Arabic dialects, the native spoken Arabic of about 250 million people, are spread over an immense, contiguous geographical area from Iran to Lake Chad, from Morocco to Yemen. Corresponding to this geographical spread is considerable linguistic diversity. An explanation for this diversity has proved elusive. The existence of variants found either in the modern dialects or in the Classical literature (or both), which are not self-evidently derivable from a normalized Classical Arabic (largely standardized by the ninth century), argues for a more diverse set of inputs into the Arabic which spread outwards from the Arabian Peninsula beginning in the seventh century. I elucidate this problem by comparing four varieties of Arabic located in widely separated areas and settled at different times. To account for the internal diversity of the areas compared, a dataset is established with 49 phonological and morphological features, which, using simple statistical procedures, permits a normalized comparison of the varieties. From this set of variables, two specific linguistic features are discussed in detail and reconstructions proposed, which place their origins in a pre-diaspora variety. It is concluded that the Arabic which preceded the Arabic diaspora of the seventh century was considerably more diverse than interpretations of the history of Arabic traditionally allow for.