Grimm's law
In De Bello Gallico, Cesar mentions tribes around the river Meuse: "Condruses, Eburones, Caerosos, Paemanos, qui uno nomine Germani appellantur“  (Condrusi, Eburones,Caerosi and Paemani, which are called Germani by name) - (Bello Gallico, 2,4,10). Today, most people think that Eburones is the only Germanic name (with shift); the others are Celtic (without shift).