This person has now been incorporated in the Clack One Name Study I am building (see .https://www.wikitree.com/wiki/Space:Clack_Name_Study) I intend to return to review the biography when I have added some other members I am aware of in my research. Meanwhile I have attached the sources I and others have identified.
Sources
1895
Birth of Forrest G Clack in Dorr, Allegan, Michigan
"United States Census, 1900," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/M91D-CR3 : accessed 17 Dec 2013), Forest G Clack in household of Darius G Clack, Dorr township, Allegan, Michigan, United States; citing sheet , family 152, NARA microfilm publication T623, FHL microfilm 1240698.
1910 (15)
Dorr, Allegan, Michigan
"United States Census, 1910," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MLP4-89L : accessed 17 Dec 2013), Forest G Clack in household of Darius C Clack, Dorr, Allegan, Michigan, United States; citing sheet , family 201, NARA microfilm publication T624, FHL microfilm 1374647.
1920 (25)
Wayland, Allegan, Michigan
"United States Census, 1920," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MZS3-7RW : accessed 17 Dec 2013), Forest Clack in household of Jones Bryson, Wayland, Allegan, Michigan, United States; citing sheet , family 172, NARA microfilm publication T625, FHL microfilm 1820754.
1930 (35)
Wayland, Allegan, Michigan
"United States Census, 1930," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/XQTK-RW7 : accessed 17 Dec 2013), Forest Clark, Wayland, Allegan, Michigan, United States; citing enumeration district (ED) 0038, sheet , family 189, NARA microfilm publication .
1931 (36)
Death of Forrest Guy Clack in Plainwell, Allegan, Michigan