Another Clark researcher and I have a bit of a mystery.
He has among his family photos, a photo of a boy photographed by J. Pollard on Broadway in Tillsonburg.
On the back is written "William Clark, Uncle John's son"

I have posted the photo on my web site at:
http://freepages.genealogy.rootsweb.ancestry.com/~marcelo/Cl...

If you recognize the boy in the photo, we'd love to hear from you!