"Darwin's Origin of Species had a good section on this; the book is quite readable." Yeah, I recall Darwin discussing at length how there's no clear difference

"Darwin's Origin of Species had a good section on this; the book is quite readable." Yeah, I recall Darwin discussing at length how there's no clear difference between a "species" and a "variety". It's a key point, so it's too bad that many people are still under the impression that "species" has an unambiguous definition.