1908] EDGERTON—ANTHRACNOSES 399 



Among the bacteria it is a common thing to speak of attenuated 

 forms; that iS; forms which have lost some of their characters, due 

 to growth on a different substratum or host from the customary one. 

 Many of the pathogenic forms have been cultivated until varieties 

 have been produced which are unable to produce the poisons charac- 

 teristic of the parent form. 



It is doubtful whether we can say for any form that it is absolutely 

 stable. Thom claims this for the species of PeniciUium; and doubt- 

 less some forms are more fixed than others. But, as Ppeffer (28) 

 suggests, the fact that a form has remained unchanged after growing 

 for two years on a certain medium does not prove that it will not vary 

 in a longer period, or does not prove that it will not vary on some other 



medium. 



category 



from those of Gloeosporium. They have been growing for ages as 

 saprophytes on various substances, and have become adapted to a 

 wide range of media, and the forms have become fairly well fixed; 

 but the forms of Gloeosporium are more plastic. Is it not just as 

 possible to have variable forms among the low, fast-growing fungi as 

 among the higher plants ? There seems to be no other explanation 

 for the condition that is present in the Glomerella type of anthracnose. 



If the forms will vary in culture under constant conditions, should 

 we not expect a greater variation in the open, where the extremes to 

 which the plants are exposed must be many times as great as in the 

 laboratory ? In nature, they must at one time or another be subject 

 to every sort of condition under which they will grow. If variation 

 is possible, we should expect to find varieties or forms built up which 

 will come true for at least several generations. 



Furthermore, if we admit the fact of variation, we can explain the 

 distribution of forms and also the constant discovery of forms of 

 -Gloeosporium on new hosts, or "new species" as they are generally 

 called. Quite often a form is discovered which has never before been 

 found, and immediately it becomes very prominent on account of 

 the great damage it is doing. As a form varies, it may adapt itself 

 to some new liost and be able to grow with some vigor. A sudden 

 mutation, like the one described above, might produce a form that 

 'would grow immediately on another host; or a form might take but 

 noorlv on a new host at first, but after many generations might become 



