You would need a time machine to know if some present-day species were going to eventually have some mutated offspring that would become yet another species. All species now existing have the potential to create mutant descendants, so the book is far from closed. But we can't turn to the back of the book to see what's going to happen.
Yes BUT, if evolution is a constant, though slow process, then there should be SOME modern indication of an intermediate species. Life forms produce mutations all the time but I know of none of those mutations that continue on as a viable species without human intervention. If species to species sexual contact cannot produce a hybrid why does it make sense that natural selection can do so. What you seem to be advocating is the scientific version of the "immaculate conception". It is rumored that Hitler tried to cross breed human and ape and it did not work, however, you can cross breed a horse and donkey and get a mule but even then the mule is sterile.