Many "older" US first-names derive from surnames, by way of middle names. What happens (or "happened" is probably more correct) is that a middle name comes in through marriage - e.g., a boy child is given his maternal grandfather's surname as a middle name. A generation or two later, that middle name is used as a first name. This has happened in my family (a few generations ago) with the names "Curtis" and "Morrison". Many first names today are fanciful spellings of more traditional names - such as "Mayree" for "Mary" or even "Shawn" for "Sean".