In 1922, Mackenzie published "Ancient Man in Britain", a work covering the history of Britain from Upper Paleolithic times, from a strong ethnological basis. The foreword of the book was written by Grafton Elliot Smith. The work covers the earliest settlement of Britain by the first modern humans from around 35,000 years ago during the Aurignacian (pp. 19–27). In the book, Mackenzie maintains that the Caucasoid Cro-Magnons who settled in Britain were dark haired and dark eyed, racially akin to the French Basques, Iberians and Berbers of North Africa (p. 25), who he theorised were one of the earliest representatives of the Mediterranean race. This indigenous proto-Mediterranean racial stock was later invaded by another "variety of the Mediterranean race" who initiated the Solutrean culture around 20,000 years ago (p. 50).