Argues against a pre-Insular Celtic division of Goidelic and Brythonic, and suggests that Celtic was brought to Britain and Ireland by a tribe that had been exposed to the influence of North-Western Germanic. Appendix contains a collection of possible Celto-Germanic isoglosses: 1. Loss of the Indo-European reflexive pronoun; 2. Irish comparative forms ending in ā; 3. The 2nd sg. of the Old Irish and Welsh suffixless preterite; 4. Notes on verbal prefixes; 5. The Welsh relative particle a.