Some people are so beguiled by the obvious—the flux of things—that they ignore how fixed things generally are. Books that are copied down a thousand different times remain subantially the same. We know how Aristotle thought, even though that thought came, not from him buty from his students. Thought has a certain fixity, because the “structure” of it must be respected, and too many changes simply destroy it, make it meaningless. A cancer is meaningless life.
One could say the same thing about a protozoa. But that does not mean that a protozoa did not evolve in ways that benefit it, or that a cancer tumor has not evolved from the hosts DNA in ways that benefit it.