The Lost Worlds of Planet Earth Ortelius hypothesized the idea that land masses may have been connected in the past, Alfred Wegener who hypothesized the idea of a super-continent Pangaea and continental drift despite the prevailing idea of flooded land-bridges at the time, and Bruce C. Heezen and Marie Tharp who discovered the Mid-Atlantic Ridge that supported the theory of plate tectonics. Tyson describes how the landmasses of the earth lay atop the mantle, which moves due to the motion and heat of the earth's outer and inner core. Tyson moves on to explain the asteroid impact that initiated the Cretaceous–Paleogene extinction event, leaving small mammals
Scottish society in the Middle Ages sense in the service of the major nobility, either in terms of landholding or military obligations, roughly half sharing with them their name and a distant and often uncertain form of kinship. Below the lords and lairds were a variety of groups, often ill-defined. These included yeomen, later called by Walter Scott "bonnet lairds", often owning substantial land. Below them were the husbandmen, lesser landholders and free tenants that made up the majority of the working population. Serfdom died out in Scotland in the fourteenth century, although through the system of courts baron landlords still exerted considerable control over their