EVOLUTIOX OF PARASITIC FUNGI. 115 



Podosjihtera oxyacanthce, quite common on certain cherries and 

 other species of the genus Prunns, which has been considered by 

 some to represent several different species, according to the 

 number and position of the appendages of the peritliecium. In 

 the genus Microspha'va there exists one of the great niycological 

 puzzles as to just what the species MicrospJuera <ihii is, so 

 variable and so intergrading are many of the forms which have 

 been considered distinct species by some. Some of these varia- 

 tions in form are concomitant with the occurrence of the fungus 

 on certain host species, and the host species in some eases has 

 served as the only criterion for the separation of forms. Some 

 go so far as to include in this M. aim all forms of variation 

 within certain limits on all hosts except the oaks, drawing the 

 line at the genus Quercus, while it is impossible from purely 

 morphological characters to separate most of the forms on the 

 oaks from this generalized concept of Mlcrosphn'ra alni. 



Tendency toavard Fixity of Form. — On the other hand 

 there is a tendency toward a fixity of form and habit in some 

 fungi. Many of the members of the family known as the Smuts, 

 the Ustilagineae, are, partly from the simplicity of their spores, 

 limited to very slight variation of form. This is well shown in 

 the case of the oat, wheat, barley, and otlier smuts. Judged 

 simply from morphological characters the variation in form 

 would not seem sufficient to Avarrant the separation into so many 

 species as are now accepted. Cultures, however, which have 

 brought out differences in the course of development as well a% 

 in the confinement to distinct hosts have shown that species are 

 to be maintained where forms were formerly alone accepted. It 

 is quite within reason that at one time several of the species now 

 considered separate were one species, with variations including 

 nearly or quite the limits of the differences exhibited by the 

 species at present, and that now certain of these characteristics 

 have become fixed along with the confinement of the form on a 

 single host. 



Physiological or Biological Forms. — Perhaps one of 

 the most interesting tendencies is that in the direction of the 

 evolution of biological or physiological forms of a given species. 

 These are forms which could not be detected from a purely 

 morphological standpoint, but are determined by the study of 

 the relationship of the species to its various hosts. 



