In this ground-breaking book, Johanna Nichols proposes means of describing, comparing, and interpreting linguistic diversity, both genetic and structural, providing the foundations for a theory of diversity based upon population science. This book will interest linguists, archaeologists, and population specialists. "An awe-inspiring book, unequalled in scope, originality, and the range of language data considered."--Anna Siewierska, Linguistics "Fascinating...A brilliant pioneering study."--Journal of Indo-European Studies "A superbly reasoned book."--John A. C. Greppin, Times Literary Supplement