From my genetics syllabus:
"In New England, polydactyly is commonplace. However, this is not reflected in its British origins. It appears that polydactyly arose as a new mutation in New England in the early 1800s. We can use this feline uniqueness to track the activities of early Bostonians. Although polydactyl cats are rare in Britain and the rest of our country, they are present in Yarmouth and Halifax, Nova Scotia. the cats probably arrived in Yarmouth with the emigration of people from coastal Massachusetts in the mid 18th century. In nearby Digby there are no polydactyl cats but then, when history is consulted, we find that the cats came from different sources--France and New York City. In contrast, the polydactyl cats in Halifax probably came, not with the emigrating people, but from Boston traders sailing there to do business with the European immigrants as well as on Boston ships transporting goods from Britain. Appropriately, these cats also reflect their Scottish ancestry. Thus, orange, non-agouti and blotchy tabby cats are characteristic of Halifax and the Scottish Isles and dilute, piebald spotting and polydactyly are common in Halifax and Boston."