In Origin of the Species, Darwin writes
"A group does not reappear after it has once disappeared; or its existence, as long as it lasts, is continuous .”
I can't tease out the meaning of the second clause. Elsewhere in the book Darwin writes
"When a species has once disappeared from the face of the earth, we have reason to believe that the same identical form never reappears." This is close to the first statement, but perfectly clear.