David Lack's theory of island biology was published in 1976. In this he stated that isolation was of little consequence as a barrier to bird dispersal. Lack's theory was singular for his lack of statistical support of his assertions. One reviewer indicated from his data that statistically significant isolation effects might be present. Here data for the Bahamas and Gulf of Guinea islands were examined. In both cases isolation from the mainland colonisation source was significantly inversely correlated with the number of breeding bird species on the islands.