314 



A CBNTURY OF PROGRESS IN THE NATURAL SCIENCES 



belong to the old composite family "Polypodiaceae," which make up the con- 

 tents of Volume 3 of the Ferns (1928). 



It is hardly surprising that Bower's treatment of the ferns in Volume 3 

 is less satisfactory than are the contents of Volume 2 because the number of 

 species is here so great and the confusion caused by parallel evolution so dif- 

 ficult to disentangle that the full technique and mental equipment of the pro- 

 fessional systematist is necessary to deal with them. This, Bower was not, and 

 the most obvious mistakes which can already be recognized as such arise 

 from this cause. For the same reason, it is precisely here that the contribution 

 of professional taxonomists in the twentieth century has been the greatest. Fore- 

 most among these has been Carl Christensen. In the Index Filicum (1905-1906), 

 the standard compilation to which (with the three supplements published in 

 1913, 1917 and 1924) all modern problems of nomenclature in the ferns are re- 

 ferred, Christensen followed Diels in general classification, although the indi- 

 vidual genera are quoted alphabetically. The growth of Christensen 's own views 

 is, however, traceable in various monographs between 1907 and 1932 and these 

 views were summed up in 1938 in the form of a short sketch of a revised clas- 

 sification, which, however, the author did not live to develop further. Some of 

 Christensen 's ideas, notably those concerning the subdivision of the vast and 

 heterogeneous genus Dryopteris were, however, already being applied, notably 

 by Ching (1935-1938), and many of them emerge again, though with additions, 

 in the two most modern revised taxonomic schemes, published independently 

 and almost simultaneously by Holttum (1946) and Copeland (1947). 



DENNSTAEDTIACEAE- 



HYMENOPHYLLACEAE 



THELYPTERIDACEAE 



ADIANTACEAE / / 



(part) 0/pterij / GRAHMITIDACEAE 



\ PLAGIOGYRIACEAE / G\eichtnia\ / / 



' / / \ \// 



MATONIACEAE primitive 



GLEICHENIACEAE 



Primitive Harginiles OSMUNDACEAE Primitive Superficialei 



Figure 3. Diagram sliowing the interrelations of tlie various groups of ferns ac- 

 cording to Holttum, 1949. 



