Of the genera listed from the Wilcox, 60% are known in the present flora of s.-e. United States, 53% are still extant in central and e. China, and 68% are found living on or near the escarpments of e. Mexico. This Mexican region has a mixture of temperate and tropical conditions. In the absence of evidence for transport of plant materials over long distances prior to fossilization, Wilcox genera such as Fagus, Betula and Sassafras, which today are restricted to temperate areas, must have had greater genetic variability with resultant greater ecological amplitude. Segregation to less general genetic types and extinction of some of them may have left modern genera with less amplitude than they had in Wilcox times.