312 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



A diagram summarizing some of these phyletic views and the facts of dis- 

 tribution in time of the known living and fossil members of the Pteridophyta 

 is reproduced as figure 1 after Zimmermann, 1938. It indicates very clearly 

 what is fact and what is hypothesis and needs no further explanation. 



One further topic must now be considered, which may perhaps form some- 

 what of an anticlimax compared with those just discussed but which must be 

 dealt with last since active work on it is still in progress as the "century" 

 closes. This is the detailed classification within the various surviving groups 

 of Pteridophyta. 



The history of knowledge on the various genera and species of Psilotales, 

 Equisetales, and Lycopodiales need not be discussed since most of the relevant 

 facts are summarized conveniently in the several chapters in Verdoorn (1938) 

 devoted to these groups. On the other hand the ferns, by their mere numbers 

 (some 10,000 known species), have necessarily developed a very considerable 

 taxonomic literature of their own. The early history is summarized in Smith 

 (1875) and the later history in Christensen (1938), in the "Introduction" to 

 Copeland (1947) and, from a different point of view, in Bower's Ferns, Volume 

 I (1923). At the beginning of our "century" in spite of the existence of im- 

 portant taxonomic works such as those of Schott (1834), Moore (1857), Fee 

 (1850-1852), Presl (1836), the prestige of the elder Hooker was so great 

 that in the middle of the nineteenth century he effectively dominated fern 

 taxonomy in a way which all recent systematists feel to have been disadvanta- 

 geous. Hooker's system of classification was summarized in Hooker and Baker's 

 Synopsis Filicum, published in 1865-1868 after Hooker's death. Now that it 

 has grown by accretion out of its original usefulness as developed in the suc- 

 cessive volumes of the Hooker's Species Filicum (1844-1864), we now see it to 

 be an unwieldy assemblage of too many species grouped in too few genera, 

 based on too few criteria in an almost Linnean arrangement. The work of turn- 

 ing this into a phylogeny has taken the whole of the century and is still incom- 

 plete. As in the larger groups, the first requirement in the consideration of 

 genera and species has been to determine the criteria which are taxonomically 

 effectual and from them to deduce which characters are most primitive and 

 which are advanced. In the effort to do this it has been necessary to disentangle 

 numerous cases of parallel evolution which make individual characters of less 

 value than at first they appeared. This has involved the use of an increasing 

 number of characters both external and internal taken together and progress 

 has naturally, at any one time, been very closely dependent on the state of 

 knowledge of the Pteridophyta as a whole, which has just been outlined. Thus 

 in Engler and Prantl's treatment of the ferns (1898-1902) already quoted (p. 

 307) the order of citation begins with the Leptosporangiatae followed by Marat- 

 tiales and Ophioglossaceae while within the Leptosporangiatae the order of cita- 

 tion is as follows: 



Hymenophyllaceae Gleicheniaceae 



Cyatheaceae Schizaeaceae 



Polypodiaceae Osmundaceae 



Parkeriaceae Salviniaceae 



Matoniaceae Marsiliaceae 



Diels, the author of most of this part of Engler and Prantl, was an evolu- 



