RETICULATE EVOLUTION IN THE APPALACHIAN ASPLENIUMS

To be able to postulate the course of evolution in a group of organisms on the basis of indirect evidence is one of the goals of phylogenetic research, since in many groups direct evidence is difficult or impossible to obtain. Occasionally a worker may be fortunate enough to confirm his indirect and comparative techniques by direct evidence, such as the discovery of a postulated ancestral type, or the production of a postulated form by hybridization experiments. In the present study of the evolution of the Appalachian Aspleniums, a complex group of small ferns of the eastern United States, the methods used have been indirect ones, morphological, anatomical, and cytological. But in future years it is to be expected that experimental proof of the conclusions of this study will be forthcoming. The indirect steps used as evidence of reticulate evolution in the Appalachian Aspleniums are thus subject to validation by direct tests. Wherry (1925, 1936) pointed out that the Appalachian Aspleniums "form a series showing intermediates between certain long-recognized species." His basic end-point species were five in number: Asplenium pinnatifidum (lobed spleenwort), A. montanum (mountain spleenwort), A. bradleyi (cliff spleenwort), A. platyneuron (ebony or brownstem spleenwort), and A. rhizophyllum (the walking-fern, usually treated as Camptosorus r-hizophyllus but for sake of simplicity retained in Asplenium here). Detailed studies of the morphology of these five end-