Further v. Farther
Apparently, further and farther come from the same source, namely the verb that we retain as further meaning "to promote". The different spellings were originally due to the general diversity of English orthography in earlier times. And the spelling was apparently not regularized because the word(s) took over as the comparative form of far, which used to be farrer. But because of the similarity of meanings, both forms seem always to have been used in the full range of adjectival and adverbial meanings, though with some probabilistic influence of far's spatial sense on the vowel.
From the 1895 OED entry for farther (not revised since then):
Middle English ferþer (whence by normal phonetic development farther ) is in origin a mere variant of further n., due probably to the analogy of the verb ferþren < Old English fyrðrian to further v. The primary sense of further, farther is ‘more forward, more onward’; but this sense is practically coincident with that of the comparative of far, where the latter word refers to real or attributed motion in some particular direction. Hence further, farther came to be used as the comparative of far; first in the special application just mentioned, and ultimately in all senses, displacing the regular comparative farrer. In standard English the form farther is usually preferred where the word is intended to be the comparative of far, while further is used where the notion of far is altogether absent; there is a large intermediate class of instances in which the choice between the two forms is arbitrary.
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