Alexander MacBain,
An Etymological Dictionary of the Gaelic Language
(1911)
“ The Gaelic languages—Irish, Manx, and Scottish Gaelic—have a much closer connection with one another than the Brittonic languages. Till the Reformation and, indeed, for a century or more thereafter, the Irish and Scottish Gaelic had a common literary language, though the spoken tongues had diverged considerably, a divergence which can be traced even in the oldest of our Gaelic documents—the Book of Deer. ”
