In The Fossil Record, A Species Is A Hypothesis
In the fossil record, a species is a hypothesis. We can't test that hypothesis in the way we do with living animals. Even in the dark, after all the paleontologists have left, the fossil bones just won't get it on. No reproduction, no test.
So, sometimes we have to live with hypotheses that we can't immediately test. Because many hypotheses are wrong, we have to keep juggling in our minds the names of more species than probably existed.
-- John Hawks
Quoted on Wed Jun 22nd, 2011