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.
Best Answer
Naming has a large arbitrary and cultural component and so is subject to the same structures as fashion. It seems for many centuries the only choices for Western Europe were Christian saint's names and for men mostly just the apostles.
But fashions change. It seems like in 19th c. Americans, the fashionable source was Old Testament prophets with z's and k's (Ezekiel, Zebulon, Zachariah).
And lately (early 21st c) American girls are named after president's last names (the aforementioned Madison, plus Kennedy, Reagan, Taylor (this is the answer to one of the questions). This is simply a current fashion trend mostly likely to be replaced soon from some new set.
One subtle pattern in English speaking areas for at least the past two centuries is to name a boy using the maiden name of the mother, usually a second or third boy since the first got the full name of the father (with Jr. or III appended).
As trends go, people sometimes follow the external view of the phenomenon (last names for first), which accounts for Jefferson Davis or (much later) Harrison Ford, whose mother's maiden names did not supply their first names. That these first names are the last names of famous people (American presidents) probably helped in their choice.