Not quite true. From Steve Jones (page 347) we have: "... I should infer from analogy that probably all the organic beings which have ever lived on this Earth have descended from some one primordial form, into which life was first breathed."
Analogy is always problematical. But by Darwin’s theory I mean what he says in “Origin of Species.” You will find, however, that all he observed as variation. He saw variations within species and assumed that over time the walls between species would break down. The analogy he was pursuing was that of selective breeding, something of which there was a great deal of information. But he knew nothing about the science of genetics and his subsequent in that field led to a dead end. A lot of details but no theory . Like Newton’s pursuit of alchemy, which did not led to chemistry because he didn’t do the right experiments, ask the “wright” questions.