The history of the French language was initially marked by Celtomania, which saw Celtic roots everywhere. When this doctrine was discredited and discarded in the XIXth century, the role of the Germanic superstrate became hypertrophied, the more so that Breton, long considered a direct descendant of the native Gaulish, was ranked in the same period as an alien language imported from Great Britain into the Armorican peninsula. Relying on modern geolinguistics, I compare ALF (Atlas Linguistique de la France) maps with Breton ones, using the data recorded in Le Roux’s Atlas Linguistique de la Basse-Bretagne and Le Dû’s Nouvel Atlas Linguistique de la Basse-Bretagne. I shall try to show that several of theses maps reveal the presence of ALF data whose origin is clearly Celtic and not Germanic. The study of the Atlas Linguarum Europae and of the Atlas Linguistique Roman has shown that borders between languages and even language families are not waterproof. It is high time to develop such comparisons to bring about a new vision of the history of languages.