In the past, for example, there were many more irregular plural nouns in English: one bōc (book), many bēc; one hand, two hend; one eye, two eyn; one cow, many kine. But gradually, ‘errors’ like ‘hands’ crept in by analogy on the regular -s plural pattern. So bēc was replaced by the ‘incorrect’ bokes (books) during the thirteenth century, eyn was replaced by eyes in the fourteenth century, kine by cows in the sixteenth.