This book presents a largely convincing case for the theory that the pre-Roman inhabitants of the British Isles were not a united Celtic race that had invaded the Isles, nor indeed were they necessarily ethnically connected to Celts in mainland Europe. The term "Celt" as applied to Scots, Welsh and Irish was not used before the eighteenth century and is posited as a reaction, based on over-zealous linguistic and historical research, to the growth of the concept of Britain as a political entity in that century. The author also raises some very interesting points about pre-historic society and how archaeologists and historians often seek to explain any changes in that society by reference to invasion from overseas, due to an instinctive belief that such societies are primitive and unchanging. The archaeological evidence from Iron Age Britain does not for the most part match that from the Celtic areas of the continent.
The author also makes the same anti-invasion argument with respect to the Saxon invasions of the 5th and 6th centuries, though I remain as yet much less convinced by it in this case. He argues that modern notions of national ethnic identity started to rise only as a response to the mass threat from Vikings in the 9th century and, even then for centuries thereafter, that the masses of the population remained largely unaffected.
The author's style is quite academic and this is not a particularly easy read, though short at 144 pages. He has a tendency to make the same points repeatedly in a slightly didactic way. But his central thesis is worthy of debate and should not be dismissed as mere anti-Scottish/Irish/Welsh politicking.