as the famous "Bullard's Fit". In 1966 Wilson published the paper that referred to previous plate tectonic reconstructions, introducing the concept of what is now known as the "Wilson Cycle". In 1967, at the American Geophysical Union's meeting, W. Jason Morgan proposed that Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, Xavier Le Pichon published a complete model based on six major plates with their relative motions, which marked the final acceptance by the scientific community of plate tectonics. In the same year, McKenzie and Parker independently presented a model similar to Morgan's using translations and rotations on a sphere to define the plate motions. From that moment onwards, discussions have been focussing on the relative role of the forces driving plate tectonics, in order to evolve from a kinematic concept into a dynamic theory. Initially these concepts were focussed on mantle convection, in the footsteps of A. Holmes, and also introduced the importance of the gravitational pull of subducted slabs through the works of Elsasser, Solomon, Sleep, Uyeda and Turcotte. Other authors evoked external driving forces due to the tidal drag of the Moon and other celestial bodies, and, especially since 2000, with the emergence of computational models reproducing the Earth's mantle behaviour to first order, following upon the older unifying concepts of van Bemmelen, authors re-evaluated the important role of mantle dynamics, . Implications for biogeography Continental drift theory helps biogeographers to explain the disjunct biogeographic distribution of present-day life found on different continents but having similar ancestors. In particular, it explains the Gondwanan distribution of ratites and the Antarctic flora. Plate reconstruction Reconstruction is used to establish past (and future) plate configurations, helping determine the shape and make-up of ancient supercontinents and providing a basis for paleogeography. Defining plate boundaries Current plate boundaries are defined by their seismicity. Past plate boundaries within existing plates are identified from a variety of evidence, such as the presence of ophiolites that are indicative of vanished oceans. Past plate motions Tectonic motion is believed to have begun around 3 to 3.5 billion years ago. Various types of quantitative and semi-quantitative information are available to constrain past plate motions. The geometric fit between continents, such as between west Africa and South America is still an important part of plate reconstruction. Magnetic stripe patterns provide a reliable guide to relative plate motions going back into