Say you have your family tree for the last 400 million years or so, back to the Devonian, the age of fishes. You take the set of ancestors that are about 400 million years old (which will be across multiple generations). The question is are they all fish?
Or, is it more complex than that? Can there somehow be non-fish in that set?
Note added later: this question is not a duplicate of another question. In the context of that other question, howewever, my question can be stated as "Is Dawkins correct in stating that my 185th million ancestor was definitely a fish, with no exceptions?"