EVOLUTION OF PARASITIC FUNGI. Ill 



ascosporous form of polymorphic fungi passes more readily into 

 the imperfect form, than the imperfect form passes into the per- 

 fect form in many cases. 



Texdexcy toavakd the Ixclusiox of Eorms IX THE Life 

 History. — As opposed to the shutting out of forms in the evolu- 

 tion of species there is a tendency among some to the inclusion of 

 forms. This is an accompaniment of the increasing polymor- 

 phism of certain fungi, or in the increase of forms in a given 

 species of fungus. By the increase in the number of forms in 

 which a fungus can grow and multiply, it increases its power 

 in the struggle for existence. Some of these additional forins in 

 certain species are probably either developed as a result of 

 the tendency of the same species to cut off an earlier form so 

 that this new form may take its place in tiding it over unfavor- 

 able periods for its growth, or in making more certain the perpet- 

 uation of the fungus in the imperfect condition. The incipient 

 stromata of many species, as well as the sclerotoid bodies, 

 segments of the mycelium which acquire a thicker wall and a 

 change in the condition of the contents so that the protoplasm is 

 more resistant, gemma?, and many bodies of these and similar 

 kinds, Avhich in many species take on more or less the form, and 

 certainly the function, of conidia, may be enumerated as coming 

 under this head. A very interesting case is that of the Monilia 

 fructigena, which causes the rot of many fruits. Under certain 

 conditions there is developed in this species an aspergillus-like 

 form of fruiting. 



Tendency to a Fluctuation in Virulence. — The ten- 

 dency to a fluctuation in virulence in certain species is a point 

 which would repay careful investigation. This tendency is 

 probably better known in the case of many bacteria than in the 

 fungi. Races of bacteria can quite easily be developed under 

 artificial conditions which are more or less permanent, according 

 to the length of time that they are subjected to the special treat- 

 ment or conditions. These races may show peculiarities of form, 

 physiological peculiarities, or peculiarities in virility as parasitic 

 germs, and partly upon the attenuation of the virility of certain 

 species does the theory of preventive inoculation rest. 



The common botrytis, which has been mentioned above, is an 

 illustration of this tendency in certain species of fungi. Under 

 some conditions it exists in our forcing houses on the dead or 



