Hi, J. N. Perkins was John N. and his last appearance on a Census was the June 1898, Garfield Co., WA Census. He was 82 and a doctor.
His wife D. A., Dorisa, or Derrissa (depending upon the Census) last appeared on the same Census.
They died before the State required death certs. Garfield County might have something about their death, http://wei.secstate.wa.gov/WEI/Garfield/Recording/
The 1889 Garfield Co., WA Census shows they had a daughter Lillie, 21, b. Oregon. However, she doesn't appear on a subsequent Census and I assume she married. You could contact the County about a marriage license too.
I also checked www.whitepages.com
for Perkins in Pomeroy but nothing appeared.
In 1880 the family lived in Columbia Co., Washington Territory. Their son Alvah was living with them there. He was 22 and b. in Oregon.
In 1870 the family lived in Weston, Umatilla Co., OR and they had a lot of daughters at that time. You'd have to trace who all the daughters married to find the descendants.