I have searched my records and looked through the Darter/Tarter book and can't find your grandfather. There were several Robert Tarters but all I have are older than your grandfather. There are so many of them and the book I have doesn't have the later descendants of lots of them. Have you checked the 1900 census to find your grandfather? I wish you knew his father's name. That might be enough to connect him.If I can be of any further help, don't hesitate to ask. So sorry that I wasn't able to be of help right now.