His birth mother was Martha Sharp nee Callan from Ayrshire. I was very lucky to receive lots of information on his birth mother who actually remarried a Alexander McMillan who may have been a relative of Donald.
1. Get the marriage certificate of Martha Sharp and Alexander McMillan. Note the names of his parents
2. Get the marriage certificate of Alexander's parents. Note the names of his grandparents.
3. Get the marriage certificate of Alexander's grandparents. Note the names of his great-grandparents
and so on until you get to 1855.
4. Get the marriage certificate of Malcolm McMillan and Maria. Note the names of his parents.
5. Get the marriage certificate of Malcolm's parents. Note the names of his grandparents.
7. Get the marriage certificate of Malcolm's grandparents. Note the names of his great-grandparents
and so on until you get to 1855.
Compare all the names of Donald's parents/grandparents/great-grandparents with the names of Malcolm's parents/grandparents/great-grandparents. If there is a match, that is your answer.
If not, it gets a bit more challenging because you need to look for post-1855 death certificates of the people married before 1855. It isn't all that difficult, because the deaths of married women are generally indexed under both their maiden and their married surnames, and their death certificates tell you whether they were married (i.e. husband still alive) or widowed. You can use this information to narrow down the search for the husband's death. Death certificates will tell you the names of the parents of the deceased, and you can then compare the names of Donald's ancestors with those of Malcolm's.
