The following response was posted on a forum (NOT here!). Thankfully, most genealogists are a little more helpful.
In the 28 generations [+ -] from Sir James "the good" Douglas to now, Sir James has roughly 20 billion descendants. When only 6 billion people live on the planet presently, you must have a "family tree" the size of a football field? :-)
Considering that the theoretical number of our ancestors is exponential, and calculating that a century is equivalent to four generations, the theoretical number of ancestors from the year 1300 to someone born in our lifetime would be well over 20 billion. The exponential growth formula being: N(t)=N(0)ert.
In short, no one has a complete Douglas family tree and you've a few more names to add in any case.