CHAPTER XLVIIl 



GENERA INCERTAE SEDIS 



A COMPARATIVE study of any large Class of Plants naturally akin, and 

 represented by many families, genera, and species, will raise more questions of 

 relationship by descent than can be finally resolved on the basis of present 

 knowledge. Even a long and more or less consecutive geological history does 

 not always suffice to decide problems of detailed affinity, though this provides 

 the most powerful and direct check upon the results of comparison of forms 

 still living. More than ever does difficulty arise when, as in the Class of the 

 Filicales, there is evidence of widespread homoplastic modification, so that 

 the final results of evolution from stocks originally distinct appear to resemble 

 one another in certain leading features, though in others they proclaim a dis- 

 tinct origin. The balance of relatively late modification against archaic features 

 may thus become highly debateable : a slightly greater weight accorded to 

 the one and less to the other b}' different writers will probably lead to 

 divergent conclusions. To the outside observer of such discussions the exact 

 place assigned to one genus or another may appear to be tinctured b}' 

 personal taste or idiosyncrasy as much as b}- scientific truth. The cynic may 

 then say that problems so apparently insoluble are best left alone: but that 

 would be a mere withdrawal from the natural field of science. Still certain 

 problems may quite legitimately be left in abe}'ance, pending the discovery 

 of new facts, or the introduction of new methods of attack. 



These remarks apply to many genera of Ferns, which may be called 

 "genera incertae sedis.'' Such genera have been accorded varying positions 

 in the classificatory system by different writers, and the synonymy of many of 

 them is a sufficient witness to the diversity of the views expressed. As to this, 

 one general remark may be made. In the past the comparisons have for the 

 most part been drawn between forms highly advanced, such as the Lepto- 

 sporangiate Ferns. Little importance was attached by the early writers to the 

 evolutionary aspect of questions of affinity. Before i860 such an aspect 

 hardly existed. Now, however, the outlook should constantly be phyletic : 

 and the first question as to any genus of uncertain affinity will be its probable 

 source by descent. How far are the characters used in comparison fundamental 

 and archaic? how far have they arisen from relatively late and perhaps homo- 

 plastic adaptation? It is hardly necessary to explain that it is the pursuit of 

 such questions which has led to the disruption of the old genera Acrostic/nun 

 and Polypodimn, as they were constituted in the Synopsis Filicuni. It is now 

 believed that the features upon which those genera were based are for the 



