farther (adj.) Look up farther at Dictionary.com
late 14c., "front;" variant of further (adj.). From 1510s as "additional;" 1560s as "more remote."
farther (adv.) Look up farther at Dictionary.com
15c. alteration of Middle English ferther (c.1300), a variant of further (adv.). There is no historical basis for the notion that farther is of physical distance and further of degree or quality.