John Stuart Blackie (1809-1895), Altavona: Fact and Fiction from My Life in the Highlands
(Edinburgh: David Douglas, 1882), p. 300:
A genuine Highlander is apt to feel towards English, as your Oxonian does to Latin; however well he knows it, it is not his mother tongue. A man may learn many languages, but he can have only one mother tongue.
This started me thinking about the phrase "mother tongue" and its equivalents, which I don't remember seeing in ancient Greek or Latin. Apparently it first appeared in the 12th century. There's a good article on the subject by Einar Haugen (1906-1994), "The 'mother tongue'," in Robert L. Cooper and Bernard Spolsky, edd., The Influence of Language on Culture and Thought: Essays in Honor of Joshua A. Fishman's Sixty-Fifth Birthday
(Berlin: Mouton de Gruyter, 1991), pp. 75-84 (some pages not visible through Google Books).