A NUMERICAL ANALYSIS OF THE PAST AND PRESENT FLORA OF THE BRITISH ISLES.

Lists of all the taxa of vascular plants recorded from deposits of Late-Devensian, mid-Flandrian, and late-Flandrian age in twelve geographical regions of Britain were compiled. From these lists coefficients of floristic dissimilarity between the regions were calculated, and two-dimensional dispositions of points representing the regions were found for each time interval by non-metric multidimensional scaling. Distributional data for the present day were analysed in a similar way. The method demonstrates a marked north-south floristic gradient in the British Isles today, and analyses of the fossil data suggest that although a gradient was present in Late-Devensian and mid-Flandrian times it only became well developed in late-Flandrian times. Phytogeographical analyses of the flora in each region at each time interval also demonstrate the existence of some west-east floristic gradients, particularly in Late-Devensian times. It is concluded that, despite the many limitations of the primary floristic data, the numerical methods have considerable potential for handling and synthesizing large amounts of historical biogeographical data. [ABSTRACT FROM AUTHOR]