FURTHER VS. FARTHER

Yesterday we discovered that toward and towards can be used interchangeably, but it's a little more complicated for further and farther. The Oxford English Dictionary and a few other places claim that there shouldn't be a distinction, but many other sources say that farther represents physical distances and further represents distances that are figurative or beyond something else. So you can run a little farther to the further field, but develop a bit further. Etymologically speaking, farther was modified from further near the end of the fourteenth century, so they're linguistically similar, at least. In Old English, it could be spelled further or forther (pronounced with a soft th), in Proto-Germanic it was furtheraz, and that traces to Proto-Indo-European per, meaning "forward". Usage of further is more than ten times as frequent as farther in English, and that gap might grow even further​.