Peter Olsen Groth, A Danish and Dano-Norwegian Grammar… (1894)
“ By mutation (Omlyd, umlaut) is understood the change of a vowel caused by assimilation to a following vowel (i, u) or consonant (j) . The sound causing the change has in the present language as a rule disappeared, but it is shown by a comparison with the earlier stages of the language. The principle of mutation is active both in inflection and in word formation. ”
