Ah.
I have also read that there was an ancient law reserving middle names for the nobility, but this was in England and would therefore not have applied in Scotland.
Also, while there are many web sites that state this as a fact, I have yet to find any credible source at all that quotes chapter and verse of when this law was passed and when it was repealed.
So I'd be very pleased to hear from anyone who can tell me more about this topic.
I have also read that, in Scotland, the churches recognised only the first given name as valid, and therefore middle names often tended not be recorded, if indeed they were bestowed at all, until the start of civil registration in 1855. Even after that, it is surprising how often people acquire in later life a middle name that is not on their birth certificate.
Again, I'd be very interesting in some more solid evidence to support or refute this assertion.