wir 



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1^ A Monograph of the Erysiphaceae 



plication of species. For example, mor6 than twenty forms 

 of Phyllactinia corylea (notwithstanding that this is the most 

 sharply characterized species in the family) have been published 

 as new species. For the most part, these "species" were sup- 

 posed to be confined to the host-plant on which they were discov- 

 ered, and after which they were named ; this was the case with 

 Erysiphe fraxim, E. betulac, E. alni, E. maii, E. fagi, E. quer- 

 ats, E. pyri, E. ilicis, E. cerasi, etc. 



^ It is interesting to note that even among the earlier authors 

 this unscientific practice of making a new species of a form be- 

 cause it occurred on a new host-plant did not pass unreproved. 

 As early as 1819 Wallroth (383, p. 18) says: "Es ist ein ganz 

 irriger Glaube und ein unverzcihlicher Fehler der FKichtiffkeit wenn 



so viel Arten der Epiphyten aufgestellt und nach den 

 Pflanzen selbst, auf denen sie hervorbrcchen, benennt finden als 

 dercn selbst sind." Again, in 1 851, when Lcveille arranged in 

 clearly defined species the great mass of named forms (often dis- 

 tinguished merely by the host on which they grew) which had 

 accumulated since Wallroth's time, the same warning is repeated; 

 "Pour les reconnaitre on devra les etudier seulement quand ils 

 auront atteint leur plus haut degre d'organisation et ne plus faire 

 attention aux vegetaux sur lesquels on les rencontre. Cette 

 maniere de les denommer est essentiellement vicieuse, elle conduit 

 a la confusion et a I'erreur." 



InSaccardo's " Sylloge " 11 1 species, and i variety, and 20 

 species " dubiae vel inquircndae," are enumerated. To this num- 

 ber must be added 8 species and 6 varieties, either accidentally 

 omitted from the Sylloge, or published subsequently. 



In the treatment of the family in the present paper 49 species 

 and 1 1 varieties are recognized. Of these three species and two 

 varieties are new to science. 



This great reduction of species will not be wholly unexpected, 

 as a similar process has been found necessary in the treatment of 

 the family by several authors, in works published subsequently to 

 the " Sylloge." Winter, for instance, in his work on the Erysi- 

 phaceae o{ Germany, reduced several of the European species enu- 

 merated in the " Sylloge." and in i887BurriIl and Earlc (61) sim- 

 ilarly accounted for many of the American species of Schweinitz 



