This may not be an English language question, but I've always wondered. In Sweden, it is very unusual to have surnames that can also be used straight up as first names. In fact, I can think of no such examples. But in English speaking countries, it seems to be rather common. List of examples:
Barry Lyndon – Lyndon Johnson
Dylan Thomas – Thomas
John Adams – Adam
Thomas Jefferson – Jefferson Davis
William Henry Harrison – Harrison Ford
John Tyler – Tyler
Chester A Arthur – Arthur
(Yes, I used a list of US presidents as reference). With the exception of James Madison, whose surname is sometimes used as a female name, I cannot think of many other such surnames which are used as female first names. Is this the case? Is the tradition to use only male names as surnames, or reuse surnames as male first names?
Of course, nowadays, people improvise a lot more, but speaking from a historical perspective.
As a side note, in Sweden we used to have the -- rather curious -- tradition of giving surnames to children based entirely on the father's first name. E.g.:
Johan Davidsson's son was named Nils Johansson
Nils Johansson's son was named Bertil Nilsson
Bertil Nilsson's son was named Karl Bertilsson
(Yes, two 's', as in "Johan's son") Something that surely makes genealogical research difficult. These surnames are used today as well. Either way, it seems to loosely be based on the same principle of using male names as surnames.